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Altitudinal migration as a factor in the nutrition of bighorn sheep Hebert, Daryll Marvin

Abstract

Altitudinal migration is a feature common to most wild ungulates along the Rocky Mountain chain. This study, conducted from 1968-70 in the East Kootenay region of British Columbia, used bighorn sheep as the experimental animal and undertook to examine various nutritional parameters which might benefit populations of migratory animals. Basic range studies were conducted to complement extensive measurements of the nutritional status of the various groups of captive sheep. Quantitative measurements were made of the species composition of several winter ranges and approximate composition of the winter and summer range diets along with phenology and forage moisture in order to assess availability, palatability and suitability for maintenance of wintering animals. Between year differences were found to influence availability of spring growth, nutrient intake and apparent digestibility. Qualitative measurements were conducted approximately monthly to determine crude protein, gross energy and phosphorus content of the winter and summer range forage. Animal trials were conducted over a two year period using two methods of diet simulation. During 1968-69, a single group of adult sheep was given forages varying in quality (crude protein) to represent the natural sequence of events on the range from early spring until late winter. Two groups of yearling sheep were used during the second year. One group, representing the control, was maintained on forage from the winter range year around. The other was given spring growth winter range forage, then changed to alpine range forage and finally returned to winter range forage during the late fall and winter period in order to simulate the normal migratory pattern. The response of each group was measured through change in body weight. The summer range forage was assessed for its ability to supply nutrients in the feed, its effect on feed intake and digestibility and its influence on digestible nutrients and nitrogen retained. It proved superior to the corresponding winter range forage for all methods of evaluation. The improved quality alpine forage given to the migratory group also found expression the following spring through better utilization of late winter forage and early spring growth. A series of predictive linear equations was established for each forage type in order to estimate such things as crude protein from forage moisture, feed intake and digestibility from nutrient content of the forage and feed and nutrient intake using fecal nitrogen techniques. They served to demonstrate differences in efficiency of forage utilization, between groups, and allowed a quantitative assessment of the effect of changes in forage quality on feed and nutrient intake. The collection of basic range data along with the animal trials allowed an estimation of the digestible energy required for body maintenance, calculation of the yearly intake of nutrients and feed and insight into nitrogen as the limiting nutrient during the critical winter period. It was found that minimum ambient temperature during the critical winter period increased feed intake even though forage quality appeared to restrict the amount of feed ingested. Also, examination of fecal and urinary pathways of protein loss indicated that the kidney was regulating nitrogen loss via the urine during periods of nitrogen shortage. Collection of data of this type provides opportunities for future experimentation involving manipulation of dietary composition, forage quality and the establishment of predictive equations and maintenance values by season. It will lead to a better understanding of carrying capacity through the comparison of feed and nutrient intake of several species on a single range, on a metabolic weight basis.

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