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 with some progestins, but not others, before first ovulation was able to eliminate the occurrence of short‐lived corpora lutea [74–76]. Specifically, progesterone administered via intravaginal implant for five days was found to be more effective in reducing incidence of premature luteolysis after GnRH‐induced ovulation than oral MGA [75].
Nutritional Optimization of Puberty
Rapid growth from birth to weaning (6–10 months) and from weaning to puberty (11–16 months) is critical for beef heifer calves to attain puberty at an early age. It is now clear that the interval between four and nine months of age is a critical period for metabolically programming the neuroendocrine axis to achieve early reproductive maturation. The requirement for rapid growth in breeding heifers is to ensure that they will reach puberty in advance of the breeding season and be bred to calve by 24 months of age. Therefore heifers must exceed both the age and weight “threshold” to attain puberty. Nutritional programs for breeding heifers are designed to grow animals at a rate that allows them to exceed the weight threshold before or soon after the age threshold for puberty is surpassed [77]. Across several breeds, heifers were 55–60% of mature body weight at puberty [78]. The practice of developing heifers to reach a target body weight (typically ≥65% of mature weight) prior to breeding is commonplace in the industry. However, one must bear in mind that the target body weight will vary by breed. Methods to estimate effects of heifer nutrition protocols have been developed. For example, residual feed intake (RFI) is the residual from a regression model regressing feed intake on average daily gain (ADG) and body weight (BW)–0.75 [79]. Body fat stores are greater in heifers with greater RFI than in their more efficient herdmates. A 1‐unit increase in RFI resulted in a reduction of 7.54 days in age at puberty in Bos taurus beef heifers. However, Bos indicus‐influenced heifers, which reach puberty at older ages, were not found to have sexual maturity influenced by selection for higher RFI [79]. From a management standpoint,
