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Seasonal Variation in Chemical Composition of Pasture Herbage and the Relation to Its Digestibility by Steers and Sheep. (Pasture Studies—XXVI)

McGill University¹

Abstract

The digestibility of the dry matter of pasture herbage under eastern Canadian conditions may vary from 80 percent in the early spring to 60 percent or less during mid-summer. The factors which cause these changes in digestibility are not fully known, but there is reason to believe that changes in digestibility reflect changes in nutritive value of feed. It is unlikely that either amount or quality of protein limit the feeding value of pasturage to grazing ruminants. The protein content of such material normally does not fall below 15 percent and ruminant digestion appears to be wholly independent of amino acids in the diet. It is presumed that synthesis of necessary amino acids may occur in the rumen from a variety of sources of dietary nitrogen.

Available energy, on the other hand, is correlated with the feeding value of any ration. Since ration energy losses through incomplete digestion of the potential energy yielding fractions are variable between different diets, it is assumed that seasonal trends in the digestibility of the energy-yielding portions of pasture herbage indicate seasonal changes in the nutritive value of such feed.

¹ Contribution from Faculty of Agriculture, McGill University, Macdonald College, Quebec, Canada. Journal Series No. 189.