Factors affecting the nutritive value of the diet and the composition of rumen fluid of grazing sheep and cattle

Abstract

The digestibility and nitrogen content of the diet and of the green fraction of the diet, and concentrations of individual volatile fatty acids in the rumen were measured in sheep and cattle grazing together on five Phalaris tuberosa pastures varying in forage availability. The sheep and cattle were aged 5 and 6 months respectively. Measurements were made at monthly intervals for 5 consecutive months during which animals were rotated between pastures. Measurements of forage availability, structure and composition were subjected to a principal components analysis from which orthogonal variables describing pasture composition were calculated. These were related to the nutritive value of the diet and the composition of rumen fluid by regression analysis. Diets selected by sheep were of higher digestibility and nitrogen content than those selected by cattle, because they contained more green material and the green material selected by the sheep was of higher digestibility and nitrogen content. The nutritive value of the diet of sheep varied with the availability of green forage, and that of cattle with both the availability of green and the bulk of forage on offer. Cattle were the more sensitive to changes in the quantity of green material available, and differences between sheep and cattle in the nutritive value of diets were reduced as the quantity of green material and the bulk of forage increased. The rumen liquor of cattle contained proportionately more acetic acid, and the proportions of other volatile fatty acids were less than in sheep. These differences were generally associated with differences in selective grazing.