Effects of genetic merit and varying dietary protein degradability on lactating dairy cows

Eighty two multiparous Holstein cows were blocked by genetic merit thigh vs , low) and assigned to one of two treatments [high rumen-undegradable prote in (RUP): rumen-degradable protein (RDP) vs. low RUP: RDP] from d 21 before to d 150 after calving to study the effects of these treatments on product ion and reproductive performance. Diets were isonitrogenous (dry cow 10.5% crude protein; lactating cow 19.3%), isoenergetic (dry cow 10.0 MJ of metab olizable energy (ME); lactating cow 10.9 MJ of ME) and fed as total mixed r ations. Feeding more RUP significantly increased dry matter intake and milk yield, reduced body tissue mobilization, and lowered concentrations of ser um nonesterified fatty acids (NEFA) and plasma urea. Expression of estrus a t first ovulation was improved, first service conception rate was higher, a nd calving to conception interval was shorter for the high RUP group. Cows of high genetic merit produced more milk, mobilized more body tissue, and h ad higher concentrations of plasma growth hormone. The dry matter intake an d concentrations of blood metabolites did not significantly differ with gen etic merit. Expression of estrus at first ovulation was significantly lower for cows of high genetic merit. Serum NEFA concentrations were significant ly higher, and estrus was not observed at first ovulation for cows of highe r genetic merit fed the low RUP diet. The interaction between dietary RUP a nd genetic merit was not significant for other measures of performance or f ertility. Feeding a low RUP: high RDP diet had negative effects on some asp ects of production and reproductive performance. The effects of diet on NEF A concentrations and estrus display were greater in cows of high genetic me rit, indicating that potential interactions should be evaluated in future r eproductive studies involving protein and fertility.