Towards a better understanding of the effect of genetic merit for milk production on post-partum cyclicity of first lactation dairy cows
Résumé
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.
Domaines
Sciences du Vivant [q-bio]
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