Pre-calving and early lactation factors that predict milk casein and fertility in the transition dairy cow

Abstract Text:

Multiparous Holstein cows (n = 82) of either high or low genetic merit (GM) (for milk fat + protein yield) were allocated to one of two diets in a 2 X 2 factorial design. Diets differed in the ratio of rumen-undegradable protein (RUP) to rumen-degradable protein (37% RUP vs. 15% RUP) and were fed from 21 d pre-calving to 150 DIM. This study evaluated the effects of these diets and GM on concentrations of milk casein (CN) variants and aimed to identify pre-calving and early lactation variables that predict milk, CN and protein yield and composition, and fertility of dairy cows. It explored the hypothesis that low milk protein content is associated with lower fertility, extending this to also evaluate the contribution of CN contents. Yields (kg/d) for CN variants were 0.49 and 0.45 of alpha CN, 0.38 and 0.34 of beta CN, 0.07 and 0.06 for kappa CN, and 0.10 and 0.09 of gamma CN for high and low RUP diets, respectively. Increased RUP increased milk, CN and milk protein yields. Increased GM increased milk and gamma CN yields and tended to increase milk protein yield. The effects of indicator variables on CN variant yields and concentrations were largely consistent, with higher body weight and alpha amino nitrogen resulting in higher yields, but lower concentrations. An increase in cholesterol was associated with decreased CN variant concentrations, while disease lowered CN variant yield. A diet high in RUP increased proportion of first services that resulted in pregnancy from 41 to 58%. Increased pre-calving metabolizable protein (MP) balance decreased the proportion of first services that resulted in pregnancy when evaluated in a model containing CN %, milk protein yield, diet, and GM. This indicates that the positive effects of a diet high in RUP on fertility may be curvilinear as cows with a very positive MP balance before calving were less fertile than those with a lower, but positive, MP balance. Prepartum MP balance was important to production and reproduction outcomes, while surprisingly, metabolizable energy balance was not. Cows producing the lowest quartile of milk protein percentage were 28% less likely to become pregnant during the first 150 DIM. Milk CN %, was similarly positively associated with improved pregnancy at first service. This study demonstrates the importance of protein metabolism to reproductive performance of the dairy cow.

Keywords: casein, fertility, protein degradability