The effects of protein degradability and food intake on milk yield and composition in cows in early lactation.

1. In two experiments measurements were made of food intake, live-weight change, milk yield and milk composition in early lactation when dairy cows were given diets containing varying proportions of protein as fish meal (low rumen degradability) or as groundnut meal (high rumen degradability). In a preliminary trial measurements were also made with cows given supplements of either fish meal or barley and fed at a restricted level of feeding. 2. When metabolizable energy (ME) intake exceeded 160 MJ/d there was no evidence of responses to changes in protein degradability, ut at ME intakes below 135 MJ/d increases in the supply of undegradable protein led to increases in fat-corrected milk yield, protein content and live-weight loss. 3. The interaction between energy intake and protein degradability is unexpected because net protein:net energy requirement increases as milk yield increases, but may be explained in terms of differential effects of changing rumen outflow rates on degradabilities.