Bovine Reproduction. Группа авторов
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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, selection for low RFI results in selection 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