Genetic (co)variance for sire fertility estimated by additive, non-additive and longitudinal models in Holstein-Zebu cross-bred cows.

For this study, we used the results of sire fertility (SF) measured on a monthly basis by the ratio between the number of pregnant cows and the number of inseminations, from a total of 905 140 inseminations carried out in Cuba between 1994 and 2003. These artificial inseminations were made using 815 sires in 3249 herds throughout the country, and were analysed using additive, non-additive linear models and a random regression model (RRM). The additive genetic ( add ), heterosis ( het ) and recombination loss ( rec ) coefficients were estimated according to the proportion of Zebu (Z) and Holstein (H) blood from the paternal and maternal origin of each cow. The mean level of SF was 48.8%, whereas het and rec were 9.6% and −8.4%, respectively. The heritability ( h 2 ) of a single insemination ranged from h 2 = 0.011 to h 2 = 0.030 for females from 0% to 100% of H genes. The additive multi-trait and RRM analyses showed the existence of heterogeneous genetic (co)variance components, as the proportion of Holstein genes in the inseminated cow increased. We found low genetic correlations for SF recorded in pure-bred and cross-bred females, with over 50% of breed differences in their additive genetic composition. The use of a RRM allows us to identify the changes in genetic (co)variance and estimated breeding values in the whole trajectory of the different proportions of Bos taurus × Bos indicus blood.