Effects of dietary protein supply on caseins, whey proteins, proteolysis and renneting properties in milk from cows grazing clover or N fertilized grass

The objective of this work was to examine whether variation in the amino ac id supply to cows could be a reason for the reduced casein content and poor er renneting properties of milk that often occur in late summer, or whether these effects are related to proteolysis in the raw milk. In a 2x2x2 facto rial design, we investigated the effects of sward (clover v. rye-grass) and supplementary feed with a high or low level of rumen-soluble N or of rumen undegradable protein on milk protein composition during the grazing season . A total of 32 Danish Holstein cows were included in the experiment. Milk protein and casein contents and the ratios casein N:total N and casein:true protein were at a minimum in late summer, whereas the contents of urea, no n-protein N and whey protein were higher during this period. These seasonal effects were unrelated to either the type of supplementary feed or the typ e of sward; neither were they clearly related to proteolysis, although case in:true protein was related to the proteose peptone content. The results in dicated that whey proteins other than alpha-lactalbumin or beta-lactoglobul in accounted for the higher proportion or concentration of whey protein in late summer. Based on a principal component analysis including variables su ch as citric acid, lactose and non-protein N, we suggest that the cows' ene rgy supply during this period may be a critical factor in determining the m ilk protein composition, although our results were not conclusive. There wa s an interaction between the supplement of rumen undegradable protein and t ype of sward. When clover was grazed, a high supplement increased the conce ntrations of protein and casein in milli and the kappa-casein:total casein ratio. When rye-grass was grazed, the opposite response was found, and over all milk protein yield was not affected. The very low N content of clover i n early summer reduced milk protein and casein protein during this period.