CONICET | Buscador de Institutos y Recursos Humanos

The objective of the present study was to assess the relationship between individual cow milk yield and fertility, accounting for the contextual effect of the herd. A dataset including 657,968 lactations from 677 dairy herds in Argentina from 2001 to 2012 was used. The odds of pregnancy by 100 DIM were assessed by a multilevel logistic model (with cow as the first and herd as the second hierarchical level), and time to pregnancy by a proportional hazards regression model. Multilevel logistic models included the fixed effects of milk yield by 80 DIM, parity, year and calving season at cow level and quartiles of herd milk yield by 80 DIM as a contextual effect. The proportional hazards model included the effect of daily milk yield as time dependent variable and herd-level milk yield as stratification variable. Cows producing 1 SD (400 kg) over the mean milk yield of their herd had 1.0 percentage points lower pregnancy by 100 DIM (from 31.4% to 30.4%, odds ratio (OR) = 0.942) when in herds in the top quartile of milk yield, whereas they increased 0.4 percentage points (from 27.9 to 28.3%) when in herds in the lowest quartile of milk yield. Only 4% of the observed variation in pregnancy by 100 DIM was explained by the random effect of the herd. Similarly, cows producing 1 SD (8 kg/d) greater than the herd daily milk had 1.3% lower hazard of pregnancy (hazard ratio (HR) = 0.987) at 63 DIM in herds in the top quartile of milk yield whereas they had 14.8% higher hazard (HR = 1.148) in herds in the lowest quartile of milk yield. The magnitude of the negative association between the cow´s daily milk yield and the hazard of pregnancy increased with DIM.In conclusion, the relationship between milk yield and reproductive performance is statistically significant but the effect size is practically small, it is modulated by herd productivity.