Additive genetic parameter estimates for heifer pregnancy and subsequent reproduction in Angus females

A primary objective of this study was to determine whether the binary trait s heifer pregnancy (HP) and subsequent rebreeding (SR) were heritable in an experimental population of Angus cattle. A second objective was to determi ne the nature of the additive genetic relationships among HP, SR, and staya bility (S(5 / 1)) in the same population. Heifer pregnancy was defined as t he observation of a heifer conceiving and remaining pregnant to palpation a t 120 d, given exposure during the breeding season. Subsequent rebreeding w as defined as the observation of a 2-yr-old conceiving and remaining pregna nt to palpation at 105 d, given pregnancy as a yearling and exposure during the breeding season. Stayability was defined as the probability of a femal e having at least five calves, given she becomes a dam as a 2 yr old. Data were analyzed using a maximum a posteriori probit threshold model to predic t breeding values on the liability scale and Method R procedures to estimat e variance components in the determination of heritability (h(2)). Additive genetic groups were used in determining the additive genetic relationships among these fertility traits. Additive genetic groups were formed on one t rait's breeding values and used in the prediction of another trait's breedi ng values. Analyses yielded h(2) estimates that were out of the parameter s pace 8.5 and 46.3% for HP and SR, respectively, and 5.9% for the reestimati on of S(5/1). The majority of point estimates outside the parameter space f or SR converged toward 0, whereas those for HP and S(5/1) primarily converg ed toward 1. From the subsamples producing h(2) estimates within the parame ter space, average h(2) for HP, SR, and S(5/1) were .21, .19, and .15, with standard deviations of .12, .14, and .08, respectively. The estimates of h (2) indicate that HP and S(5/1) were heritable and should respond-favorably to selection; however, SR aid not appear heritable due to the large number of subsamples producing h(2) estimates out of the parameter space. Fixed e ffect estimates for age of dam were significant for HP. From the analyses u sing additive genetic groups, the relationship among HP and S(5/1) appeared to be nonlinear. This potential nonlinear relationship seen between HP and S(5/1) indicates that selection for improved female fertility would be mos t effective by having predictions on both traits.