Genetic parameters estimates for gestation length in beef cattle

Variance components were estimated for gestation length fitting the additive direct effect of calf maternal genetic effect and sire effect as random effects. The statistical models also included the fixed effects of contemporary group, that included the date of breeding (AI), date of birth, and sex of calf, and the covariate age of dam at calving (linear and quadratic). Two different models were used, model 1 considering GL as a trait of the calf and model 2 considering GL as a trait of the dam. The means of gestation length for the purebred animals were 294.55 days (males) and 293.34 days (females), while for the crossbred animals they were 292.49 days (males) and 292.55 days (females). Variance components for the purebred animals, fitting model 1, were 14.47, 72.78 and 57.31, for the additive genetic total phenotypic (sigma(2)(p)) and residual (sigma(2)(e)) effects, respectively, with a heritability estimate of 0.21. For the crossbred animals, variance components for sigma(2)(a), sigma(2)(p), sigma(2)(e) were 90.40, 127.35 and 36.95, respectively, with a heritability of 0.71. Fitting model 2, the estimated variance components for the purebred animals were 12.78, 5.01, 74.84 and 5 7.05 for sigma(2)(a) sire of calf (sigma(2)(asire)), sigma(2)(p), and sigma(2)(e), respectively. The sire effect accounted for 0.07 (c(2)) of the phenotypic variance and the coefficient of repeatability was 0.17. For the crossbred animals, the variance components were 22.11 (sigma(2)(a)), 22.97 (sigma(2)(asire)), 127.70 (sigma(2)(p)) and 82.61 (sigma(2)(e)), while c(2) was 0.18 and repeatability was 0.17. Therefore, regarding selection of beef cattle, it is suggested to use the heritability estimate obtained by model 1, where GL is considered as a trait of the calf.