Towards a better understanding of the effect of genetic merit for milk production on post-partum cyclicity of first lactation dairy cows

Milk genetic merit is known to affect commencement of luteal activity (CLA) in dairy cows. This effect is considered to be due to energy exported in milk production. We hypothesize that cows with genetic predisposition to export milk energy through milk yield would have degraded cyclicity compared with those exporting energy through fat and protein contents. This study aimed to identify and quantify the effect of breed and genetic groups on postpartum cyclicity of primiparous dairy cows. From 2006 to 2013, an experiment was conducted on 194 primiparous dairy (Holstein) and dual purpose (Normande). Within breeds, cows were classified into 2 groups with regards to their estimated breeding value (EBV). Cows with high EBV for milk yield were included in a “Milk” group (MG) and those with high EBV for fat and protein content were included in a “Content” group (CG). Within breeds, exported milk energy and weight loss were similar for cows in both MG and CG groups. Interval from calving to CLA was studied performing survival analyses (Weibull regression). Progesterone profile, milk yield and body condition were analyzed using Chi2-test and ANCOVA. Holstein cows produced more milk (+1450 kg, P < 0.001) and lost more body weight (BW: −1.4kg/wk, P < 0.01) than Normande ones. Normande and Holstein cows had different baseline hazard functions for CLA, Normande cows having earlier CLA than Holstein ones. Within breeds, cows in the CG group had earlier CLA (associated HR = 2.0, P = 0.001) than cows in the MG group. For Holstein only, BW loss from wk 1 to 14 of lactation tended to be associated with later CLA (P < 0.1). Within breeds, no effect of milk yield on CLA was observed. Prolonged luteal phases were frequent (18% of cows) and neither associated with breed nor with differences between MG and CG. Interovulatory intervals were longer for Holstein than for Normande cows (+1.7 d, P < 0.001) because of a longer luteal phase and a longer interluteal interval. To conclude, this study showed that cows with genetic predisposition to export milk energy through fat and protein contents had earlier CLA than those exporting milk energy through yield