Martin L C, Brinks J S, Bourdon R M, Cundiff L V
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506.
J Anim Sci. 1992 Dec;70(12):4006-17. doi: 10.2527/1992.70124006x.
Significant genetic variation exists within and between breeds of beef cattle for age at puberty (AP). In general, faster-gaining breed groups of larger mature size reach puberty at a later age than do slower-gaining breed groups of smaller mature size; breeds selected for milk production reach puberty at younger ages than do those breeds not selected for milk production. Heterosis, independent of heterosis effects on weight, influences most measures of puberty in females and scrotal circumference (SC) in males. Crossbred heifers reach puberty at younger ages and heavier weights than their straightbred counterparts. Scrotal circumference has been shown to be an excellent indicator of AP in yearling bulls. Furthermore, a favorable genetic relationship exists between SC in bulls and AP of female offspring. Beef cattle breeders may take a direct approach to breeding for AP and subsequent reproduction by directly selecting for measures of fertility such as SC. However, an indirect approach, involving selection for an array of traits that provide an appropriate "genetic environment" for the expression of fertility (i.e., size, milk production, calving ease) may be preferred. Although seedstock producers are limited to making change through within-breed selection, commercial producers can take advantage of both within- and between-breed selection as well as crossbreeding to achieve the same goal.
肉牛品种内和品种间在初情期年龄（AP）方面存在显著的遗传变异。一般来说，成熟体型较大、生长速度较快的品种组比成熟体型较小、生长速度较慢的品种组初情期来得晚；选择用于产奶的品种比未选择用于产奶的品种初情期来得早。杂种优势，独立于其对体重的杂种优势效应，影响雌性的大多数初情期指标以及雄性的阴囊周长（SC）。杂交小母牛比其纯种对应个体初情期来得早且体重更重。阴囊周长已被证明是一岁公牛初情期年龄的一个极好指标。此外，公牛的阴囊周长与雌性后代的初情期年龄之间存在良好的遗传关系。肉牛育种者可以通过直接选择诸如阴囊周长等繁殖力指标，采取直接的方法来培育初情期年龄及后续繁殖性能。然而，一种间接的方法，即选择一系列为繁殖力表达提供适宜“遗传环境”的性状（即体型、产奶量、产犊难易度）可能更受青睐。尽管种畜生产者限于通过品种内选择来进行改良，但商业生产者可以利用品种内和品种间选择以及杂交来实现相同的目标。
