Читать книгу Bovine Reproduction - Группа авторов - Страница 364

Early studies reported a favorable correlation between yearling scrotal circumference (SC) in bulls and age at puberty in half‐sib heifers in populations of purebred animals [52, 53]. Estimates of genetic correlation between SC in yearling bulls and age at puberty in half‐sib heifers of −0.71 and −1.07 (favorable) have been reported by Brinks et al. [52] and King et al. [53] In a study designed to estimate genetic correlations between testicular measurements and female reproductive traits in Hereford cattle, the authors concluded that heritability of female reproductive traits tended to be low to moderate but “selection for increased testicular size would lead to improvement in female reproduction, particularly an increase in calving rate and a decrease in age at first breeding” [54]. A favorable relationship between SC in Brahman bulls and age at puberty in Brahman heifers reared under subtropical conditions has also been reported [55]. However, more recent work suggests that the correlation between genetic response in female reproductive traits (including age at puberty) and sire yearling SC may be expected to be less than previously reported in the literature. The study by Martinez‐Velazquez et al. [56] calculated the correlation between yearling bull SC and age at puberty in daughters to be only −0.15. Morris et al. [57], in a 14‐year study involving Angus cattle, reported a correlation of −0.25 between sire SC and age of puberty in heifer offspring. In an Australian study, no significant relationship was found between age at puberty in heifers and age and SC at puberty in related bulls [58]. Results of a subsequent experiment designed to investigate this correlation using Limousin bulls bred to crossbred cows indicated that selection of resulting replacement heifers based on sire SC phenotype did not significantly influence heifer age at puberty [59]. However, when sire SC estimated progeny difference was used instead, selection resulted in a significant reduction in heifer age at puberty. The authors concluded that when sires are selected for high SC estimated progeny difference within a crossbreeding system, a large percentage of heifers should reach puberty early enough to calve at two years of age or younger. Nonetheless, it has been reported that age of puberty in bulls, based on attainment of a critical SC, is correlated with age at puberty in female offspring [60]. One explanation of the conflicting data may be that heterosis has a significant effect on the percentage of heifers reaching puberty by 368, 410, and 452 days, with the greatest effect at the younger age [61].