Bovine Reproduction. Группа авторов

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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].

      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].

      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].

      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.

      Unlike other domestic species (sheep, goats, swine), exposure to a bull has no effect on the incidence of precocious puberty [99].

      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


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