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Investigating reduced rate of nitrogen application and delayed harvest of orchardgrass as a means of reducing the amount and altering the form of nitrogen ingested by lactating dairy cattle Groenenboom, Pierre Sebastiaan

Abstract

Rations for lactating dairy cattle should supply adequate amounts of rumen undegradable protein without oversupplying rumen degradable protein. As forage grasses can supply too much rumen degradable protein to the ration, it is beneficial to produce forage grasses with higher proportions of rumen undegradable protein while maintaining acceptable yields and fibre levels. Forage grasses have many different types of protein with differing ability to withstand ruminal degradation. Non-protein nitrogen is very rapidly degraded in the rumen whereas true protein consists of a range of proteins, some of which are rapidly degraded while others may escape ruminal degradation. The goal of the present studies was to reduce rumen degradable protein or non-protein nitrogen concentrations in orchardgrass (Dactylus glomerata L. "Benchmark"). One study examined three nitrogen application rates (0, 50, and 100 kg N ha⁻¹) and three growth periods (41, 49, and 59 d) in the spring of 1996 for their effect on fresh orchardgrass yield, fibre content, true protein content, and crude protein content. Samples were incubated in the rumens of ruminallyfistulated Holstein cows to examine dry matter and protein degradation in the rumen. In this study, increasing the growth period of orchardgrass by 15 d proved ineffectual in significantly reducing rumen degradable protein (%crude protein). Increasing nitrogen fertilization rate caused an increase in rumen degradable protein expressed as either %crude protein or as %dry matter without significantly altering rumen undegradable protein (%dry matter). Another study examined three nitrogen application rates (0, 50, and 100 kg N ha⁻¹) and three growth periods ranging from 29 to 59 d after nitrogen fertilization in the spring, early summer and late summer from 1995 to 1997 for their effect on fresh orchardgrass yield, fibre content, true protein content and crude protein content. In all seasons increasing nitrogen fertilization rate caused a significant reduction in the percentage of crude protein that was true protein. In both studies, increasing nitrogen fertilization caused an increase in dry matter and protein yields. It was concluded that incorporating forage grass that has received less nitrogen fertilization into dairy cow rations could reduce ration rumen degradable protein concentrations. However, decreased forage grass yields will require producers to either use additional land for grass production, or to purchase additional forage.

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