Читать книгу Bovine Reproduction - Группа авторов - Страница 366

Consideration should be given to time of year when calving occurs. Although cattle are not considered to be strictly seasonal breeders, there is a seasonal effect on reproduction. For example, it has been reported that fertility, cycle length, and postpartum anestrous period length vary with season. More specifically, a winter environment (northern hemisphere) delays onset of puberty. Schillo et al. [65] reported that fall‐born heifers were younger at puberty than those born in spring, and that heifers exposed to simulated changes in daylength from spring to fall after six months of age showed advanced onset of cyclic ovarian activity. The effect of season is likely mediated through photoperiod and is not a direct consequence of improved nutrient availability. As further evidence of this effect, puberty can be advanced in heifers by administration of melatonin‐containing implants [66]. Treatment of three‐ to four‐month‐old winter‐born heifers with exogenous melatonin for a period of five weeks at the beginning of summer significantly increased the incidence of animals attaining puberty by March of the following year (58 vs. 17%) [66]. Exposure of prepubertal heifers from 22 or 24 weeks of age until first ovulation to an artificially extended photoperiod will also advance first ovulation [67].