Bovine Reproduction. Группа авторов
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anestrous interval. Heifers that have an earlier age at puberty generally have greater lifetime productivity. Age at puberty is moderately heritable, so selection over multiple generations should reduce the number of days to puberty. We now have the technology to evaluate genetic markers for age at puberty in cattle. For example, quantitative trait loci have been identified that predict male reproductive traits including age at puberty in cattle [47]. Similarly, random amplified polymorphic DNA markers have been used for identifying Nelore bulls with early (precocious) or late (non‐precocious) puberty [48]. In heifers, an association weight matrix (AWM) has been constructed based on 22 related traits with single nucleotide polymorphisms [49]. The AWM results recapitulated the known biology of puberty, captured experimentally using validated binding sites, and identified candidate genes and gene–gene interactions for further investigation. Takada et al. [50] evaluated Nelore heifers to detect known polymorphisms in candidate genes related to sexual precocity and identified five genes that influence sexual precocity. Advances in genomic technologies will likely provide a powerful tool for selecting heifers at birth that will have a greater probability of being reproductively successful if managed correctly [51].
Correlation Between Sire Scrotal Circumference and Age at Puberty of Daughters
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].
Management Options to Influence Age at Puberty
Management of replacement beef heifers should focus on factors that enhance physiological processes that promote puberty [62]. Procedures that ultimately affect lifetime productivity and reproductive performance of heifers begin before birth and include decisions that involve growth‐promoting implants, feeding, breed selection, calving and weaning date, social interaction, sire selection, and exogenous hormonal treatments to synchronize or induce estrus. This is especially relevant in systems where heifers are expected to calve by two years of age or where the breeding period is restricted [36]. Heifers that calve as two year olds produce more calves in their lifetime than heifers that calve as three year olds [63]. Breed and postweaning rate of gain have a large influence over onset of puberty [36]. For optimal fertility, heifers should not be bred at their pubertal estrus as calving rate has been reported to be 21% less than for heifers bred at third estrus [64]. This implies that heifers should reach puberty within one to three months before the age at which they will be bred in order to mitigate the reduced fertility associated with breeding at pubertal estrus.
Photoperiod
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].
Growth‐Promoting Implants
Growth‐promoting implants are widely used in the beef industry. However, in heifers to be retained for breeding, implants containing estradiol, zernol, or trenbolone acetate inhibit the development of a mature reproductive endocrine system when administered to suckling beef heifers and consequently delay onset of puberty. For example, zeranol implants at birth delay the onset of puberty and decrease uterine horn diameter. Furthermore, zeranol‐implanted heifers have lower pregnancy rates and higher rates of abortion than non‐implanted herdmates [68]. In contrast, Rosasco et al. [69], in a study comparing performance of crossbred Angus heifers implanted with Synovex® C (Zoetis, Parsippany, NJ, USA) at three months of age, reported no deleterious effects on reproductive performance through four calving seasons (note: product label specifies for heifers 45 days and older up to 400 pounds). The implanted calves were heavier at weaning than control heifers such that producers who do not make selection of replacement heifers until weaning may expect some profit advantage from heifers not retained as replacements. Replacement heifers that are identified early in life should not be implanted as there is no benefit in terms of age at puberty or incidence of dystocia.
Use of Progestins to Advance Onset of Puberty
It is well established that progesterone sensitizes the hypothalamus to the positive feedback effect of estradiol‐17β that results in the preovulatory LH surge. Progestin administration hastens puberty by accelerating the peripubertal decrease in estradiol‐17β negative feedback effect, thereby facilitating the LH surge [72]. Furthermore, progesterone priming has been shown to enhance follicular estradiol‐17β synthesis in ewes [70] and it has also been demonstrated that circulating estradiol‐17β concentrations were higher at the first ovulatory estrus than in the preceding silent estrus [71]. As an example of the use of an exogenous progestin, melengestrol acetate (MGA) fed for eight days followed by withdrawal, in turn, enhanced onset of puberty by stimulating pulsatile LH secretion that accelerated follicle growth to the preovulatory stage [73]. In synchronization programs for heifers that may be peripubertal and prone to premature luteolysis, the incorporation of a progestin improves pregnancy rates. Treatment