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

Abstract

The objective of this work was to examine whether variation in the amino acid supply to cows could be a reason for the reduced casein content and poorer renneting properties of milk that often occur in late summer, or whether these effects are related to proteolysis in the raw milk. In a 2×2×2 factorial 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[ratio ]total N and casein[ratio ]true protein were at a minimum in late summer, whereas the contents of urea, non-protein N and whey protein were higher during this period. These seasonal effects were unrelated to either the type of supplementary feed or the type of sward; neither were they clearly related to proteolysis, although casein[ratio ]true protein was related to the proteose peptone content. The results indicated that whey proteins other than α-lactalbumin or β-lactoglobulin accounted for the higher proportion or concentration of whey protein in late summer. Based on a principal component analysis including variables such as citric acid, lactose and non-protein N, we suggest that the cows' energy supply during this period may be a critical factor in determining the milk protein composition, although our results were not conclusive. There was an interaction between the supplement of rumen undegradable protein and type of sward. When clover was grazed, a high supplement increased the concentrations of protein and casein in milk and the κ-casein[ratio ]total casein ratio. When rye-grass was grazed, the opposite response was found, and overall milk protein yield was not affected. The very low N content of clover in early summer reduced milk protein and casein protein during this period.

Effects of the hot season on milk protein fractions in Holstein cows

“…The reduction of casein content in summer milk has also been reported by some authors [14,25]. In a previous study we observed lower colostrum casein contents in cattle exposed to a hot environment during the last three weeks of pregnancy [33].…”

“…In the present study, we did not observe a seasonal effect on α-lactalbumin and β-lactoglobulin concentrations, but serum proteins increased. Hermansen et al [14] reported a higher content of whey proteins in milk produced in the late summer and that the result was not related to proteolysis.…”

“…Hot season and milk protein fractions 29 [8,14,25,40]. Coulon et al [8] reported positive effects of the amount and type of energy and nitrogen supply on milk protein content but no or slight effects on the casein/protein ratio.…”

“…Coulon et al [8] reported positive effects of the amount and type of energy and nitrogen supply on milk protein content but no or slight effects on the casein/protein ratio. Hermansen et al [14], Mackle et al [25] and Todorova [40] found the reduction of caseins by dietary changes consisting of a lower grass availability and consequent lower energy and nitrogen content of the summer diets. Hermansen et al [14] found a significant positive effect on true protein and casein contents in response to concentrate supplementation.…”

“…Hermansen et al [14], Mackle et al [25] and Todorova [40] found the reduction of caseins by dietary changes consisting of a lower grass availability and consequent lower energy and nitrogen content of the summer diets. Hermansen et al [14] found a significant positive effect on true protein and casein contents in response to concentrate supplementation. Mackle et al [25] observed a reduction of casein concentration in cows subjected to restricted pasture intake.…”