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Establishment and evaluation of cover crops underseeded in sweet corn in Delta, British Columbia Ismail, Aweis Aware Issa 1994

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ESTABLISHMENT A N D E V A L U A T I O N OF C O V E R CROPS UNDERSEEDED IN SWEET CORN IN DELTA , BRITISH C O L U M B I A by Aweis Aware Issa Ismail L A U R E A ( A g r i c ) , Somalia National University, 1985 A T H E S I S S U B M I T T E D IN P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S (Department of Soi l Science) We accept this thesis as conforming to the required T H E U N I V E R S I T Y O F BR IT ISH C O L U M B I A October 1994 ® Awe is Issa, 1994 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree, that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of S c i ^ S> 0^ Oj -The University of British Columbia Vancouver, Canada Date N j MM J DE-6 (2/88) 11 A B S T R A C T A two year field experiment was carried out in Delta Municipality, British Columbia. The study was designed to investigate the effects of dates of underseeding different cover crops such as crimson, red clover and alsike clovers and annual ryegrass and fall rye with sweet com. The experiment was conducted as a split plot, randomized complete block design with 12 treatment combinations and four replicates. Ma in plots were the two dates of cover crop planting, one shortly after emergence and the second at sidedressing time (~ 30 cm). Subplots were comprised of an unseeded control plus five different cover crops seeded under sweet com. In the 1992-1993 and 1993-1994 growing seasons, the effects of cover crops on fresh arid dry cob yield of sweet com were not significant. In the 1992-1993 experiment the type of underseeded cover crops had no effect on either fresh or dry stalk yield, while in the 1993-1994 growing.season stalk yields were reduced by fall rye and annual ryegrass relative to red clover. Early planting of cover crops significantly reduced the fresh and dry stalk yield of sweet com. There were no differences due to cover crops in the com ear leaf nitrogen concentration in the 1992-1993 growing season. However, in the 1993-1994 growing season, sweet com/fall rye had significantly lower ear leaf nitrogen concentrations than sweet com/red clover. In the 1993-1994 growing season the ear leaf nitrogen concentrations of early underseeded sweet com were significantly lower than ear leaf Ill nitrogen concentration of late underseeded sweet corn. In the 1992-1993 growing season, red clover produced the highest cover crop dry matter yield. Nitrogen concentrations in alsike and red clovers were higher than that of annual ryegrass. In 1993-1994 growing season, crimson clover produced the highest dry matter yield. The nitrogen content of crimson clover was higher than that of alsike clover, fall rye and annual ryegrass in that year. In both 1992-1993 and 1993-1994 growing seasons, annual ryegrass had the highest percent cover compared to the other treatments. Fresh and dry cob yields of sweet corn were not affected by date of seeding nor type of cover crop underseeded. Early underseeded cover crop appeared to compete with sweet corn for nitrogen as compared to late underseeding. Despite low dry matter production which may be attributed to different growth habits, annual ryegrass and alsike clover look promising soil cover crops because they gave higher percentage of soil cover. iv T A B L E O F CONTENTS page ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES vii LIST OF APPENDICES viii ACKNOWLEDGEMENTS ix 1.0 INTRODUCTION 1 1.1 The importance of cover crops underseeded in sweet corn 1 1.2 Objectives 3 2.0 LITERATURE REVIEW 4 2.1 Historical background of cover crops 4 2.2 Resources for underseeding cover crops 6 2.1.1 Light 6 2.1.2 Water and nutrient use 7 2.1.3 Effect of cover crops in underseeding on plant population 8 2.3 Effect of cover crops in underseeding on insects and diseases 9 2.4 Effect of cover crop in underseeding on weed control 12 2.5 Legume cover crops in underseeding 14 2.6 Non legume cover crops in underseeding 17 2.7 Effects of cover crop underseeding on soil properties 19 V 2.7.1 Accumulation of organic carbon and nitrogen 19 2.7.2 Soil aggregation and aggregate stability 21 2.7.3 Soil water and temperature regimes 22 2.8 Effect of underseeding on socio-economic aspects 24 3.0 MATERIALS AND METHOD 26 3.1 Site Description 26 3.2 Experimental Layout 27 3.3 Field Sampling 28 3.4 Crop Harvest 29 3.5 Laboratory Methods 30 3.5.1 Plant Analysis 30 3.5.2 Soil Analysis 30 3.6 Statistical Analysis 31 4.0 RESULTS AND DISCUSSIONS 4.1 Weather and Soil Conditions 32 4.2 1992-1993 and 1993-1994 Experimental Results 33 4.2.1 Cob yield of sweet com 33 4.2.2 Stalk yield of sweet com 36 4.2.3 Ear leaf nitrogen of sweet com 40 4.2.4 Dry matter production, nitrogen concentration and nitrogen uptake 43 4.2.5 Ground cover (% cover) 48 vi 5.0 CONCLUSIONS 50 6.0 RECOMMENDATIONS 50 7.0 LITERATURE CITED 52 APPENDICES 60 Vl l LIST O F TABLES Page Table 1. Effects of date of planting cover crops on fresh and dry cob yield of sweet corn in 1992-1993 and 1993-1994 growing seasons 34 Table 2. Fresh and dry cob yields of sweet corn underseeded with five different cover crops in 1992-1993 and 1993-1994 growing seasons 35 Table 3. Effects of date of planting cover crops on fresh and dry stalk yield of sweet corn in 1992-1993 and 1993-1994 growing seasons 38 Table 4. Fresh and dry stalk yields of sweet corn underseeded with five different cover crops in 1992-1993 and 1993-1994 growing seasons 39 Table 5. Effects of date of planting cover crops on ear leaf nitrogen content of sweet corn in 1992-1993 and 1993-1994 growing seasons 41 Table 6. Ear leaf nitrogen concentration of sweet corn underseeded with five different cover crops in 1992-1993 and 1993-1994 growing seasons 42 Table 7. Cover crop yield and nitrogen concentration and uptake at two planting dates for 1992-1993 and 1993-1994 growing seasons 44 Table 8. Cover crop yield and nitrogen concentration and uptake (averages of two planting dates) for 1992-1993 and 1993-1994 growing seasons 47 Table 9. The establishment and percent cover of different cover crops in sweet corn 49 viii LIST O F APPENDICES Page Appendix 1. Some chemical properties of composite soil samples taken from plots in early spring 60 Appendix 2. Mean monthly air temperatures (°C) and precipitation (mm) during the 1992-1993 growing season compared with the mean data for the 1951-1980 period 61 Appendix 3. Mean monthly air temperatures (°C) and precipitation (mm) during the 1993-1994 growing season compared with the mean data for the 1951-1980 period 62 Appendix 4. Analysis of variance for fresh cob yield of sweet com underseeded with five different cover crops in 1992 growing season 63 Appendix 5. Analysis of variance for fresh cob yield of sweet com underseeded with five different cover in 1993 growing season 64 Appendix 6. Analysis of variance for dry cob yield of sweet com underseeded with five different cover crops in 1992 growing season 65 Appendix 7 Analysis of variance for dry cob yield of sweet com underseeded with five different cover crops in 1993 growing season 66 Appendix 8. Analysis of variance for fresh stalk yield of sweet com underseeded with five different cover crops in 1992 growing season 67 Appendix 9. Analysis of variance for fresh stalk yield underseeded with five different cover crops in 1993 growing season 68 Appendix 10. Analysis of variance for dry stalk yield of sweet com underseeded with five different cover crops in 1992 growing season 69 Appendix 11. Analysis of variance for dry stalk yield underseeded with five different cover crops in 1993 growing season 70 Appendix 12. Analysis of variance for ear leaf nitrogen status of sweet com 1992 growing season 71 Appendix 13. Analysis of variance for ear leaf nitrogen status of sweet com 1993 growing season 72 IX Appendix 14. Analysis of variance of cover crop biomass planted on 29 May and 22 June 1992 growing season. Cover crop was sampled on 3 March 1993 73 Appendix 15. Analysis of variance for cover crop biomass planted on 24 June and 20 July 1993 growing. Cover crop was sampled on 24 November 1993 74 Appendix 16. Analysis of variance of cover crop nitrogen concentration underseeded in sweet corn in 1992 growing season 75 Appendix 17. Analysis of variance of cover crop nitrogen concentration underseeded in sweet corn in 1993 growing season 76 Appendix 18. Analysis of variance for nitrogen uptake by cover crop planted 29 May and 22 June 1992 growing season. Cover crop biomass was sampled on 3 March 1993 77 Appendix 19. Analysis of variance for nitrogen uptake by cover crop planted 24 June and 20 July 1993 growing season. Cover crop biomass was sampled on 24 November 1993 78 A C K N O W L E D G M E N T S x Sincere gratitude is expressed to my graduate supervisor, Dr. Art Bomke to whom I pay glowing accolade for his patience, guidance and assistance. I am also grateful to the other members of my committee: Drs Brian Holl and Mahesh K. Upadhyaya for their assistance and constructive criticism of this thesis. Special appreciation is also extended to Dr Wayne Temple, UBC agronomist in Delta for his unlimited assistance throughout the study period and for the review and suggestions. Special appreciations is also extended to my friends Bandi, Sandy, Soenarto, Fremont and F. Wanju for their contributions. I would like to thank my parents for their love and the special memories of my late father. Lastly I wish to thank the governments of Canada and British Columbia for the funding my study at UBC through the Soil and Water Conservation Accord. 1 Chapter One INTRODUCTION 1.1. The importance of cover crops underseeded in sweet corn in Delta. Delta municipality is part of South Coastal British Columbia and has some of Canada's most productive land in agriculture. The major cash crops grown in Delta are sweet com, vegetables and potatoes and many are harvested late in the season. Present farming in Delta is far below its potential crop production. This can be attributed to a number of factors, e.g. declining soil organic matter and soil compaction. Over the past two decades the Delta farming community has switched from dairy farming to intensive vegetable production. As a result, organic matter inputs have decreased since crops such as peas, beans and potatoes return little crop residue. Declining soil productivity has been compensated by heavy fertilization (Bomke, personal communication, 1991). However, heavy usage of fertilizers can be decreased if farmers use grasses and legumes for underseeding (cover crops). Legumes act as cover crops, and fix nitrogen from the air, which may became available to subsequent crops. Agricultural crop production practices in Delta have intensified soil degradation processes such as soil erosion and compaction. Soil compaction occurs because the local farmers in the valley often till wet soils in early spring and late fall as required for spring seeded crops. In addition the winter precipitation causes leaching of soil nutrients and soil erosion. Underseeding with cover 2 crops may reduce nutrient leaching during winter. The introduction of different cover crops like red clover, crimson clover, alsike clover, fall rye and annual ryegrass as relay cropping in the farms in South Coastal climatic region, encompasses many important considerations. For instance cover crop establishment could be carried out during the drier months of the year and thus reduce soil compaction. The presence of cover crops in the systems will help to reduce soil compaction and erosion. Cover crops as a green manure are an alternative source of soil organic matter. The climate in Delta is exceptional with respect to precipitation and temperature. Because of the mild temperature, cover crops generally remain vegetative over the winter months and recommence growth in early spring. On average Delta, B.C. receives about 1000 mm of precipitation per annum, which is about half of that in the eastern parts of the lower Fraser Valley. The longest frost-free period in Canada, extends from April 15 to October 21 (Temple, 1992). The soils in Delta are inherently fertile, heavy textured and deep; consequently the soil has a good water retention capacity and a potential to sustain crop production on a year-round basis. Agronomic information pertaining to cover crop underseeding in sweet corn is lacking locally; however, information from other regions with similarities in climate is available in the literature. Such available information, however, may not be directly applicable under Delta, B.C conditions because differences in soil conditions and slight climatic differences may have considerable consequences. Therefore, the success of cover crops in sweet corn in Delta would demand that information be developed locally. The research work reported in this thesis investigated the establishment and evaluation of 3 cover crops in sweet corn production. 1.2 Objectives 1. To investigate the biomass production of three species of clover and two grasses underseeded with sweet corn. 2. To investigate the effects of cover crop type and date of seeding on sweet corn yield and the nitrogen content of both the cover crops and the sweet corn. 3. To investigate the effectiveness in providing soil cover prior to winter of underseeding sweet corn with five cover crops at two different dates. 4 Chapter Two L I T E R A T U R E REVIEW 2.1. Historical background of cover crops. Cover cropping is the practice of growing pure or mixed stands of annual or perennial herbaceous plants to cover the soil of croplands for part or all of the year. The plants may be left to cover the top soil or incorporated into the soil by tillage as green manure. In the history of agriculture, legumes and animal manure have been the major source of soil nitrogen. Although animal wastes, nonsymbiotic fixation, and atmospheric fixation can be significant sources of nitrogen, a large fraction can be attributed to legumes through symbiotic fixation. The importance of green manure can be traced back to early Mediterranean civilization, as early as the writings of Xenophen, who lived from 434 to 355 B.C. (Wedderbuan and Collingwood, 1976). In a review of historic agricultural practices, Semples (1928), cited several writers who had discussed the use of legumes for soil improvement. There is evidence in literature that bean crops were used as green manure by farmers in Macedonia and Thessely as early as 373 B.C. Comparisons among different types of legumes for soil improvement have also been reported (Smith et al. 1987). According to Pieters (1927), Chinese Civilization had known the importance of legumes in increasing crop production for more than 2000 years. Using legumes in crop rotation is among the oldest agricultural management 5 practices used to enhance soil fertility and crop production. Ancient Roman and Greek writers documented the importance of faba bean, vetch and other leguminous species as cover crops and rotation crops with grains (Smith et al. 1987). From the above discussion, it can be seen that the modern agricultural practice of green manuring is an ancient invention. At the beginning of modern agricultural science, Lawes and Gilbert conducted experiments at Rothamsted to measure and understand the significant contributions of legumes to soil fertility (Russell, 1966). By the 1930s, the mechanism by which legumes improve soil nitrogen availability, nitrogen fixation, and organic nitrogen mineralization, had become reasonably well understood (Waksman and Starkey, 1931; Fred et al, 1932). In early American agriculture it is difficult to determine how widely green manuring and cover cropping were practised. According to Pieters and Mckee (1929), these practises were known but not common in the colonial era. As soil fertility was depleted, the value of legume green manures should have become more apparent, but perhaps this problem was more commonly solved by long-term pasture rotations and application of animal manure to grain crops. The use of green manure and cover crops had yet not been appreciated, for example in 1936, which was suggested as the heyday of green manuring, there were 55 million hectares of cropped land in 12 southern states but only 0.8 million were seeded to winter cover crops (Pieters and McKee, 1938). Although there was 5.9 million hectares of green manure crops in the Southeast in 1940, the acreage declined significantly after that time (Rogers and Giddens, 1957). The decline can be attributed to the widespread availability of synthetic nitrogen fertilizer and the 6 economic advantage of its use in continuous grain crop production systems. After World War II, agronomists did not completely ignore winter cover crops but, for the most part, farmers did. A lot of research was conducted between 1940 and 1965 (Nelson, 1944; Evans et al, 1954; Beale et al, 1955; Kamprath et al, 1958; Benoit et al, 1962) but the practice was rather limited. From the historical background, it would certainly be difficult to claim that legume cover crops and legume intercropping are new ideas. However, during the last 10 years both researchers and producers have shown tremendous interest in this old practice. This can be explained by three major factors: (i) Large increases in the cost of fossil fuels and the related increase in the price of nitrogen fertilizer experienced in the 1970s and early 1980s. Although the costs of both commodities have recently stabilized or even decreased, the perception remains that over the long-term these are likely to become more expensive or more limited in supply, (ii) Increased concern about soil erosion and the more general concern about the effects of agricultural practices on environmental quality, (iii) Rapid adoption of no-tillage and conservation tillage practices by crop producers in many regions of United States and throughout the world. 2.2. Resources for underseeding cover crops 2.2.1. Light Light is an important resource and an inadequate supply becomes a limiting factor in achieving optimum yields. Therefore, successful mixed cropping systems may reduce 7 competition for light. This can be minimized through various possibilities such as planting the dominant crop in double rows, orientation of rows in east-west direction, increasing leaf inclination of the dominant crop, and growing shade tolerant crops. Willey (1979) conducted experiments at the International Crop Research Institute For The Semi-Arid Tropics and found that in a pearl millet- groundnut intercropping system, the much slower development of the groundnut canopy was apparent as it intercepted only 45% of incident light at 40 days after sowing as sole crop as compared to 80% by pearl millet at this growth stage. The interception in intercrop was intermediate between the sole crops. Pendleton et al. (1963), who studied intercropped maize-soybean sensitivity to reduced light intensity reported that east-west direction of rows reduced shading of groundnuts and led to yield increases. Other plant species like cocoyam, yam, cassava and cowpea can adapt to low light conditions (Steiner, 1984). 2.2.2. Water and Nutrient use It is believed that mixed cropping systems make better use of soil resources than sole crops because component crops can exploit different layers. Several authors have shown greater uptake of nutrients by intercrops than by sole crops (Liboon and Harward, 1975; De 1980, Natarajan and Willey, 1980). Reddy and Willey (1981) reported Land Equivalent Ratios (LER) values of 1.25, 1.28 and 1.26 for uptake of N, P and k respectively at final harvest in pearl-millet/groundnut intercrop and attributed higher yield of intercrop to those factors. The water use efficiency has also been studied in intercrops by some workers. 8 Baker and Norman (1975) while studying sorghum-pigeonpea intercrop found that better water use was probably a common cause of increased yields in the semi-arid tropics where water is a limiting production factor. 2.2.3. Effect of cover crops in underseeding on plant population Plant population refers to both the concepts of number of plants per unit area and the spatial arrangements to accommodate that population. It is generally reported that the total optimum population of mixed crops may be higher than that of sole crops. Willey (1979) confirmed these results in maize/bean mixed cropping. Results from mixed cropping in India indicated that in a mixed cropping system comprising of an 80-90 day cereal and 150-180 day pigeonpea, optimum plant population could be increased to full sole crop optimum of each crop (Freyman and Venkateswarlu, 1977). Similar results were reported from cassava/legume mixed cropping (Thung and Cock, 1979). Component populations generally have a direct bearing on the yield contributed by each in mixed cropping. This relationship however, is influenced by the relative competitiveness. Willey (1979), reported that component crops become relatively more competitive if they form a larger proportion of the trial population. Planting patterns or spatial arrangements are of equal importance to the relative population proportions in the mixed cropping because of competition for light, water and mineral nutrients. In general where a shorter crop is susceptible to shading, planting mixed cropping in multiple rows, alternate rows, or in some grouping has given higher yields than mixed intercropping (Dalai, 1977; Willey, 1979). In cereal based 9 intercropping systems in India, De (1980) showed that maize, sorghum or millet could be grouped in closer rows without any adverse effect on yield, leaving larger spaces for shade sensitive species like groundnut, and in this way achieve highest LERS. He also found no adverse effect on dominant crops by reducing inter-plant spacing within the rows. In the intercropping systems, where cover crops have been used as underseedings, it is reported that the cover crops had no effect on the yields of the dominant crops. For example, Palada et al. (1983) working at Rodale Research Centre, found no reduction in grain yields in corn underseeded with legumes. Nanni and Baldwin (1987) in Ontario found that different clover species underseeded in corn did not affect the corn grain yield. Mt. Pleasant (1982), working in New York, saw that corn yields were not affected by red clover intercrop during establishment provided that corn was 0.5 to 0.30 m high at the time of cover crop establishment. Wall et al. (1991) working at Guelph, concluded that intercropping silage corn with red clover can provide soil erosion protection without sacrificing silage corn yields. 2.3. Effects of underseeded cover crops on insects and diseases Cover cropping systems constitute agricultural systems diversified in time and space. There is evidence that this vegetational diversity often results in significant reductions of insect pest problems (Altieri et al. 1978). Research on the effects of cover crops on weeds, pathogens and nematodes has started to emerge, and studies indicate that their 10 populations change in response to diversification of cropping systems (Egunjobi, 1984). The effects of intensive systems on pests and weeds can neither be generalized nor predicted because of the enormous variety of systems utilized throughout the world. As the temporal and spatial dimensions of vegetation diversity change, so does the magnitude of the effects on pest population (Perrin and Philips, 1978). For the majority of cover crops, the residue remains on the soil surface following herbicide application, increasing the overall diversity in the agroecosystem. The most pronounced effects are seen early in the season prior to, or immediately following, herbicide application. Several studies have shown that there is an increase in the arthropod fauna, most notably soil predators, herbivores, and decomposers, with the use of cover crops. It has also been observed that with different types of cover crops there is an increase in arthropod diversity. House and Alzugary (1989) found that hairy vetch supported higher below ground arthropod densities and more diverse fauna than crimson clover or wheat. Smith et al. (1988) in Ohio, found that potato leafhopper populations in soyabeans were consistently lower when rye cover crop residues remained on the soil surface. Highest numbers of potato leafhoppers were found in rye-free plots or where rye was plowed in. This negative impact on potato leafhoppers from the presence of grassy residues corresponds to studies with alfalfa (Lamp et al. 1984a; Lamp et al. 1984b; Oloumi-Sadeghi et al. 1989) and soybeans relay-intercropped into winter wheat (Hammond, 1990) where lower leafhopper populations were found in mixed grass/legume systems. Potato leafhopper population was found to be lower in mixed grass and legume 11 in an experiment where soybean was relay-intercropped into winter wheat. Grasses are not hosts for potato leafhopper but induce behavioral changes in the leafhopper (by a mechanism that is not completely understood) which in turn reduces leafhopper numbers. These changes apparently can occur whether the grass is living or, in the case of cover crops, dead or dying from a recent herbicide application. Smith et al. (1988) found that numbers of Japanese beetles and bean leaf beetles were slightly higher in the rye cover crop plots. The impact that cover crops have on soil and foliar arthropods depends not only on the types of arthropods and cover crops and main crop agroecosystem but also on the type (grass versus legume) and management of the cover crops. Knowledge of influence of the cover crop on the arthropods may assist the farmer in better management of their cropping systems. Steiner (1984) has given a list of cases where component crops have been used successfully in controlling pests in a wide variety of crop combinations. The mechanisms that have played a role are reported to be visual effects on insects, impediments in dispersal of larval stages of insects, increased abundance of natural enemies and feeding inhibition. Despite this broad situation, the pest incidence/damage may also be influenced by crop species or variety and location interactions. Natural ecosystems can be regarded as models for pest management strategies in agroecosystems. Some rural societies simulate forest conditions in their farms to obtain the beneficial effects of forest structures. Farmers in Central America imitate the structure and species diversity of tropical forests by planting a variety of crops with different growth habit. By keeping diversity at the highest possible level, small scale 12 farmers have minimized the threat of unstable condition (such as pests) while obtaining a stable source of income and nutrition and maximizing returns under low levels of technology. Taylor (1977) reported that in maize/cowpea combination, the stalk borer (Chilpartellus) damage in maize was 50 to 60 percent less than in the case of the maize sole crop. However, pod damage in cowpea by Maruca, was the same in both the intercrop and sole crop in variety Tvu4557, but was reduced by nearly 50% in intercrop as compared to the sole crop for variety Ife brown. Chad and Sharma (1977) reported significant reduction of borers {Chilpartellus) in maize incidence in intercropped (maize/beans) as compared to sole crop. Altieri (1978) observed that populations of several important bean pests were reduced in maize/bean crop combinations due to increases in predator populations. However, Steiner (1984) reported that dry season planting of maize with cotton increased the abundance of boll worm (Helothesis armergia) in cotton as the pest could multiply on maize and migrate to cotton without being checked by enemies. 2.4. Effect of cover crop in underseeding on weed control Weeds are a major limiting factor in crop production and have a significant influence on yields as well as the area that farmers can cultivate. Most farmers are concerned with reducing negative impacts of weeds on crop production and the losses that they suffer from weeds. Often they cannot kill or effectively suppress the weeds. Presently, farmers in the United States spend more than $6.2 billion annually controlling 13 weeds in crop production and pastureland. This includes an estimated $3.6 billion used for nearly 200 million kilograms of herbicide (Shaw, 1982; Pimental and Levitan, 1980). About 50% of all tillage operations in the United States are carried out specifically for weed control (McWhorter and Chandler, 1982), and one to three cultivations are common in many row crop production systems (Zimdahl, 1981). Over the last several decades, farmers in Canada and United States have used more herbicides in weed control. This has made tillage operations shift towards reduced or minimum tillage systems (Koskinen and McWhorter, 1986). More reliance on herbicide has resulted in increased farm sizes and a decreases in crop diversity within farms (USDA, 1973). A large number of researchers and farmers look at herbicides as a main ingredient for effective weed control and increased profit. Despite the current emphasis on herbicides in North American agriculture, several factors (e.g. environmental cleanliness, quality of produce and herbicide resistant etc.) have recently led to a reappraisal of their use. Secondly some farmers are faced with financial difficulties and are led into consideration that farm profitability might be increased by reducing inputs such as herbicide, if less costly alternatives were available (Papendick, 1987; Francis and King, 1988). Lastly, researchers and farmers have become increasingly aware that full-time farms can operate profitably with little or no use of herbicides (Thompson and Thompson, 1984). Based on experience of farmers and research results, we can see that some biological and physical practices may reduce heavy dependence on herbicides and potentially improve farm profits and environmental quality. These practices include using 14 allelopathic cover crops, intercropping, and crop rotation. Cover cropping in row crop production that combines short-term crop rotation has a potential in reducing use of herbicides for weed control in conservation tillage. Cover crops are generally established prior to the fall to provide a dense soil cover during winter and spring, and to suppress weed germination and establishment. 2.5. Legume cover crops in underseeding Different legume species are available in many different parts of the world. Some of these legume species are used to feed livestock as in grazing or silage, green chop and hay. Many of these species have played a significant role in soil conservation tillage practices. Generally speaking, the diverse climatic and soil conditions across both temperate and tropical regions necessitate this diversity in legume resources. Therefore, no specific species is dominant in a particular region. Red clover (Trifoliumpratense L.) is one of the most important legumes grown in United States and Canada because of its winter hardiness and fixation of substantial amounts of nitrogen and significant biomass production. The total area covered in both countries is 5 million hectares of land (Smith et al. 1985). Red clover is grown for pasture, hay, improvement of soil structure in a four year crop rotation. White clover (Trifolium repens L.) is another important legume cover crop in temperate regions (Carlson et al. 1985). White clover is a legume grown by farmers and ranchers in many different parts of the United States annually. 15 Other important legumes are crimson clover (Trifolium incarnatum L.), vetch (Vicia spp), rose clover (Trifolium hirum L.) and alsike clover (Trifolium hybridwn). Other species are important in many different parts of United States. As an example, Roter and Kretschemer, have a large program involving over 4,000 accessions of tropical legumes in Florida (Pederson and Knight, 1984). Biologically, most legume species are annuals or biennials. Their adaptation ranges from semi-temperate for hairy vetch and crimson clover to temperate for winter pea, sweet clover and alfalfa. Dry matter production of these legumes ranges from 2.3 t/ha for sweet clover to 10 t/ha for hairy vetch and alfalfa (Palada et al. 1982). Most legume cover crops cannot tolerate dry and acid soil conditions, while some are known to be tolerant to shade and field traffic, which are ideal characteristics of intercropping. Resistance to severe winter frost is important if the legumes are grown for soil nitrogen. Winter survival and spring regrowth seem to be fair with selected species. In spring 1978, research was conducted in many different parts of the United States to observe the crop establishment and growth characteristic of six legume sod species (Palada et al. 1982). These species were medium red clover, crownvetch, short vetch, Nolan improved Louisiana white clovers, strawberry clover and sweet clover. The legumes were seeded without companion crops and managed as if they were grown for hay production. Strawberry clover and crownvetch were totally destroyed by tillage operations, while short white clover and Nolan improved Louisiana white clover showed significant resistance. Of the six species, only medium red clover and short white clover survived the winter. This research helped to identify species that are suitable for 16 overseeding and interplanting. If the cover crop used is a legume or a mixture including a legume, it can provide the additional benefit of contributing a substantial amount of biologically fixed nitrogen to subsequent crops. In association with appropriate Rhizobium bacteria, legumes are capable of fixing atmospheric nitrogen, which becomes available to other plants through mineralization. Certain species of legumes are genetically more efficient than others at fixing nitrogen. Given well-inoculated leguminous plants, the amount of biologically fixed nitrogen supplied by a particular legume cover crop is affected mainly by the amount of growth of the legumes, particularly the aboveground growth. According to Allison (1957), average values for N 2 fixation by legume crops are usually in the range of about 60 to 110 kg/ha, but more than 225 kg/ha of N may be fixed by certain legumes. The amount of nitrogen produced is a function of the dry matter yield and the nitrogen content of the legume. Therefore, any factor limiting dry matter production by the legume decreases the amount of nitrogen produced. Results with legumes have shown that about 80% of the nitrogen is contained in the above-ground portion of the cover crops. Van Doren, (1979) observed that it did not matter whether the legume stand was weedy as long as there was a reasonable population of vigorous legume plants in the stand. Several factors affect the amount of nitrogen produced by the legumes. Slow growth in the spring resulting from cold and dry weather or from some other environmental factor may severely limit nitrogen production by legumes. Killing legume stands too early will limit the nitrogen production by the legume cover crop. Legumes that are poorly adapted to some specific localities will perform poorly in fixing and 17 providing nitrogen to the subsequent crop if grown in these localities. Bomke et al. (1993), while investigating the effect of a wide range of fall-seeded cover crops on nitrogen cycling in the South Coastal region of British Columbia, found that by spring plow-down time only crimson and red clovers and the low yielding forage kale had nitrogen concentrations in excess of 2%, the approximate level required for net nitrogen mineralization for the succeeding summer crop. Although red clover had the highest nitrogen concentration, its vigor and dry matter accumulation were so low that it was not expected to make a significant contribution to available nitrogen during the subsequent growing season. 2.6. Nonlegume cover crops in underseeding The value of nonlegumes as cover crops has been recognized for many years. Generally speaking, nonlegume cover crops are classified into two major families Graminaceae and Cruciferae. In the Graminaceae family, the majority of research has centred on the use of cereal rye (Secale cereale L.), although many other grasses such as barley (Hordeum vulgare L.) and wheat (Triticum aestivum) have been successfully used (Hargrove and Frye, 1987). Most of the remaining nonlegume cover crops are members of the genus Brassica and include such crops as mustard (Chapman et al, 1949). The ability of nonlegumes to prevent nitrogen leaching is related to their ability to develop rapidly and their dry matter production under cool conditions. Grasses have been used extensively as cover crops because they are hardy under a wide range of 18 environmental conditions. The ability of cover crops to improve soil structure and to take up and conserve residual nitrogen prior to the winter rainy period is directly related to their ability to accumulate biomass (Bomke et al. 1993). However, non leguminous cover crops like cereals and annual ryegrass, which had total nitrogen concentrations at plowdown time of 0.9 to 1.3% may immobilize soil available nitrogen in direct proportion to their dry matter yields. A field study was conducted on the Atlantic coastal plains of Maryland in which com (Zea mays L.) was fertilized with 336 kg N/ha and an unfertilized rye cover crop was planted in early October (Meisinger et al. 1990). The com was intentionally over-fertilized to ensure a large pool of fall NOj-N to test the capacity of the rye to use residual nitrogen. Shallow groundwater wells 1.5 m deep, were installed in replicate plots in November before recharge season, and water well samples of recent percolation drainage into these wells were collected throughout the winter and spring. The average N0 3-N concentration below the no-cover controls was 17 ppm, while the concentration below the rye cover was 12 ppm. Therefore, the rye cover crops reduced the concentration of N0 3 entering shallow groundwater by 29%. It was not possible to measure drainage volumes in the study, but it is clear that the rye cover crop had a beneficial impact on groundwater quality. Other researchers monitored during fall and winter the soil N0 3 content in fields seeded with cover crops (Neilsen and Jensen 1985; Staver et al. 1990). In the studies, it was observed that there was a marked reduction in the size of the mobile NO3-N pool 19 below grass cover crops. In the State of Maryland, a rye cover crop reduced the NO3-N content below the 0-30 cm soil surface layer from 58 to 13 N kg/ha during the winter. Neilsen and Jenson (1985) in Denmark found that, an annual ryegrass cover crop reduced the N0 3-N pool in 100 cm of soil by 33 kg N/ha, which represented a 62% reduction in potentially leachable nitrogen. From the above studies, it can be seen that grass cover crops are effective in reducing the mass and concentration of the leachate N0 3. The somewhat smaller percent reduction in the N0 3 concentration stems from the fact that as cover crops take up N the mass of potentially leachable N decreases; but the N0 3 concentration in the soil solution may not decrease. Because of simultaneous use of NO^ and water by cover crops a larger percentage reduction could be expected in the mass compared with the concentration of N lost. Among the grass cover crops studied so far, it seems that cereal rye is best in many environments for the improvement of water quality. 2.7. Effects of cover crop underseeding on soil properties 2.7.1. Accumulation of organic carbon and nitrogen It can be stated as common knowledge that legumes and grasses in rotations will increase soil organic matter, or at least maintain it at relatively higher levels than under row crops. Increased organic matter could be beneficial to crop growth by enhancing soil physical and chemical properties, water retention capacity and nutrient reservoirs. 20 Kamprath et al. (1958) in North Carolina measured the effects of oats or hairy vetch winter covers with conventionally tilled corn and various nitrogen fertilizer rates on changes in soil carbon and nitrogen over eight years at four sites in North Carolina. In general, soil organic matter declined without cover crops but tended to increase with either vetch or oats plus nitrogen fertilizer. Touchton et al. (1984) in Georgia concluded that winter legumes caused no measurable changes in soil carbon or nitrogen, but research data indicated strong trends for a relative increase with crimson clover or common vetch. Hargrove (1986) measured soil carbon and nitrogen before and after three years of no-tillage grain sorghum with several different winter cover treatments in Georgia. He found that organic matter declined in winter fallow treatments but was generally maintained or declined less with cover crops. The differences were consistent only above 15 cm soil depth. There was little evidence that soil organic matter accumulation was highly sensitive to type of cover crop or residue used. Legumes result similiar soil organic carbon contents as equivalent quantities of higher C:N materials, such as grass or wheat straw. Larson et al. (1972) added into the soil different crop residues for 11 consecutive years. For a given mass of a residue, soil carbon accumulation was comparable for legumes, straw, and even sawdust. Soil nitrogen increases were also surprisingly similar for all materials except sawdust an extremely low N substrate. Kamprath et al. (1958) observed no consistent differences in soil carbon and nitrogen between hairy vetch and oats if adequate fertilizer nitrogen was supplied for good crop growth. Hargrove (1986) found that rye covers resulted in just as much soil nitrogen accumulation as crimson clover, and at least as much soil carbon, even though 21 the nitrogen content of the crop residue of the former is less than a quarter of the latter. This indicates that the retention of both organic carbon and organic nitrogen in the soil is independent of crop residue. However, hairy vetch, which contained slightly more carbon and nitrogen, than crimson clover, resulted in significantly greater soil carbon and nitrogen. Beale et al. (1955) observed more soil nitrogen after 10 years with minimum tillage cover crops than moldboard plowed cover crops. Such observations may reflect greater loss of soil organic matter with greater tillage, and not relatively less effect of cover crop residue on soil organic matter in plowed systems than minimum tillage systems. Utomo (1986) in Kentucky observed that there was a greater difference between organic carbon in hairy vetch and winter fallow treatments for no-tillage than for conventional tillage. Vetch had a small effect in conventional tillage, but a significant effect in no-tillage. 2.7.2. Soil aggregation and aggregate stability Many of the effects of legumes on soil physical properties are exemplified by their effect upon soil aggregation and aggregate stability. Tisdall and Oades (1982) indicated that soil aggregation is influenced by three types of agents: (1) transitory materials, such as polysaccharides, that are usually products of microbial activity, (2) temporary effects through binding action of fungal hyphae and plant roots, and (3) persistent effects resulting from the action of polyvalent cations and strongly adsorbed organic polymers. They concluded that total quantity of soil organic matter present has a major influence on aggregation and aggregate stability. Therefore, use of legume cover 22 crops in a cropping system could affect aggregation through changes in soil organic matter content and microbial activity. Because of the relative narrow C:N ratio of legume residues, microbial biomass may be temporarily increased, increasing aggregation due to hyphal binding. On the other hand, grass roots are usually more fibrous than those of legumes, hence aggregation resulting from root binding may be greater under grasses than under legumes. Strickling (1950) observed these same effects of cropping systems on soil organic matter and aggregation. In general, the water-stable aggregates (greater than 0.25 mm) were closely related to soil organic matter content. Aggregation in soil in continuous bluegrass was much greater than that for any other treatments. For cultivated soils, aggregation was greatest for a rotation containing two years of alfalfa-grass hay. Continuous ryegrass was intermediate and continuous com was very low in aggregation; however, lowest values were reported for com and soybean hay. 2.7.3. Soil Water and Temperature Regimes Legumes in crop rotations have some effects on the soil water and temperature regimes. Legume cover crops lower soil temperature by acting as mulch (live or dead). The insulating effect of legume residues on the soil surface is no different from the nonlegume residues. Utomo et al. (1987) found that soil temperatures under no-till hairy vetch residue and com stover were respectively 1.5° and 1.2°C lower than for clean, cultivated com. The main effect of legumes on reducing temperature and potential evaporation rates results from the fact that legume cropping systems often provide more ground cover than occurs under normal cultivation. However, a living mulch of legumes 23 reduces soil water content, thereby reducing the heat sink in the soil. The effects of legume cover crops on soil water were discussed by Hargrove and Frye (1987). They found that when used as a cover crop, legumes utilized stored soil water during the noncrop period of the grain crop with which the cover crop was associated. This can have a positive, negative, or no effect on the following grain crop. In poorly drained soils, when excessive precipitation was received during the noncrop period, use of legume cover crop reduced soil water content, thereby reducing the adverse effect of the excess water on crop growth. The cover crop also reduced the likelihood of nutrients and pesticides leaching into ground water. For drier climates, however, legume cover crops can reduce soil water content to such an extent that the following grain crops suffer. For example, Koerner and Power (1987) showed that under Eastern Nebraska conditions hairy vetch, if not properly managed as a winter cover crop, reduced soil water storage and increased competition, reducing yield of the following corn crop. In the wheat growing regions of the northwestern United States, various legumes are frequently grown in different types of rotation with winter wheat. Elliot et al. (1987) showed water storage at wheat seeding time varied with the legume used. Water storage was decreased most with a spring pea rotation, and least with red clover or hairy vetch in rotation. Legume dry matter production and amount of nitrogen fixed by the legume generally increased, except for spring pea. These results indicated that legume species differ significantly in their water requirements as well as in nitrogen fixation. 24 In drier regions, use of legumes in crop rotations is often restricted because of water availability. Haas et al. (1976) showed that deep rooted legumes such as alfalfa or sweet clover, when grown in rotation with wheat in North Dakota, frequently depleted soil water reserves to 2 m or greater. As a consequence, the following grain crops had no subsoil reserve of soil water, and yields for the first several years after plowing up the sod suffered accordingly. Brown (1964) came to a similar conclusion after summarizing long term data from legume based rotations at a number of locations throughout the north American Great Plains. 2.8. Effect of underseeding on socio-economic aspects Although increased productivity is one of the major advantages of mixed cropping, there are equally important socio-economic considerations which induce farmers to adopt these cropping systems in preference to sole cropping. Norman (1977) and Francis and Sanders (1987) reported that mixing maize with legumes gave comparable returns to sole crops. The crop mixtures are also considered as a risk minimization mechanism. Rao and Willey (1980) studied stability of mixtures as compared to sole crops by determining the probability of crop failure. In no case did mixtures show a higher probability of a return below the sole crop mean. In northern Guinea, savanna mixtures showed a much reduced risk of crop failure. Jodha (1977) reported that intercropping is predominant in low rainfall/high risk areas. Similar observation were made by Dichel (1981) in Southern 25 Guinea, where risk of crop failure is high due to lack of rain. Farmers there grow mixtures of drought resistant crops in order to have some yield in dry years. Another important economic factor that has influenced perpetuation of crop mixture is diversified and continuous food supply over prolonged periods. Steiner (1984) considered this as important in humid areas, where storage of harvested produce is difficult. In south coastal British Columbia as with other regions of a similar climate, the heavy precipitation can leach most of the residual nitrogen after harvest. The use of cover crops as nitrogen scavengers can alleviate the problem particularly if they follow a crop associated with moderate levels of nitrogen mineralization after harvest, such as early potatoes, beans and peas (Temple, personal communication). Clearly there is a need to develop innovative methods to increase organic matter inputs in order to increase crop yields and reduce agrochemical costs. Successful practices must be easily incorporated into the current cropping systems and compatible with profitable farming. 26 Chapter Three MATERIALS AND M E T H O D S 3.1. Site Description The two year (1992-1993) and (1993-1994) field study was carried out in Delta Municipality, approximately 30 km south of Vancouver, British Columbia in cooperation with John Malenstyn, Jowkema farms. Delta was chosen as the location for the study because of its proximity to UBC and the availability of a farmer willing to cooperate in the study. Sweet com variety 'Jubilee', the crop chosen for the study, is grown in the region mainly for canning and freezing. The soil classified as a Crescent silty clay loam, Orthic Gleysol, whose parent material is deltaic alluvial deposits (Luttmerding, 1981). The climatic data was provided by Environment Canada, Delta Ladner Weather Station. Drainage is the major problem in the study area. The 1992 experiment was conducted on a field with surface drainage to a ditch, while the 1993 experiment was conducted on a site with subsurface drains. The preceding crop on the former site in 1991 was potato and on the latter site peas in 1992. 27 3.2. Experimental layout The experiment was conducted as a split plot, randomized complete block design with 12 treatment combinations and four replicates. Main plots were two dates of cover crop planting, shortly after emergence or sidedressing time ( — 30 cm), and subplots were comprised of an unseeded control plus five different cover crops seeded under sweet corn. Main plots measured 8 m x 48 m and sub-plots 8 m x 8 m. During the 1992-1993 growing season, the dates for the early and late underseeding of cover crop were 29/5/92 and 22/6/92, whereas during the 1993-1994 growing season, the respective dates were 24/6/1993 and 20/7/1993 respectively. Two weeks before planting, a preemergence herbicide vernolate (surpass) was applied at a rate of 5.5 1/ha. Planting of sweet corn was done by the farmer at the of 60,000 plants/ha using a row width of 1.0 m. The seeding rates of cover crops used in both years were: crimson clover (12 kg/ha), red clover (12 kg/ha), alsike clover (7 kg/ha), annual ryegrass (20 kg/ha) and fall rye (80 kg/ha). The red clover, annual ryegrass and fall rye cultivors were Pacific double cut, aubade and Danko, respectively, while common seed was used for crimson and alsike clovers. Red clover and alsike clover seeds were inoculated with the appropriate Rhizobium just before planting and broadcast seeded by hand. In the 1992-1993 growing season, urea was side banded by hand alone the corn rows at the rate of 104 kg N ha"1 and cover crops were broadcast by 28 hand. In the 1993-1994 growing season, the experiment was repeated and ammonium nitrate applied at the rate of 102 kg N ha"1 when the sweet com was 30 cm in height. 3.3. Field Sampling Weed identification was done in July and August by locating randomly within each plot. The weeds were clipped at ground level and separated into grasses and broad leaves for identification. During the study, the four centre rows of the experimental plots were hand weeded. Soil samples for site characterization were collected just prior to the first date of underseeding cover crops in sweet com. Six composite soil cores were taken randomly within each plot at one depth (0-20 cm) using an Oakfield 2.5 cm diameter sampling probe. The soil samples were placed in labelled polythene bags and transferred in a cooler to the laboratory where they were stored in refrigerators at 4° C. NH 4 and N0 3 -N were extracted within 24 hours. Four bulk density samples were taken in each site at the time soils were sampled for chemical characterization. A cylindrical core (7.3 cm diameter, 7.6 height) was inserted vertically on the soil surface. The core was then dug out using a spade, the excessive soil was trimmed. The bulk density was determined by oven drying the samples at 105°C for 48 hours. The sweet com ear leaves were sampled immediately after silking, (R stage) for analysis of the nitrogen status. Twenty ear leaves were randomly selected from four center rows of each plot on 30/7/1992 and 27/8/1993. During the growing season, 29 establishment of cover crops was assessed by two methods. The first was through field observation, which involved rating the cover crops on the following scale: F (fast), S (slow), M (medium), (Temple, 1991). The second assessment was percentage of ground cover estimated by stretching a string with 25 points along the inner six rows (Laften et al., 1981 ). 3.4. Crop harvest Harvest of the experimental plots was done by hand, using machetes. Samples were taken from 1 meter lengths of the two centre rows of each plot. Fresh weights of stalks and cobs were obtained. Following weighing in the field, three stalks were randomly subsampled from each plot for dry weight determinations of both stalks and cobs after being dried at 65°C. About two weeks before harvesting, the sweet corn plants were topped by a contractor in order to facilitate combine harvesting. The process of topping involved the removal of the top 50 cm of the plants and that portion of the crop was not included in the total stalk weight recorded at harvests on 28/9/92 and 27/9/93. Cover crops were sampled randomly from a 0.5 m 2 quadrat on every plot by clipping at ground level. Assessments of cover crops were done at two different times in spring 1993 (1992-1993 growing season) and fall 1993 (1993-1994 growing season). Spring cover crop sampling was done on 3 March 1993 because the soil was too wet on November 1992 to sample without serious disturbance. However, in November 1993 the 30 soil was not wet. The sample were taken on 2 November 1993, before the field had been cultivated. A 1994 spring assessment was not made because the farmer inadvertently cultivated the site before it could be done. 3.5. Laboratory methods 3.5.1. Plant analysis Sweet com and cover crop plant samples were taken to the Totem field laboratory for drying at 65°C in a forced air oven for 72 hours. After dry weight determination, cover crop plant material was ground using a stainless steel Wiley mill to pass a 2 mm sieve. Samples of 0.5 g were digested following the procedure outlined by Parkinson and Allen (1975) and total N concentration was determined colorimetrically using a Technicon Autoanalyzer II (Technicon, 1974). 3.5.2. Soil analysis Soil samples were mixed in their respective polythene bags before extraction. Soil water contents of the samples were determined by oven drying a 30 g subsample of soil at 105°C for 24 hours and reweighing (Gardner, 1986). Bulk density samples were treated in a similar manner. Field moist 10 g samples were extracted for NH4 -N and N0 3-N by shaking with 100 mL of 2 M KC1 four one hour (Keeney and Nelson, 1982). After settling, the supernatant was filtered through Whatman No.42 filter paper. Two drops of toluene were 31 added to extracts stored in 60 mL bottles at 2 °C awaiting analysis. NH4 -N and N0 3 -N concentrations were determined colorimetrically using a Technicon Autoanalyzer II, coupled with a cadmium reduction column for NO3-N (Technicon, 1977). The soil samples used to describe the study site (Appendix 1) were extracted using the Kelowna extractant (0.015 M NELF+0.25M HOAC) and available nutrients in the soil determined by procedures outlined by Gough (1991). 3.6. Statistical analysis Data from each growing season were subjected to analysis of variance following procedures outlined by Little and Hills (1978) using a computer program Proc. G L M (SAS Institute, 1988). Orthogonal contrasts were used to partition main effects and interaction sums of squares into single degree of freedom contrasts. Statistical significance was determined at the probability level of 5%. Duncan's multiple range test was used to compare means following a significant F-value. 32 Chapter Four RESULTS AND DISCUSSION 4.1. Weather and soil conditions Results of soil nutrient analysis conducted on samples taken from the two experimental sites are presented in (Appendix 1) and are interpreted by using the soil interpretations recommended by the British Columbia Ministry of Agriculture (Gough, 1991). Soil pH is relatively low. Soil organic matter and total nitrogen concentrations at both sites were low. The concentrations of phosphorus, potassium and magnesium were very high. The bulk density of the soil 0-20 cm layer was found to be about 1.2 Mg/m3. Mean monthly air temperatures (°C) and precipitation (mm) during 1992-1993 and 1993-1994 seasons are presented in Appendices 2 and 3, respectively. The average precipitation from 1951-1980 was 1133 mm (Appendix 2,3). In the 1992 growing season, sweet com was planted in May. Cover crops were underseeded in May (Early) and June (Late). The amounts of precipitation in the months of May and June 1992 were 15.8 and 96.4 mm, respectively. May precipitation was lower than the average of 1951-1980 (30 years) which was 51.6 mm, while June precipitation was higher than the 30 year average of 45.2 mm. In the winter months (December-March) of 1992, average precipitation was lower than average of 30 years. In 1993, sweet com was planted in June. Cover crops were underseeded in June 33 (Early) and July (Late). The amount of precipitation in the months of June and July 1993 were 72.2 and 34.2 mm, respectively. June precipitation was higher than the 30 year average (45.2 mm). July precipitation close to the long average term of 32.0 mm. In the fall of 1993, average precipitation was lower than that of 30 year average. 4.2. 1992-1993 and 1993 and 1994 Experimental Results Analysis of variance tables for soil, sweet com and cover crop variables are presented in Appendices 4-19. 4.2.1. Cob yield of sweet corn Fresh cob yield of sweet com is the most important attribute that determines its economic return. The data in Table 1 show that date of planting of cover crops in sweet com had no significant effect on fresh or dry cob yield. The data in Table 2 indicate that the five different cover crops underseeded in sweet com in both growing seasons did not have any significant effect on the fresh yield of the sweet com. The data for dry cob yield of sweet com in both 1992-1993 and 1993-1994 growing seasons also show that there were no statistically significant differences among the five cover crops (Table 2). 34 Table 1. Effects of date of planting cover crops on fresh and dry cob yield of sweet corn in 1992-1993 and 1993-1994 growing seasons. Fresh cob yield Dry cob yield Treatments (t/ha) (t/ha) 1992/93 1993/94 1992/93 1993/94 Early planting 14.5" 14.2a 3.3a 3.1s Late planting 16.3" 17.9a 3.5" 3.7" C.V.(%) 14.5 12.9 21.1 18.0 Note: means within a column with the same superscript are not significantly different (P>0.05). 35 Table 2. Fresh and dry cob yield of sweet corn underseeded with five cover crops in 1992/93 and 1993/94 growing seasons. Fresh cob yield Dry cob yield Treatments (t/ha) (t/ha) 1992/93 1993/94 1992/93 1993/94 Unseeded 15.6* 16.6" 3.5a 3.4a Crimson clover 15.5" 15.5a 3.3a 3.3a Alsike clover 14.6a 15.9a 3.2a 3.3a Red clover 15.6a 17.3a 3.3a 3.1a Fall rye 15.1a 14.8a 3.1a 3.4a Annual ryegrass 15.8a 16.4a 3.8a 3.8a C.V.(%) 14.5 12.9 21.1 18.0 Note: means within a column with the same superscript are not significantly different (P>0.05). 36 The results presented above on fresh and dry cob yield of sweet corn clearly point out that legume and grass cover crops underseeded with sweet com had no effect on sweet com yield. This agrees with findings from other researchers working with cover crops underseeded or overseeded in com. For example, Palada et al. (1982) reported that there was no reduction in grain yields in com which was overseeded with legumes. Nanni and Baldwin (1987) found that different clover species underseeded in corn did not have an effect on com grain yield. Mt. Pleasant (1982) noted that com yields were not affected by red clover intercrop during the establishment provided that com was 0.15 to 0.30 m in height at the time of cover crop establishment. Wall et al. (1991) reported that intercropping silage com with red clover can provide soil erosion protection without significant effect on silage com yields. -4.2.2. Stalk yield of sweet corn Early planting of the cover crops significantly reduced the fresh or dry stalk yield of sweet com (Table 3). Early planted cover crops may have competed with sweet com for the mineral nutrients required for the stalk formation, while this may not be the case with the late planted cover crops. Ear-leaf N concentration was less in com with early underseeding as compared to late underseeding (Table 5). This is because, establishment of the early planted cover crops was closer to the vegetative establishment of the sweet com. Table 4 indicates that the type of underseeded cover crop had no effect on both fresh and dry sweet com stalk yield in the 1992-1993 growing season. In the 1993-1994 37 growing season however, the effect of red clover on fresh stalk yield was significantly higher than fall rye and annual ryegrass. During the 1993/1994 growing season, the red clover was affected by powdery mildew disease in August and there was no biomass harvested. The above ground biomass, which would have otherwise persisted until winter returned back to the soil during the corn growing season. This may have supplied some nutrients particularly nitrogen to the soil in the plots underseeded with red clover. Consequently, the supplied nutrients could have been taken up by the sweet corn. This could possibly explain the higher sweet corn stalk yield in the red clover treatment as compared to the two non-legumes and crimson clover. 38 Table 3. Effects of date of planting cover crops on fresh and dry stalk yield of sweet corn in 1992-1993 and 1993-1994 growing seasons. Fresh stalk yield Dry stalk yield Treatments (t/ha) (t/ha) 1992/93 1993/94 1992/93 1993/94 Early planting 42.4" 29.9b 6.6b 5.1" Late planting 50.8a 41.2" 7.6a 6.2a C.V.(%) 13.7 11.9 12.8 12.6 Note: means within a column with the same superscript are not significantly different (P>0.05). 39 Table 4. Fresh and dry stalk yield of sweet corn underseeded with five cover crops in 1992/93 and 1993/94 growing seasons. Fresh stalk yield Dry stalk yield Treatments (t/ha) (t/ha) 1992/93 1993/94 1992/93 1993/94 Unseeded 43.7a 36.5ab 6.7a 5.8ab Crimson clover 47.6" 34.2" 7.4a 5.5b Alsike clover 46.8a 35.6ab 7.3a 5.7ab Red clover 45.2a 39.9a 6.8a 6.3a Fall rye 47.0a 33.3" 7.2a 5.42" Annual ryegrass 49.2a 33.9" 7.3a 5.3a C.V.(%) 13.7 11.9 12.8 12.6 Note: means within a column with the same superscript are not significantly different (P>0.05). 40 4.2.3. Ear leaf nitrogen of sweet corn From Table 5, there was no significant difference in the ear leaf N concentration in the sweet com early and late underseeded with the cover crops for the 1992-1993 growing season. In the 1993-1994 growing season the ear leaf N concentration of the sweet com early underseeded was significantly lower than the ear leaf N concentration in the sweet com late underseeded. This difference may be due to the fact that early planting of cover crops was closer to the vegetative establishment of the sweet com and hence competition for N. From Table 6, it Can be observed that there was no difference due to cover crop in the ear leaf N concentration in the 1992-1993 growing season. It can also be seen that the ear leaf nitrogen concentrations were below the critical nitrogen range (28-35 g/kg) for maxiumn yield (Tisdale et al. 1993). In the 1992-1993 growing season, the preceding crop was potato which may have depleted the N from the soil. In the 1993-1994 growing season, the sweet com/fall rye had the lowest ear leaf N concentration. The reason could be possibly be due to the effect of the preceding crop (peas) planted at the experimental site prior to the 1993-1994 growing season. Overall 1993-94 ear leaf N concentration appeared to be higher than in 1992-93. Since peas are a legume there is a possibility that pea residue provided more N to the subsequent crop in the 1993-1994 growing season. 41 Table 5. Effects of date of planting cover crops on ear leaf nitrogen concentration of sweet corn in 1992-1993 and 1993-1994 growing seasons. Ear leaf nitrogen Treatments (g/kg) 1992/93 1993/94 Early planting 22a 23a Late planting 21a 28b C.V.(%) 11.9 5.9 Note: means within a column with the same superscript are not significantly different (P>0.05). 42 Table 6. Ear leaf nitrogen concentration of sweet corn underseeded with five cover crops in 1992/93 and 1993/94 growing seasons. Ear leaf nitrogen (g/kg) Treatments 1992/93 1993/94 Unseeded 21" 26a Crimson clover 21" 25 a b Alsike clover 22a 26a Red clover 21a 26a Fall rye 22a 24b Annual ryegrass 21a 26a C.V.(%) 11.9 5.9 Note: means within a column with the same superscript are not significantly different (P>0.05) 43 4.2.4. Dry matter production, nitrogen concentration and nitrogen uptake of cover crops. 4.2.4.1. Effect of date of planting In the 1992-1993 growing season the fall rye did not establish and crimson clover was grazed by migratory birds. There was no difference in 1992-1993 between early and late planting of cover crops in sweet corn on cover crop dry matter production, nitrogen concentration, and nitrogen uptake (Table 7). From the foregoing, it can be seen that there was more precipitation at planting (cover crops) time during 1993-1994 than 1992-1993. This might have resulted in better germinations for the 1993-1994 cover crops than the 1992-1993 crops; resulting in slightly higher yields for the 1993-1994 cover crops than the 1992-1993 cover crops, as can be seen in Table 7 and 8. In 1993-1994, the effects of early and late planting of cover crops in sweet corn, dry matter production and nitrogen concentration were not significant (Table 8). However, nitrogen uptake by cover crops in early planting was 19% higher than late planting. 44 Table 7. Cover crop yield and nitrogen concentration and uptake at two planting dates (29 May 1992 and 22 June 1992) for 1992-1993 growing seasons sampled on 3 March 1993. Dry Matter N Cone. N Uptake Treatments (t/ha) g/kg (kg/ha) Early planting 2.3a 25" 59.2a Late planting 2.2a 25a 55.7a C.V.(%) 20.2 9.5 15.7 Alsike clover 2.1" 32a 66.6b Red clover 2.7 a 31' 83.6a Annual Ryegrass 2.0b l l b 22. l c C.V.(%) 20.2 9.5 15.7 Note: means within column with the same superscript are not significantly different(P>0.05). 45 4.2.2.4. Cover crop dry matter production, nitrogen concentration and nitrogen uptake. In the 1992-1993 growing season, red clover produced the highest dry matter yield by spring 1993 (Table 7). It was found to have 28 and 35% more dry matter than alsike and annual ryegrass respectively. Nitrogen concentrations in alsike and red clover were 190 and 180% higher than in annual ryegrass. Red clover and alsike clover N concentrations were not significantly different. Nitrogen contents in red clover and alsike clovers were significantly higher than annual ryegrass. This may be due to of the ability of legumes to fix nitrogen. Red clover had the highest nitrogen uptake. It was found to have 26% and 278% more nitrogen than alsike and annual ryegrass respectively. Both biomass production and nitrogen concentration of the cover crops obtained in this study compare very well with those obtained in the screening trials of the cover crops conducted by Temple (1992) and Bomke et a/.(1993). During the first year of the study, the main problem encountered was how to control weeds successfully and get the cover crops established. Weeds found in the experiment in both years were redroot pigweed (Amaranthus retroflexus), lambsquarter (Chenopodium album), common chickweed (Stellaria media), common groundsel (Senecio vulgaris), common pepper-grass (Lepidium densiflorium), shepherd's purse (Capsella bursa-pastoris), and corn spurry (Spergula arvensis). Where weeds were successfully controlled, the cover crops did not compete with nor reduce the yields of the sweet corn. Fall rye did not establish well in 1992-1993 and produced virtually no biomass in fall or spring, while crimson clover did well in the fall, but could not survive 46 wet overwinter conditions on the site. Dry matter production of crimson clover in the fall (November) following the 1993 growing season was significantly higher than annual ryegrass but not alsike clover or fall rye (Table 8). Data on dry matter of red clover was not included in this analysis, because the plants were infected by powdery mildew and there was no biomass to be harvested. Crimson clover produced more dry matter than annual ryegrass. Crimson and alsike clovers N content were significantly higher than grasses. The N uptake by crimson clover was 29, 87 and 139% higher than that by alsike, fall rye and annual ryegrass respectively. 47 Table 8. Cover crop yield and nitrogen concentration and uptake at two planting dates (24 June 1993 and 20 July 1993) for 1993-1994 growing seasons sampled on 2 November 1993. Combination Dry Matter N Cone. N Uptake Treatments (t/ha) (g/kg) (kg/ha) Early planting 2.9a 21a 61.2a Late planting 2.5a 19a 49.3" C.V.(%) 17.5 14.1 25.0 Crimson clover 3.1a 81.1' Alsike clover 2.6ab 24" 62.7b Fall rye 2.7ab 16b 43.4° Annual ryegrass 2.5b 14" 33.9C C.V.(%) 17.5 14.1 25.0 Note: means within a column with the same superscript are not significantly different (P>0.05). 48 4.2.5. Ground Cover (percent cover) Some cash crops like sweet corn in Delta are harvested late in the growing season. It is therefore advantageous to plant cover crops as relay crops so that they can establish and become beneficial to the soil. Some cover crops, e.g. clover, establish too slowly for late fall seeding. At time of harvesting, the farmers are so busy that they are not able to plant the cover crops. Many of the cover crops when planted late, after the third week of September, are subjected to intense grazing by migratory birds. Underseeding cover crops may be more effective as far as soil conservation is concerned than planting cover crops after the cash crops has been harvested. Winter annual cover crops provide plant cover and root mass during winter and spring, which effectively reduce the soil erosion during wet winter seasons. Table 9 shows both the mode of establishment and percent soil cover of the five different cover crops underseeded in sweet corn. In the 1992-1993 growing season, annual ryegrass had the highest percent cover. It was found to be 10 and 40 % more than alsike and red clover respectively. In the 1993-1994 growing season, annual ryegrass had the highest percent cover. It was found to be 20, 20 and 40 % more than fall rye, alsike and crimson clover respectively. 49 Table 9. Establishment and percent cover assessed in spring 1993 for year and fall 1993 for year 2 of different cover crops in sweet com. Treatments Establishment % Cover 1992- 1993 Crimson clover M * Alsike clover S 80 Red clover M 50 Fall rye F * Annual ryegrass F 90 1993- 1994 Crimson clover M 60 Alsike clover S 80 Red clover M * Fall rye F 80 Annual ryegrass F 100 * = No stand F (fast), M (medium), S (slow) 50 Chapter Five C O N C L U S I O N S 1. The study revealed that the fresh and dry cob yields of sweet corn were not affected by date of seeding nor type of cover crop underseeded. 2. Early underseeded cover crops appear to reduce corn stalk yields and compete with sweet com for nitrogen. However, late underseeding of cover crops does not compete for nitrogen. This study has shown that late planting of cover crops in sweet com increases ear leaf nitrogen concentration relative to early seeding. 3. In both experiments annual ryegrass and alsike clover were promising to use as soil cover during winter. Their percent covers were higher than that of other cover crops. This is may due to the different types of growth habit. 51 RECOMMENDATIONS The findings reported here emerged from an experiment which was conducted for two seasons at two locations. In order to make definite conclusions, it would be essential to repeat these investigations over a number of seasons at different locations. This programme should include different farms in Fraser Valley in order to have more realistic information under actual farm situations. Nonetheless some of the salient implications of the present study with regard to future work are indicated below: 1. It is suggested that a study on the residual effect of underseeded cover crops on nitrogen uptake of the subsequent crops should be made. 2. Studies should be emphasized on weed control in underseeding cover crop research since weeds were a major problem during the present experiments. 52 L ITERATURE CITED Allison, F .E . (1957) Nitrogen and Soil Fertility. In Soil: The 1957 Yearbook of Agriculture. Washington, D.C. U.S. Government printing Office, pp. 179-191. Altieri, M.A. (1978) A review of insect prevalence in Zea mays L. 2) and bean {Phaseolus vulgaris L.). polycultural systems, Field crop research 1: 33-49. Baker, E.F.J, and Norman.D.W (1975) In: Proceedings of South Asia Cropping Systems Net workshops held at iRRI. 18-20th March 1975. Los Banos, Philippines. Beale, O.W. G.B. Nutt, and T .C. Peele (1955) The effects of mulch tillage on runoff, erosion, soil properties and crop yields. Soil Sci. Soc. Am. Proc. 19: 244-247. Benoit, R.E., N.A. Wilits, and W.J. Hanna (1962) Effect of rye cover crops on soil structure. Agron. J. 54: 49-420. Bomke. A. A., L .E . Lowe, M.D. Novak, and W.D. Temple (1993) Annual Report of the UBC Soil and Water conservation Group and Delta Farmers' Soil Conservation Group, August 1993. Brown, P.L. (1964) Legumes and grasses in dryland cropping systems in the northern and central Great plains, U.S. Dept. Agr. Misc. Publ. 952. 64 pp. Carlson, G.E., P.B. Gibson, and A.A. Baltensperger (1985) White clover and other perennial clovers. In: M.E. Heath, D.S. Metcalf, and R.F. Barnes [eds]. pp. 118-127 Forages: The Science of Grassland Agriculture. Iowa State Univ. Press, Ames. Iowa, USA. Chad, P. and N.N. Sharma (1977) Influences of crop associations on insect pest incidence. Proc. India. Nat. Sci. Acad. 43 (part B) 108-114. Chapman, H.D., G.F. Liebig, and D.S. Ranger (1949) A lysimeter investigation on nitrogen gains and losses under various systems of cover cropping and fertilization and discussion of error sources. Hilgardia 19: 57-95. Dalai, R.C. (1977) Effect of intercropping of maize with soybean on grain yield crop. Agri. (Trin) 54: 189-191. 53 De, R. (1980) Role of legumes in intercropping systems. In: Nuclear techniques in the development of management practices for multiple cropping system. Proceeding: Advisory group meeting on nuclear techniques in development of fertilizer and water management practices for multiple cropping systems.8-12th Oct. 1979, pp. 73-84 Ankara, Turkey. IAEA. Wein. Dichel, L. (1981) Small holders farming systems with yam in the Southern Guinea and Savanna of Nigeria Diss. Univ. Hohenheim, Germany. Egunjobi, O.A. (1984) Effects of intercropping maize with grain legumes and fertilizer treatment on populations of Pratylenchus brachyurus (Nematoda) and on the yield of maize (Zea mays L.) Prot. Ecol. 6: 153-167. Elliott, L .F. , R.I. Papendick, and D.F. Bezdicek (1987) Cropping practices using legumes with conservation tillage and soil benefit. In: J. Power [ed]. The Role of Legumes in Conservation Tillage Systems, pp. 81-89. Soil Conserv. Soc. Am., Ankeny, IA. USA. Evans, H.H. , D.D. Cockran, and M.E. Harwood (1954) Vegetable crop production as affected by cover crops. North Carolina Agronomy Research Report 12. Francis, C.A., and J.W. King (1988) Cropping systems based on farm-derived, renewable resources Agric. Sys. 27: 67-77. Francis, C.A. and J.W. Sanders (1978) Economic analysis of bean of maize systems: Monoculture versus associated cropping. Field Crops Res. 1: 319-355. Fred, E.B., L L . Baldwin, and E .M. McCoy (1932) Root nodule bacteria and leguminous plants. Wisconsin University Studies in Sciences, Vol. 5. Freyman, S. and Venkateswarlu, J. (1977) Intercropping on rainfed red soils of the Deccan Plateau, India. Can. J. of Plant Sci, 57: 679-705. Gardner, W.H. (1986) Water content. In: A. Klute [ed.]. Methods of Soil Analysis. 2nd edition, pp.493-544. Agronomy No. 9, part 1. Amer. Soc. Agron., Madison, Wis., U.S.A. Gough, N. (1991) Soil and plant tissue testing methods and interpretations of their results for British Columbia agricultural soils. (In press). Hammond, R.B. (1990) Influence of cover crops and tillage on seed corn maggot (Diptera anthomyiidae) populations in soybeans. Environ. Entomol. 13: 302-305. 54 Hargrove, W.L. (1986) Winter legumes as a nitrogen sources for no-till grain sorghum. Agron. J. 78: 70-74. Hargrove, W.L., and W.W. Frye (1987) The need for legume cover crops in conservation tillage production. In: J.F. Power [ed]. The Role of Legumes in Conservation Tillage Systems, pp. 1-5. Soil Cons. Soc. Ames, Ankeny, Iowa. Hass, H.F., J.F. Power, and G.A. Reichman (1976) Effect of cover crops and fertilizer on soil nitrogen, carbon, and water content, and on succeeding wheat yields and quality, pp 21. ARS, USDA, North Central Region. House, G.J., and M.D.R. Alzugary (1989) Influence of cover cropping and no-tillage practices a community composition of soil arthropods in a North Carolina agroecosystem. Environ. Entomol. 18: 302-307. Jodha, N.S. (1977) Resources base as a determinate of cropping patterns. In: Proceedings, Symposium on Cropping Systems Research and Development for the Asian Rice Farmers. 21-24. Sept. 1976. pp. 101-126. IRRI Los Banos, Philippines. Kamprath, E.J. , W.V. Chandler, and B.A. Krantz (1958) Winter cover crops: Their effects on corn yields and soil properties, pp.129. North Carolina Agricultural Experimental Station Technical Bulletins. Keeney, D.R., and D.W. Nelson (1982) Nitrogen-inorganic forms. In: A.L.Page, R.H. Miller and D.R. Keeney [eds.]. Methods of Soil analysis. 2nd edition. Agronomy No. 9, Part 2.pp.643-698 Amer. Soc. Agron., Madison, Wis., U.S.A. Koerner, P.T., and J.F. Power (1987) Hairy vetch winter cover crops for continuous corn in Nebraska. In: J.F. Power [ed]. The Role of Legumes in Conservation Tillage Systems, pp.57-59. Soil and Water Cons. Soc. Am., Ankeny, IA, USA. Koskinein, W.C., and C.G. McWhorter (1986) Weed control in conservation tillage. J.Soil Water Conser. 41:365-70. Lamp, W.O., J. Barney, E.J. Amrbust, and G.Kapusta (1984a) A selective weed control in spring-planted alfalfa: Effect on leafhoppers and planthoppers (Homoptera: Auchenorrhyncha), with emphasis on potato leafhopper. Environ. Entomol. 13: 207-213. 55 Lamp, W.O. J.M. Morris, and E J . Armbrust (1984b) Suitability of common weed species as host plants for the potato leafhopper (Emposasca fabae). Entomol. Exp. App. 36: 125-131. Laften, J .M. , M.A. Amemiya, and E.A. Hintz (1981) Measuring crop residue cover.J. Soil Water Conserv. 36:341-343. Larson, W.E., C.E. Clapp, W.H. Pierre, and Y.B. Morachan (1972) Effects of increasing amounts of organic residue on continuous com: II Organic carbon, nitrogen, phosphorus and sulfur. Agron. J. 64: 204-208. Liboon, S.P. and R.R. Harwood (1975) Nitrogen responses in com soybean intercropping. Paper presented at 6th Annual Scientific Meeting of the Crop Sci Soci of the Philippines, 8-10th May, 1975. Bacolod City, Philippines. Little, T .M . , and F.J. Hills (1978) Agricultural Experimentation. Design and Analysis. John Wiley and Sons, Inc., N.Y., USA. Luttmerding, H. (1981) Soils of the Langley- Vancouver Map Area. RAB Bulletin 18, Report No. 15, British Columbia Soil Survey, Vol. 3. Description of Soils. Min. of Envir. Assessment and Planning Div., Kelowna, B.C., Canada. McWhorter, C.G. and J.M. Chandler (1982) Conventional weed control technology. In: R. Charudata and H.L. Walker [eds] Biological Control of Weeds with Plant Pathogens, pp.5-24. John Wiley and Sons Inc., N.Y. USA. Meisinger, J.J., P.R. Shippley, and A .M. Decker (1990) Using winter crops to recycle nitrogen and reduce leaching. In: J.P. Muller and M.G. Wagger [eds]. Conservation Tillage for Agriculture in the 1990s, pp.3-6. Spec. Bull.90-1. N. Carolina State Univ., Raleigh, USA. Mt. Pleasant, J. (1982) Com polyculture systems in New York. M.S. thesis. Cornell Univ., Ithaca, NY. Nanni C. and C.S. Baldwin (1987) Interseeding in com. In: J.F.Power [ed]. Proceedings of a national conference, University of Georgia, Athens, April 27-29. Soil Conser. Soci. of America. Natarajan, M . and R.W. Willey (1980) A sorghum-pigonpea intercropping and the effects of plant population density. 1. Growth and yield. J. Agric. Sci. (Camb.) 5: 51-58. 56 Nelson, M . (1944) Effect of the use of winter legumes on yields of cotton, com and rice. Arkansas Agri. Exper. St. Bulletin 451. Neilsen, N.E, and H.E. Jensen (1985) Soil mineral nitrogen as affected by undersown catch crops. In: Assessment of Nitrogen Fertilizer Requirement. Proc. NW-European study for the Ground Assessment of Nitrogen Fertilizer Requirement. Netherlands Fert. Inst., Haren, The Netherlands. Norman, D.W. (1973) Economic analysis of agricultural production and labour utilization among the Hausa in north of Nigeria. Africa Rural Employment Paper 4. Dept. of Agric. Economics. Michigan State University. East Lansing, Michigan. Oloumi-Sadeghi, H.L.R. Zavlelta, W.O. Lamp, and E.J. Armbrust,and G.Kapusta (1989) Effects of potato leafhopper (Homoptera:Cicadellidae) and weed control on alfalfa yield and quality. J. Eco. Entomol. 82: 923-931. Palada, M.C., S.Ganser, R.Hofsteter, B.Volak and M.Culik (1982) Association of interseeded legume cover crops and annual row crops in year-round cropping systems.In: W. Lockeretz [ed]. Environmentally Sound Agriculture, pp. 193-213. New York: Praeger. Papendick, R.I. (1987) Why consider alternative production systems? Ames J. Alt. Agric 2: 83-86 systems. Parkinson, J.A., and S.E. Allen (1975) A wet oxidation procedure suitable for the determination of nitrogen and nutrients in biological material. Commun. Soil Sci. Plant Anal. 6: 1-11. Pederson, G.A., and W.E. Knight (1984) Legume germplasm improvement in the South eastern U.S. In: Proc. Am. Forage and Grassland Conf., Am. Forage and Grassland Cong, pp.256-260. Lexington, Ky. Pendleton, J.W., C D . Bolen, and R.D. Seif (1963) Alternating strips of com and soybean vs solid plantings. Agro. J. 55: 385-393. Perrin, R.M.,and M.L. Philips (1978) Some effects of mixed cropping on the population dynamics of insect pests, Ent. App. 24: 385-393. Pieters, A.J. (1927) Green Manuring: Principles and Practices. Wiley, New York. Pieters, A.J. , and R. Mckee (1929) Green manuring and its application to agricultural practices. Agro. J. 21: 985-993. 57 Pieters, A.J. and R. Mckee (1938) The use of cover and green manure crops. In: Soils and Men. U.S.D.A. Yearbook of Agriculture. Pimental, D., and Levitan (1980) Pesticides: Amounts applied and amounts reaching pest. Biosci. 36: 86-91. Rao, M.R. and R.W. Willey (1980) Evaluation of yield stability in intercropping: studies on sorghum/pigeonpea. Expl. Agric. 16: 105-116. Reddy, M.S. and R.W. Willey (1981) Growth and resource use studies in an intercrop of pearl-millet groundnut. Field crops Res. 4: 13-24. Rogers, T .H . , and J.E. Giddens (1957) Green manuring and cover crops. In A. Stefferund [ed]. The yearbook of Agriculture, pp. 252-257. U.S.D.A. Russell, E.J. (1966) A history of Agricultural Science in Great Britain, 1620-1954. George Allen and Unwin, Ltd., London. SAS Institute. (1988) SAS/STAT User's guide. Release 6.03 edition.Cary, N.C. USA. Semples, E.C. (1928) Ancient Mediterranean Agriculture. Part II. Manuring and seed selection. Agric. Hist. 2 : 129-156. Shaw, W.C. (1982) Integrated weed management system technology for pest management. Weed Sci. 30 (Suppl.): 2-12. Smith, H.H., R.B. Hammond, and B.R. Stinner (1988) Rye cover crop management: Influence on soybean foliage arthropods. Environ. Entomol. 17: 109-114. Smith, M.S., W.F. Wilbur, and J.J. Varco (1987) Legume winter cover crops. Adv. Soil Sci. 7: 95-138. Smith, R.R., N.L. Taylor, and S.R. Bowely (1985) Red clovers. In: N.L. Taylor [ed]. Clover Sciences and Technology. Am. Soc. Agron. pp. 457-470, Madison, WI. Staver, K.,R. Brinsfield, and J.C. Stevenson (1990) The effect of best management practices on nitrogen transport into Chesapeake Bay. In: J.B. Summers and Toxic substances in agricultural water supply and drainage.Proc. 2nd Pan-Am. Reg. Conf. on Irrig. and Drainage, pp. 163-180. U.S. Comm. Irrig. Drainage, Denver. CO. USA. Steiner, K.G. (1984) Intercropping in Tropical Small Holder Agriculture with Special Reference to West Africa, pp.9-304. Deutsche GTZ Eschborn. 58 Strickling, E. (1950) The effect of soybeans on volume weight and water stability of soil aggregates, soil organic matter content and crop yield Soil Sci. Soc. Am. Proc. 15: 30-34. Taylor, T.A. (1977) Mixed cropping as an input in the management of crop pests in Tropical Africa. A M . Environ. 2/3 (4/1): 111-126. Technicon. (1974) Technicon autoanlyzer. U. Methodology of Nitrogen and Phosphorus in BD acid digest. Industrial Method No. 334-74 A. Technicon Industrial Systems, Terrytown, N.Y. Technicon. (1977) Technicon autoanlyzer. II. Nitrate and Nitrite in Soil Extract. Industrial Method No. 487-77A Technicon Industrial Systems, Terrytown, N.Y. Temple, W.D. (1992) Report of The Delta Farmers Soil Conservation Group, March 1992. Thompson, D., and S. Thompson (1984) Farming without chemicals. EPA J. 10: 33-34. Thung, M. and J.H. Cock (1979) Multiple cropping and field beans: status of present work at CIAT. In: E. Weber, B.Nestel and M. Campbell [eds]. Intercropping with cassava. Proc. Internat. Workshop. 27 Nov.-l. Dec 1978. Trivandrum, India. IDRC, Ottawa: 7-16. Tisdall, J .M. , and J.M. Oades (1982) Organic matter and water stable aggregates in soils. J.Soil Sci. 33: 141-63. Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin (1993) Soil Fertility and Fertilizers, pp. 417. Macmillan Publ CO. New York. Touchton, J.T., D.H. Ricker, R.H. Walker, and C.E. Snipes (1984) Winter legumes as a nitrogen source for no-tillage cotton. Soil Tillage Res. 4: 391-401. USDA. (1973) Monoculture in agriculture: Extent causes and problem: Reports for the task force on spatial heterogeneity in agricultural landscape and enterprises USDA, Washington, D.C. Utomo, M. (1986) Role of legume cover crops in no-tillage and conventional Tillage corn production, PH.D. Thesis, University of Kentucky, Lexington. KY. USA. Utomo, M. , R.L. Blevins, and W.W. Frye (1987) Effect of legume cover crops and tillage on soil water, temperature, and organic matter. In: J.Power [ed] The Role of legumes in conservation tillage system, pp.5-6. Soil Cons. Soc. Am., Ankeny, IA. 59 Van Doren, D.M. Jr. (1979) Legumes supply nitrogen for no-tillage corn. Ohio Report (Nov-Dec), pp. 83-85. Waksman, S.A., and R.L. Starkey (1931) The Soil and the Microbe. Wiley, N.Y. Wall, G.J., E.A. Pringle., and R.W. Sheard (1991) Intercropping red clover with silage corn for soil erosion control. Can. J. Soil Sci. 71: 137-145. Wedderbuan, C.H.M. , and W.G. Collingwood (1976) The Economist of Xenophon. Lenox Hill Publ. and Distributing Co., N.Y. Willey, R.W. (1979) Intercropping. Its importance and research studies need. Part 2. Agronomy and research approaches. Field Crop Abstracts 32: 72-85. Willey, R.W. and Osiru, D.S.O. (1972) Studies on mixture of maize and beans (Phaseolus vulgaris) with particular reference to plant population. J. Agric. Sci. (Camb.) 79: 517-529. Zimdahl, R.L. (1981) Extent of mechanical, cultural and other non-chemical methods of weed control, pp.73-83. In: D. Pimental. [ed]. CRC Press, Boca Raton, FL . 60 Appendix 1: Some chemical properties of composite soil samples taken from plots 29 May 1992 and 24 June 1993. Soil Parameter 29/May/1992 24/June/1993 0-20 0-20 depth (cm) depth (cm) pH (H20) 5.5 5.9 Organic matter (%) 2.6 2.8 NH 4 -N (mg/kg) 0.2 0.4 N0 3-N (mg/kg) 0.9 1.6 Total N (%) 0.1 0.1 Phosphorus (mg/kg) 97.0 135.0 Potassium (mg/kg) 270.0 448.0 Magnesium (mg/kg) 223.8 173.8 Calcium (mg/kg) 1675.0 1750.0 Sodium (mg/kg) 28.8 14.8 61 Appendix 2: Mean monthly air temperatures (°C) and precipitation (mm) during the 1992-1993 growing season compared with the mean data for the 1951-1980 period. Precipitation (mm) Temperature (°C) Deviation Deviation from mean fiommean Month 1951-1980 1992 1951-1980 1992 January 153.8 281.4 16.0 2.5 5.8 3.3 February 114.7 87.8 -26.9 4.6 6.6 2.0 March 101.0 25.9 -75.1 3 5.8 8.5 2.7 April 59.6 126.2 66.6 8.8 10.6 1.8 May 51.6 15.8 -35.8 12.2 13.6 1.4 June 45.2 96.4 51.2 15.1 17.2 2.1 July 32.0 42.0 10.0 17.3 18.4 1.1 August 41.1 23.2 -17.9 17.1 17.8 0.7 September 67.1 48.2 -18.9 14.2 13.9 0.3 October 114.0 109.1 -4.9 10.0 11.3 1.3 November 150.1 168.3 18.2 5.9 6.4 0.5 December 182.4 117.8 -64.6 3.9 1.9 2.0 62 Appendix 3: Mean monthly air temperatures (°C) and precipitation (mm) during the 1993-1994 growing season compared with the mean data for the 1951-1980 period. Precipitation (mm) Temperature(°C) Deviation Deviation from mean from mean Month 1951-1980 1993 1951-1980 1993 January 153.8 103.4 50.4 2.5 -0.4 2.1 February 114.7 11.4 -103.3 4.6 3.5 -1.1 March 101.0 115.2 14.2 5.8 7.4 1.6 April 59.6 126.9 67.3 8.8 10.0 1.2 May 51.6 100.8 49.2 12.2 14.7 2.5 June 45.2 72.2 27.0 15.1 15.9 0.8 July 32.0 34.3 2.3 17.3 16.4 -0.9 August 41.1 19.0 -22.1 17.1 17.6 0.5 September 67.1 2.1 -65.0 14.2 14.8 0.6 October 114.0 73.1 -40.9 10.0 11.4 1.4 November 150.1 6.1 -87.0 5.9 4.5 -1.4 December 182.4 162.3 -20.1 3.9 4.5 0.6 Appendix 4: Analysis of variance for fresh cob yield of sweet corn underseeded with five different cover crops in 1992 growing season. Source of variation df MS F-value Probability BLock 3 23.520 2.35 0.252 Date 1 38.880 3.88 0.143 MP error (a) 3 10.011 Cover crop 5 1.672 0.34 0.887 1 vs 2+3+4+5+6 1 0.353 0.07 0.792 2+3+5 vs 4+6 1 0.641 0.13 0.722 2 vs 3+5 1 0.880 0.18 0.677 3 vs 5 1 4.000 0.80 0.377 4 vs 6 1 2.481 0.50 0.485 Date*cover crop 4.559 0.92 0.483 (dl/d2)*(cl/23456) 1 0.254 0.05 0.823 (dl/d2)*(c235/46) 1 0.345 0.07 0.794 (dl/d2)*(c2/35) 1 0.255 0.05 0.822 (dl/d2)*(c3/5) 1 20.250 4.08 0.053 (dl/d2)*(c4/6) 1 1.690 0.34 0.564 SP error (b) 30 4.969 Corrected Total 47 114.760 64 Appendix 5. Analysis of variance for fresh cob yield of sweet corn underseeded with five different cover crops in 1993 growing season. Source of variation df MS F-value Probability BLock 3 16.067 3.99 0.1438 Date 1 171.839 42.66 0.007 MP error (a) 3 4.028 Cover crop 5 6.075 1.41 0.248 5 vs1+2+3+4+6 1 2.436 0.57 0.458 1+2+3 vs 4+6 1 3.760 0.87 0.357 1 vs 2+3 1 7.130 1.66 0.208 2 vs3 1 7.563 1.76 0.195 4 vs 6 1 9.486 2.21 0.148 Date*cover crop 2.415 0.56 0.729 (dl/d2)*(c5/12346) 1 8.694 2.02 0.165 (dl/d2)*(cl23/46) 1 2.197 0.51 0.480 (dl/d2)*(cl/23) 1 0.005 0.00 0.973 (dl/d2)*(c2/3) 1 1.000 0.23 0.633 (dl/d2)*(c4/6) 1 0.176 0.04 0.841 SP error (b) 30 4.301 Corrected Total 47 247.172 Appendix 6: Analysis of variance for dry cob yield of sweet corn underseeded with five different cover crops in 1992 growing season. Source of variation df MS F-value Probability BLock 3 0.3460 0.81 0.567 Date 1 0.5830 1.36 0.327 MP error (a) 3 0.428 Cover crop 5 0.455 0.90 0.494 1 vs 2+3+4+5+6 1 0.118 0.23 0.632 2+3+5 vs 4+6 1 0.346 0.68 0.415 2 vs 3+5 1 0.002 0.00 0.955 3 vs 5 1 0.047 0.09 0.762 4 vs 6 1 1.762 3.48 0.072 Date*cover crop 0.425 0.84 0.532 (dl/d2)*(cl/23456) 1 0.029 0.06 0.813 (dl/d2)*(c235/46) 1 0.192 0.38 0.524 (dl/d2)*(c2/35) 1 0.068 0.13 0.716 (dl/d2)*(c3/5) 1 1.672 3.30 0.079 (dl/d2)*(c4/6) 1 0.166 0.33 0.571 SP error (b) 30 0.506 Corrected Total 47 7.144 66 Appendix 7. Analysis of variance for dry cob yield of sweet corn underseeded with five different cover crops in 1993 growing season. Source of variation df MS F-value Probability BLock 3 1.631 2.46 0.239 Date 1 4.774 7.21 0.075 MP error (a) 3 0.662 Cover crop 5 0.360 1.41 0.453 5 vs 1+2+3+4+6 1 0.029 0.97 0.782 1+2+3 vs 4+6 1 1.108 0.08 0.094 1 vs 2+3 1 0.011 2.98 0.867 2 vs3 1 0.124 0.03 0.568 4 vs 6 1 0.527 0.33 0.243 Date*cover crop 0.533 1.42 0.240 (dl/d2)*(c5/12346) 1 0.305 1.44 0.372 (dl/d2)*(cl23/46) 1 0.250 0.82 0.419 (dl/d2)*(cl/23) 1 0.019 0.67 0.823 (dl/d2)*(c2/3) 1 0.869 0.05 0.137 (dl/d2)*(c4/6) 1 1.224 2.34 0.076 SP error (b) 30 0.372 Corrected Total 47 13.627 67 Appendix 8: Analysis of variance for fresh stalk yield of sweet corn underseeded with five different cover crops in 1992 growing season. Source of variation df MS F-value Probability BLock 3 169.966 4.25 0.133 Date 1 846.216 21.16 0.019 MP error (a) 3 39.993 Cover crop 5 29.813 0.73 0.607 1 vs 2+3+4+5+6 1 82.204 2.01 0.166 2+3+5 vs 4+6 1 24.691 0.60 0.443 2 vs 3+5 1 12.886 0.32 0.579 3 vs 5 1 9.970 0.24 0.625 4 vs 6 1 19.316 0.47 0.497 Date*cover crop 44.138 1.08 0.391 (dl/d2)*(cl/23456) 1 60.964 1.49 0.232 (dl/d2)*(c235/46) 1 26.017 0.64 0.431 (dl/d2)*(c2/35) 1 3.451 0.08 0.773 (dl/d2)*(c3/5) 1 130.131 3.18 0.085 (dl/d2)*(c4/6) 1 0.126 0.00 0.956 SP error (b) 30 40.867 Corrected Total 47 1540.749 68 Appendix 9. Analysis of variance for fresh stalk yield underseeded with different cover crops in 1993 growing season. Source of variation df MS F-value Probability BLock 3 72.744 34.49 0.008 Date 1 1521.564 721.48 0.000 MP error (a) 3 2.109 Cover crop 5 46.848 2.61 0.045 5 vs 1+2+3+4+6 1 7.975 0.44 0.510 1+2+3 vs 4+6 1 86.100 4.79 0.037 1 vs 2+3 1 66.505 3.70 0.064 2 vs 3 1 72.750 4.02 0.054 4 vs 6 1 1.410 0.08 0.781 Date*cover crop 11.804 0.66 0.658 (dl/d2)*(c5/12346) 1 4.888 0.27 0.606 (dl/d2)*(cl23/46) 1 1.240 0.07 0.795 (dl/d2)*(cl/23) 1 31.688 1.76 0.194 (dl/d2)*(c2/3) 1 19.141 1.07 0.310 (dl/d2)*(c4/6) 1 2.066 0.12 0.737 SP error (b) 30 17.959 Corrected Total 47 1966.291 Appendix 10: Analysis of variance for dry stalk yield of sweet corn underseeded with five different cover crops in 1992 growing season. Source of variation df MS F-value Probability BLock 3 0.633 4.25 0.133 Date 1 10.056 21.16 0.019 MP error (a) 3 0.489 Cover crop 5 0.656 0.73 0.607 1 vs 2+3+4+5+6 1 1.822 2.01 0.166 2+3+5 vs 4+6 1 0.063 0.60 0.443 2 vs 3+5 1 0.502 0.32 0.579 3 vs5 1 0.797 0.24 0.625 4 vs 6 1 0.096 0.47 0.497 Date*cover crop 1.049 1.08 0.391 (dl/d2)*(cl/23456) 1 1.468 1.49 0.232 (dl/d2)*(c235/46) 1 1.126 0.64 0.431 (dl/d2)*(c2/35) 1 0.049 0.08 0.773 (dl/d2)*(c3/5) 1 1.995 3.18 0.085 (dl/d2)*(c4/6) 1 0.608 0.00 0.956 SP error (b) 30 0.830 Corrected Total 47 22.239 70 Appendix 11: Analysis of variance for dry stalk yield underseeded with five different cover crops in 1993 growing season. Source of variation df MS F-value Probability BLock 3 1.410 4.41 0.127 Date 1 15.675 49.07 0.006 MP error (a) 3 0.319 Cover crop 5 1.071 2.12 0.090 5 vs1+2+3+4+6 1 0.113 0.22 0.639 1+2+3 vs 4+6 1 2.356 4.67 0.039 1 vs 2+3 1 1.505 2.98 0.094 2 vs3 1 1.278 2.53 0.122 4 vs 6 1 0.106 0.21 0.651 Date*cover crop 0.482 0.96 0.460 (dl/d2)*(c5/12346) 1 0.006 0.01 0.915 (dl/d2)*(cl23/46) 1 0.227 0.45 0.508 (dl/d2)*(cl/23) 1 1.135 2.25 0.144 (dl/d2)*(c2/3) 1 0.846 1.68 0.205 (dl/d2)*(c4/6) 1 0.198 0.39 0.436 SP error (b) 30 0.504 Corrected Total 47 27.231 Appendix 12: Analysis of variance for ear leaf nitrogen status of sweet corn 1992 growing season. Source of variation df MS F-value Probability BLock 3 0.627 1.74 0.329 Date 1 0.159 0.44 0.554 MP error (a) 3 0.360 Cover crop 5 0.045 0.72 0.615 1 vs 2+3+4+5+6 1 0.006 0.10 0.751 2+3+5 vs 4+6 1 0.002 0.03 0.862 2 vs 3+5 1 0.055 0.89 0.354 3 vs 5 1 0.128 2.05 0.163 4 vs 6 1 0.032 0.52 0.477 Date*cover crop 0.036 0.58 0.713 (dl/d2)*(cl/23456) 1 0.135 2.17 0.151 (dl/d2)*(c235/46) 1 0.003 0.04 0.838 (dl/d2)*(c2/35) 1 0.000 0.00 0.977 (dl/d2)*(c3/5) 1 0.041 0.66 0.424 (dl/d2)*(c4/6) 1 0.003 0.04 0.843 SP error (b) 30 0.062 Corrected Total 47 1.694 Appendix 13: Analysis of variance for ear leaf nitrogen status of sweet corn 1993 growing season. Source of variation df MS F-value Probability BLock 3 0.017 0.34 0.798 Date 1 3.456 69.63 0.004 MP error (a) 3 0.050 Cover crop 5 0.042 1.86 0.132 5 vs1+2+3+4+6 1 0.068 3.00 0.094 1+2+3 vs 4+6 1 0.021 0.90 0.349 1 vs2+3 1 0.005 0.24 0.629 2 vs 3 1 0.001 0.06 0.805 4 vs 6 1 0.116 5.09 0.032 Date*cover crop 0.031 1.36 0.269 (dl/d2)*(c5/12346) 1 0.009 0.39 0.536 (dl/d2)*(cl23/46) 1 0.055 2.41 0.131 (dl/d2)*(cl/23) 1 0.039 1.72 0.199 (dl/d2)*(c2/3) 1 0.019 0.83 0.369 (dl/d2)*(c4/6) 1 0.032 1.83 0.242 SP error (b) 30 0.023 Corrected Total 47 3.976 73 Appendix 14. Analysis of variance of cover crop biomass planted on 29 May and 22 June 1992 growing season. Cover crop was sampled on 3 March 1993. Source of variation DF MS F-value Probability Block 3 0.665 4.88 0.113 Date 1 0.052 0.38 0.580 MP error (a) 3 0.136 Cover crop 2 1.095 5.13 0.025 3 5/6 1 0.785 3.68 0.079 3/5 1 1.404 6.58 0.025 Date*Cover crop 2 0.088 0.41 0.670 (dl/d2)*(c3 5/6) 1 0.130 0.61 0.450 (dl/d2)*(c3/5) 1 0.046 0.22 0.650 SP error (b) 12 0.213 Corrected Total 23 4.614 Appendix 15. Analysis of variance for cover crop biomass planted on 24 June and 20 July 1993 growing season. Cover crop was sampled on 24 November 1993. Source df MS F-value Probability Block 3 0.313 1.09 0.474 Date 1 1.272 4.42 0.126 MP error (a) 3 0.289 Cover crop 3 0.456 2.02 0.148 1+2 vs 4+6 1 0.466 2.06 0.169 1 vs2 1 0.766 3.38 0.082 4 vs 6 1 0.137 0.60 0.447 Date*Cover crop 0.125 0.55 0.653 (dl/d2)*(cl2/46) 1 0.104 0.46 0.507 (dl/d2)*(cl/2) 1 0.060 0.27 0.613 (dl/d2)*(c4/6) 1 0.212 0.94 0.346 SP error (b) 18 0.226 Corrected Total 31 4.426 Appendix 16: Analysis of variance of cover crop nitrogen concentration underseeded in sweet corn in 1992 growing seasons. Source of variation df MS F- value Probability Block 3 0.186 6.75 0.077 Date 1 0.001 0.03 0.874 MP error (a) 3 0.028 Cover crop 2 11.388 210.19 0.000 3 5/6 1 22.770 420.25 0.000 3/5 1 0.006 0.12 0.737 Date*cover crop 2 0.004 0.08 0.922 (dl/d2)*(c3 5/6) 1 0.005 0.10 0.762 (dl/d2)*(c3/5) 1 0.004 0.07 0.801 SP error (b) 12 0.054 Corrected Total 23 Appendix 17. Analysis of variance of cover crop nitrogen concentration underseeded in sweet corn 1993 growing season. Source of variation df MS f-Value Probability Block 3 0.014 0.27 0.843 Date 1 0.112 2.17 0.237 MP error (a) 3 0.051 0.64 0.598 Cover crop 3 3.012 37.69 0.000 1+2 vs 4+6 1 8.496 106.34 0.000 1 vs2 1 0.276 3.45 0.080 4 vs 6 1 0.263 3.29 0.087 Date*Cover crop 3 0.158 1.97 0.154 (dl/d2)*(cl2/46) 1 0.000 0.00 0.946 (dl/d2)*(cl/2) 1 0.040 0.50 0.488 (dl/d2)*(c4/6) 1 0.432 5.41 0.032 SP error (b) 18 0.080 Corrected Total 31 12.934 Appendix 18: Analysis of variance for nitrogen uptake by cover crop planted 29 May and 22 June 1992 growing season. Cover crop biomass was sampled 3 March 1993. Source of variation df MS F-value Probability Block 3 200.242 5.30 0.102 Date 1 74.836 1.98 0.254 MP error (a) 3 37.779 Cover crop 2 8078.237 99.98 0.000 3 5/6 1 15001.834 185.67 0.000 3/5 1 1154.640 14.29 0.003 Date*Cover crop 2 32.706 0.40 0.676 (dl/d2)*(c3 5/6) 1 48.642 0.60 0.453 (dl/d2)*(c3/5) 1 16.769 0.21 0.657 SP error (b) 12 80.796 Corrected Total 23 16729.029 Appendix 19: Analysis of variance of nitrogen uptake by cover crop planted on 24 June and 20 July 1993. cover crop biomass was sampled on 24 November 1993. Source of variation df MS f-value Probability Block 3 171.676 2.53 0.233 Date 1 1124.210 16.57 0.027 MP error (a) 3 67.851 Cover crop 3 3517.294 18.44 0.000 1+2 vs 4+6 1 8835.525 46.32 0.000 1 vs 2 1 1362.164 7.14 0.016 4 vs 6 1 354.192 1.86 0.190 Date*cover crop 117.073 0.61 0.615 (dl/d2)*(cl2/46) 1 141.751 0.74 0.400 (dl/d2)*(cl/2) 1 127.295 0.67 0.425 (dl/d2)*(c4/6) 1 82.174 0.43 0.520 SP error (b) 18 190.734 Corrected Total 31 16091.939 

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