Modelling fertility traits under natural mating conditions in beef cattle

Abstract

Fertility traits are the most economically important traits in beef cattle and in spite of having low heritabilities they ought to be included in breeding programs. The genetic selection programs in Spain are based on improving growth traits, neglecting the fertility traits up to now. In this study, a first analysis of the calving interval between first and second parities (CI₁₂) was carried out to establish a suitable selection criterion. The selected breeds were two local breeds Avileña-Negra Ibérica (ANI) and Retinta (RT), raised in extensive production with natural mating. Two models including cow and bull effects were studied. –Systematic effects were herd-year-season of first calving, age of cow at first calving and age of bull at second calving and regressions on inbreeding coefficients for cows and bulls. Then, a full model (FULL) included additive genetic and permanent environmental components for bull and an additive genetic effect for cows. A reduced model (RED) was the same as FULL but with the additive genetic component for bull excluded. Both models provided similar estimates of systematic and cow additive genetic components. Inbreeding depression was found for bulls (0.79 and 0.83 days/% inbreeding in ANI and RT, respectively) but nearly null regression coefficients on inbreeding were obtained for cows. In both breeds, heritability estimates were 0.05 for cows. For FULL, heritability estimates for bull effects were larger than the cows heritability, 0.08 for ANI and 0.10 for RT. The bull permanent component was large, with values of 0.20 and 0.31 for ANI and RT, respectively. Herd fertility in these populations can be substantially improved by bulls screening. Genetic selection can be practiced but with low efficiency, as it is commonly the case for fertility traits