Genotype

The breed composition of heifers alters age at puberty, because size at puberty is determined genetically. Numerous studies have demonstrated differences in age and weight at puberty among various breeds. In general, European (Bos taurus) dairy and beef breeds attain puberty earlier than do those of Zebu origin (Bos indicus). Breed of sire and dam alters age at puberty, and crossbreeding reduces age at puberty in heifers.

Although timing of puberty is altered by many factors, as discussed later, age at puberty is influenced by selection. Breeds selected for milk production as well as for size (dairy and beef breeds) reach puberty earlier than do breeds of similar mature size and retail product growth potential but not selected for milk yield. Inbreeding delays puberty in heifers by retarding rate of growth. Estimates of heritability for age at puberty range from 0.41 to 0.64, two- to threefold higher than the estimates of heritability of milk production. With such relatively high heritabilities, practices of selection for earlier puberty in genotypes with traditionally later sexual development could prove beneficial, particularly in some beef breeds in which puberty often is delayed. Heifers sired by bulls with large scrotal circumferences reach puberty at younger ages and rebreed sooner after their first parturition than do heifers sired by bulls with small scrotal size. Studies also have demonstrated that heifers that are heavier at 6 months of age (before puberty) reach puberty at younger ages and also are heavier at calving. Although beef heifers that are heavier at weaning reach puberty at younger ages, these same heifers experience longer intervals to estrus after their first parturition than do their lighter-weight contemporaries.

Season

Although cattle are not considered to be seasonal breeders, evidence indicates that season of birth may alter timing of puberty. When heifers were fed similarly in controlled environmental chambers, those born in the autumn reached puberty earlier than those born in the spring.² Heifers born late within a spring calving season, however, were younger and lighter at puberty, probably because of improved forage quality and its effects on milk availability from their dams and digestibility of forages obtained by their grazing.

Body Weight and Nutrition

Although age and weight are correlated somewhat with the onset of sexual maturity, body weight is the major factor affecting age at puberty in cattle. Generally, the onset of puberty occurs when the heifer reaches approximately 40% to 50% of her mature weight. Unintended pregnancies may be initiated in heifers younger than 6 months of age when they are maintained in the presence of breeding bulls. Initiation of puberty normally occurs in yearling heifers, but age at puberty may range from 4 months to more than 2 years. Puberty is delayed in heifers that are maintained on low planes of nutrition during the prepubertal growth phase. Onset of puberty is dictated by body weight for the breed. Although age at puberty is influenced by nutritional factors, body weight at puberty is unaffected by nutrition. Correlations between gain in body weight and age at puberty indicate that increased growth rate of heifers results in younger age at puberty. In short, heifers on various planes of nutrition will reach puberty at different ages, but at similar relative stages of physical development. Furthermore, subsequent adequate weight gains are required once heifers reach puberty to ensure that they continue to have normal estrous cycles.

Underfeeding or overfeeding heifers has significant consequences on their development. Underfeeding of growing heifers may result in delayed puberty, subnormal conception rates, underdeveloped mammary glands, and greater incidence of calving problems.³ Overfeeding often results in weak expression of estrus, subnormal conception rates, high embryonic mortality, decreased mammary gland development, and reduced milk production.³ Dairy heifers fed 115% of all 1989 National Research Council nutrient requirements for heifers to gain 0.7 kg/day from 3 to 24 months of age were younger at puberty, were heavier at calving, had larger heart girth, were longer, were not excessively fat at calving, and calved at younger ages. Nutrient requirements suggested by the National Research Council should serve as minimum guidelines.

Social Rearing Environment

In some feral mammalian species, the physical presence of the male decreases age at puberty in contemporary females. Presence of mature bulls and adult cycling cows has altered age at puberty of heifers in some but not all studies. Increased proportions of prepubertal beef heifers treated oronasally with bull urine reached puberty earlier than water-treated controls.³

Pregonadotropin Surge

The period of the pregonadotropin surge begins with the demise of the corpus luteum and ends with the preovulatory surge of LH and FSH, which initiate ovulation of a mature follicle 24 to 30 hours later (Fig. 35-2). Concentrations of progesterone rapidly decline as those of estradiol begin to increase concurrently with the accelerated growth of the preovulatory follicle. As progesterone declines, the baseline concentration of LH increases,⁵ the frequency of LH pulses increases from about one pulse every 4 to 6 hours to one pulse every hour, but the amplitude of LH pulses declines⁵ (all because of reduced negative feedback by progesterone on the hypothalamus and pituitary). This change in LH secretion, which coincides with a parallel pulsing of estradiol secreted into the venous effluent of the ovary bearing the preovulatory follicle, can be monitored in jugular blood by frequently collected blood samples (see Fig. 35-2). Although follicular growth may be stimulated by FSH alone, a combination of LH and FSH has been shown to induce maximal synthesis of estradiol in vitro.⁶ Thus, both gonadotropins influence follicular development and subsequent secretion of estradiol. Estradiol appears to exert inhibitory effects on FSH release, because when it is administered to ovariectomized heifers, FSH is reduced to precastration concentrations. Furthermore, when titers of estradiol are at their peak just before the preovulatory surge, concentrations of FSH are lowest.⁵

Fig. 35-2 Concentrations of progesterone (P₄), estradiol (E₂), and luteinizing hormone (LH) during the pre- and postgonadotropin surge period of the bovine estrous cycle. During diestrus (because of high concentrations of progesterone secreted by the corpus luteum), the frequency of LH pulses in jugular blood is low (approximately one per every 4 to 6 hours) and pulse amplitude is high, similar to that of estradiol in the ovarian venous effluent. As the corpus luteum dies and progesterone declines to baseline concentrations, the pulse frequency of LH increases to approximately one per hour (decreased amplitude) followed by correlated secretion of estradiol. The LH surge results from increased LH pulse frequency sufficient to sustain a peak (surge) of 8 to 12 hours induced by maximal titers of estradiol. These two events are coincident with the onset of estrus, which is followed by ovulation in approximately 27 hours.

A carefully orchestrated cascade of increasing LH pulses and elevated titers of estradiol eventually culminates in the onset of estrus, with estradiol triggering the initiation of both estrous behavior and the preovulatory surge of LH (see Fig. 35-2). Estradiol apparently triggers more frequent pulsatile GnRH secretion by the hypothalamus and enhanced pituitary responsiveness to each priming GnRH pulse. Blocking the rise in estradiol by chemical or immunologic means can block the occurrence of the gonadotropin surge.⁵ Likewise, concentrations of progesterone must be low before estradiol can induce the gonadotropin surge, because administering progesterone during the period before the surge can block it. The elevation of LH and FSH (surge) usually endures for 8 to 10 hours. Termination of the surge is due to refractoriness of the pituitary to GnRH (decreased concentration [down-regulation] of pituitary GnRH receptors) and depletion of the releasable pool of pituitary gonadotropin. This hypothesis is substantiated by two observations: (1) When heifers are ovariectomized shortly after the onset of estrus and the preovulatory surge, a delay in the classic postcastration rise in LH is observed, and (2) injections of GnRH within 12 to 16 hours of the onset of the surge result in the release of only marginal amounts of gonadotropin.

Postgonadotropin Surge

The postgonadotropin surge period is characterized by declining titers of estradiol in blood, after their peak at the onset of estrus, as follicular luteinization ensues (Fig. 35-3). The process of ovulation has been likened to mechanisms associated with an inflammatory reaction.⁷ The process of follicular rupture is regulated by a host of intrafollicular mediators. For example, ovulation can be blocked by intrafollicular administration of various prostaglandin enzyme inhibitors or antiserum to various prostaglandins or their regulatory enzymes.⁵ Inter–theca cell capillaries and the basement membrane of the follicle are disturbed at ovulation, accounting for some hemorrhage, and thus allowing both capillaries and theca cells to pervade the ruptured follicle. Follicular content of estradiol declines rapidly as progesterone content begins to increase, because both theca and granulosa cells differentiate into luteal cells (luteinize) that form the corpus luteum. Theca cells appear to develop into smaller-diameter cells, known as small luteal cells, and the granulosa cells become large luteal cells as the corpus luteum develops.⁸

Fig. 35-3 Diameters of two dominant follicles and their respective subordinate follicles from each of two cohorts of follicular waves during the bovine estrous cycle. Peak concentrations of follicle-stimulating hormone (FSH) are detected 1 to 2 days before the recruitment of each follicular wave. The first peak is nearly coincident with ovulation during metestrus. During diestrus, when elevated concentrations of progesterone (P₄) are secreted by the corpus luteum, pulses of luteinizing hormone (LH) are sufficient to allow selection of dominant follicles but insufficient to allow maturation of a dominant follicle until progesterone returns to basal concentrations after spontaneous luteolysis (increased uterine-secreted prostaglandin F_2α [PGF_2α]) or in response to exogenous injections of PGF_2α. After luteolysis, during proestrus, LH pulse frequency and estradiol (E₂) increase sufficiently to induce behavioral estrus and the LH surge.

(Adapted from Thatcher WW: Fundamentals of dairy reproduction. The 100-day contract: Fundamentals for achieving reproductive efficiency CD-ROM. Pharmacia Animal Health, Kalamazoo, MI.)

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