Genetic analysis of female fertility traits in Canadian Simmentals

Abstract

Calving to first insemination (CFI) (probability that a cow will produce a calf from her first service) and days to calving (DC) (interval between the first exposure to the bull and calving) defined separately for heifers and multi-parity cows were analyzed with a 4-trait linear animal model. The data were 108,206 records on 56,879 Canadian Simmental females with artificial insemination (AI) and natural service (NS) breeding records from 1974 to 2010. The model included fixed effects of year-season of breeding, age of cow by season of breeding, and age of cow's dam. Random effects were: contemporary group defined as herd within breeding year-season by breeding status for the first insemination (AI or NS), service sire by year of breeding (for CFI only), animal genetic, and cow permanent environmental (PE) effects. Bayesian methods were used to estimate covariance components and genetic parameters. Three specific models with different levels of heterogeneity for covariances in NS and AI data were tested. Results are reported for the most plausible specification which was the model that assumed different residual covariances for NS and AI records. The data exhibited substantial heterogeneity of residual covariance components with respect to the type of first breeding, with AI records having larger residual variances for both traits. Pooled estimates (SD) of heritability ranged from 1.8% (0.33) for CFI in heifers to 3.5% (0.47) for DC in multi-parity cows. Interval trait (DC) had higher heritability compared with CFI; both traits expressed in older cows were more heritable than corresponding traits for heifers. Contemporary group contributed up to 58% (0.51) of the total variance for DC in heifers by NS breeding. Service sire and the PE were relatively larger (smaller) sources of variation compared with additive genetic (contemporary group) effects. All traits were highly genetically correlated: from 0.69 (0.06) for CFI in heifers and multi-parity cows to 0.92 (0.04) for DC in heifers and multi-parity cows. Environmental correlations ranged from −0.73 (0.01) between CFI in heifers and DC in older cows, to 0.91 (0.01) for DC in heifers and multi-parity cows. Results indicated that CFI and DC expressed different aspects of female fertility in beef cattle, and fertility of heifers and multi-parity cows seemed to be different traits genetically. All four traits could be used in a form of selection index in Canadian Simmentals to breed for better fertility. Genetic progress, however, would be challenging given low values of heritability.