Prediction of the protein requirements of farm ruminants and implications of these predictions for diet formulation.

Methods of determining protein requirements are reviewed and recent proposals of the Agricultural Research Council working party on nutrient requirements of ruminants outlined. Needs of the rumen microorganisms for degradable nitrogen to achieve optimum rumen digestion of feed are predicted. The extent to which milk production and live-weight gain can be sustained by microbial protein alone is estimated. Higher milk yields and rates of growth require dietary protein that escapes degradation in the rumen but is digested in the small intestine. Small changes in degradability of dietary protein are predicted to have a large effect on the dietary crude protein requirement. Although there is still inadequate data for precise prediction, the concepts of the metabolic approach have been valuable in understanding those physiological situations where protein is most likely to be limiting, where use of protected proteins and urea might be most appropriate, in the planning of critical experiments and in the design of new methods of feeding or management of ruminants.