Genetic parameters of reproductive traits in beef cattle

Records from 12 Bos taurus breeds collected at MARC (USDA, ARS) for scrotal circumference (SC), age at puberty (AP), age at first calving (AFC), and pregnancy, calving or weaning status following first breeding season (PR1, CR1, and WR1, respectively), and pregnancy, calving or weaning status following second through tenth breeding seasons (PR2, CR2, and WR2, respectively) were used to estimate heritabilities and genetic correlations. Direct heritability estimates with a single trait animal model were .15 ± .03, .15 ± .03, and .11 ± .03 for PR1, CR1, and WR1, respectively. Direct heritability estimates for PR2, CR2, and WR2 were nearly zero. Management was an important factor affecting pregnancy, calving and, weaning status especially after the first breeding season. Analyses with a three trait animal model included SC, AP, and as a third trait AFC, PR1, CRI, or WR1. Direct heritability estimates were .41, .16, .08, .14, .14, and .12 for SC, AP, AFC, PR1, CR1, and WR1, respectively. Direct genetic correlations between SC and the female traits ranged from −.15 to .15 indicating that genetic response in female traits by selecting sires on SC would not be expected to be effective. Analyses with a two trait animal model combined pairs of SC, AP, and WR1 for five groups of breeds. Charolais, Limousin, and MARC I composite (Group 1); Simmental, Gelbvieh, Braunvieh, and MARC II composite (Group 2); Hereford, Angus, Pinzgauer, Red Poll, and MARC III composite (Group 3); Charolais, Limousin, Simmental, Gelbvieh, Braunvieh, Hereford, Angus, Pinzgauer, and Red Poll (Group 4), and MARC I, MARC II, and MARC III composites (Group 5). Direct genetic correlations between SC and AP and between SC and WR1 were −.41 and .22 (Group 1), −.09 and −.03, (Group 2), .19 and .04 (Group 3), .03 and −.02 (Group 4), and −.28 and .20 (Group 5). Results suggest favorable genetic correlations between SC and AP or WR1 for Groups 1 and 5. Overall, these results suggest genetic change for female traits through direct or indirect selection would be slow