Genetic parameters and correlations between days open and production traits across lactations in pasture based dairy production systems

The aim of this study was to estimate the additive genetic correlations, heritabilities and repeatabilities of days open (DO), milk (MY), fat (FY) and protein yields (PY), using data from pasture based dairy systems of Uruguay, and to explore whether DO should be interpreted as a different trait across lactations or as a trait with repeated measures. The database contained 500, 412 and 294 thousand records of first, second and third lactation, respectively. Cows were offspring of 7747 sires. Fertility records lower and upper limits were 42 and 250 days, respectively. In a first approach (Mdiff) we estimated variance components and covariances over lactations, assuming that the traits were different at each lactation. In a second approach (Mrep) estimations were carried out considering each trait as a repeated measure along lactations. In Mdiff, DO with a production trait was analyzed considering each lactation as a different trait. Three six-variate linear models were analyzed (DO-MY, DO-FY, DO-PY, and lactations first to third). In the Mrep procedure, DO, MY, FY and PY were analyzed together with a multiple trait repeatability model. For all models, the fixed effects were herd-year-season and lactation-age classes. Animal and the permanent environment effect were included as random effects. The additive genetic correlations between DO and yield traits by Mdiff were between +0.39 and +0.78; by Mrep, they ranged from +0.44 to +0.55. Heritabilities of DO by Mdiff were between 0.04 and 0.06 and 0.05 by Mrep. The additive genetic correlations of DO between lactations ranged from +0.76 to +0.91. Heritabilities of MY, FY and PY were 0.23, 0.21 and 0.21, respectively. Repeatabilities obtained were 0.10, 0.49, 0.47 and 0.49 for DO, MY, FY and PY, respectively. We concluded that the heritability of DO was low but enough to consider the trait in selection programs. We confirmed unfavorable additive genetic correlations between DO and yield traits for the Uruguayan pasture systems, which supports the importance of considering fertility in selection programs to reduce or avoid a decline in reproduction. The additive genetic correlations of DO between lactations were high, suggesting that a plausible model should consider DO records of a given animal as repeated measures.