of leaner heifers that reach puberty at older ages [79]. Therefore, at some point, it becomes counterproductive to select replacement heifers on the basis of low RFI. This is because reproduction has been reported to be five times more important to commercial cattle producers than growth rate or milk production [80], and heifers that calve early in their first calving season tend to calve early throughout their lives and have greater lifetime calf production [81].
Precocious puberty (<300 days of age) in beef heifers can be induced by early weaning and continuous feeding of a high‐concentrate diet. As in the case of progestin administration, puberty is preceded by increasing frequency of LH pulses [82]. Heifers experiencing induced precocious puberty weigh significantly less at puberty than their traditionally weaned and fed counterparts [82]. Furthermore, it has been determined that feeding a high‐concentrate diet from 126 days (after weaning at 112 days) through 196 days was as effective at inducing precocious puberty as continuous high‐concentrate feeding [83]. Taken together, data indicate that high preweaning growth rate and heavy weaning weights are associated with early puberty and heavier weight at puberty [84].
The concept of fetal programming was proposed by Barker [85] to explain a geographical association between poor maternal physique and health, poor fetal growth, and high death rates from cardiovascular disease in adult humans. Similarly, there are numerous studies in animals that demonstrate that transient adverse events in prenatal or early postnatal life have enduring and profound effects on physiology, although such effects may remain latent until the animal is mature. One example of this comes from experiments in which the nutrition of pregnant and lactating rats was manipulated. The adult body size of the offspring was more powerfully determined by their mothers' nutritional status during pregnancy and lactation than by their genetic constitution [86]. Prenatally undernourished female rats displayed delayed puberty which was associated with decreased hypothalamic kisspeptin action [87]. It is now established that maternal nutrient status can cause epigenetic alterations to the genome of the developing fetus, which can potentially impact future generations. Epigenetics is defined as heritable changes in gene expression resulting from alterations in chromatin structure but not in DNA sequence. Epigenetic programming can have lasting effects on future generations through intergenerational influences. These are described as factors, conditions, exposures, and environments in one generation that impact the health, growth, and development of subsequent generations. Three main mechanisms cause epigenetic changes to the genome: DNA methylation, histone modification, and non‐coding microRNAs [88]. Non‐coding RNAs are RNA transcripts that are not transcribed into proteins, but have been shown to regulate transcription, stability, or translation of protein‐coding genes [89]. These processes regulate both the intensity and timing of gene expression during cell differentiation [90, 91]. In cattle, nutrient restriction of gestating cows resulted in heifer offspring with reduced wet ovarian weight and decreased luteal tissue mass compared with heifers born to control‐fed cows [92]. Funston et al. [93] also reported that heifers born to cows protein‐supplemented during the last one‐third of pregnancy attained puberty 14 days earlier than heifers from non‐supplemented dams. In sheep, fetal growth restriction altered pituitary LHβ expression and number of follicles in the fetal ovary [94]. Similarly in cattle, nutrient restriction for the first 110 days of gestation resulted in calves with fewer antral follicles compared with calves born to non‐restricted cows [95]. This may impact the onset of puberty in heifers as there is evidence from ultrasonographic studies that development of ovarian antral follicles and tubular genitalia occur in parallel [95] and this development is necessary for puberty.
In dairy heifers it appears that factors including colostrum intake, preweaning growth rate, and body composition influence age at puberty. As an example of the effect of preweaning (0–42 days) growth rate, heifers fed an intensive milk replacer diet were 15 days younger at first pregnancy and 14 days younger at calving than heifers fed a conventional milk replacer diet [96]. The conventional diet consisted of a standard milk replacer (21.5% crude protein [CP], 21.5% fat) fed at 1.2% of BW on a dry matter basis and starter grain (19.9% CP) to attain 0.45 kg of daily gain. The intensive diet consisted of a high‐protein milk replacer (30.6% CP, 16.1% fat) fed at 2.1% of BW on a dry matter basis and starter grain (24.3% CP) to achieve 0.68 kg of daily gain [96].
Finally, one should bear in mind that consumption of certain feedstuffs may actually be deleterious to attainment of puberty. One such example is endophyte‐infected tall fescue. Cattle consuming this forage are prone to decreased calving and growth rates, delayed onset of puberty, and impaired function of corpora lutea [97].
Effect of Heifer Temperament on Age at Puberty
A study by Cooke et al. [98] evaluated the influence of temperament on various performance measures including age at puberty in Bos indicus heifers. Bos indicus heifers classified as “excitable” (based on chute exit velocity) had reduced growth, increased plasma cortisol concentrations, and hindered puberty attainment compared to heifers classified as “adequate” or less excitable temperament.
Influence of Bull Exposure on Age at Puberty
Unlike other domestic species (sheep, goats, swine), exposure to a bull has no effect on the incidence of precocious puberty [99].
Influence of Age of Dam on Follicular Reserves
Walsh et al. [100] reported that maternal age affected the number of antral follicles detectable by ultrasonography in the ovaries of the daughters at a year of age. Holstein heifers that were born to heifers had fewer antral follicles detectable by ultrasonography at a year of age than Holstein heifers that were born to multiparous cows. Similarly, Angus heifers with diminished numbers of antral follicles detectable by ultrasonography (14.5 ± 0.8 follicles) had dams that were younger than the dams of Angus heifers with increased numbers of antral follicles (31.1 ± 0.8 follicles) [101]. These studies suggest that the lesser number of antral follicles detectable by ultrasonography in heifers born to primiparous dams is due to fewer ovarian follicle reserves. Selecting replacement heifers from mature dams may result in daughters with greater fertility and reproductive longevity; however, further research is necessary to determine if interactions between size of the ovarian follicle reserve and age at puberty influence fertility and reproductive longevity in replacement heifers.
Monitoring Heifers for Attainment of Puberty
The rearing of replacement heifers is a major financial investment for both beef and dairy cattle producers. The investment expenses do not begin to be recovered until after the first calf is weaned and sold, or in the case of dairy heifers, the onset of lactation, so having heifers calve at an optimal age is paramount to enterprise profitability. For this to occur it is essential that operators know when their heifers have attained puberty and become eligible for breeding. This is most critical for herds using a restricted breeding season.
Observation of signs of estrus can predict onset of puberty but is impractical for application to larger herds. Another observational tool, reproductive tract score (RTS), is a useful predictor of heifer fertility [102]. There is a positive correlation between high RTSs in heifers and percentage of heifers conceiving by artificial insemination [103]. RTS is a subjective estimate of sexual maturity based on ovarian follicular development and diameter of the uterine horns. An RTS of 1 is assigned to heifers with infantile tracts, as indicated by small, toneless uterine horns and small ovaries devoid of significant structures. Heifers with an RTS of 1 are likely the furthest from puberty at the time of examination. Heifers assigned an RTS of 2 are thought to be closer to puberty than those scoring 1, due primarily to larger uterine horns and ovaries. Those heifers assigned an RTS of 3 are thought to be on the verge of estrous cyclicity based on uterine tone and palpable follicles. Heifers assigned a score of 4 are considered to be cycling, as indicated by uterine tone and size, coiling of the uterine horns, and presence of a preovulatory‐sized follicle. Heifers assigned an RTS of 4 do not have an easily distinguished corpus luteum. Heifers with an RTS of 5 are similar
