of LH pulses also increases during this time, but increased pulse frequency appears to be critical for initial ovulation [17]. Follicle‐stimulating hormone (FSH) concentrations in blood do not fluctuate as much as LH in the peripubertal period, suggesting that FSH may play more of a permissive role in the initiation of puberty [18]. Other hormones undoubtedly play a role in initiation of puberty, but LH appears to be of paramount significance. Prolactin may play a role in heifer puberty, but blood concentrations do not change at puberty as they do in bulls. There is abundant evidence to suggest that heifers are actually capable of ovulating from early on in life but fail to do so because of insufficient gonadotropic stimulation. In fact, McLeod et al. [25] were able to induce preovulatory gonadotropin surges in GnRH‐treated five‐month‐old heifers. Even more impressive was the research of Seidel et al. [26] who induced ovulations in one‐month‐old heifers with gonadotropin administration. Although ovulations can be artificially induced with gonadotropins, it is notable that the hypothalamus–pituitary becomes increasingly sensitive to GnRH stimulation as the time of puberty approaches. It has been shown that the positive feedback effect of estradiol‐17β on surge LH release becomes functional between three and five months of age [27]. Collectively, available data indicate heifers are capable of ovulating long before puberty but fail to do so spontaneously until the inhibitory effect of estradion‐17β on GnRH release wanes.
Development of the Female Reproductive Tract
Overall, body growth and development of the reproductive tract occur in an asynchronous pattern. For example, the ovaries grow at a rate 2.7 times faster than the body until puberty, whereas the tubular reproductive tract grows at about the same rate as body growth until about six months of age, and then enters a period of accelerated development until puberty [28]. No ovarian follicles are macroscopically visible at birth, but their numbers increase to maximal at four months, decrease through eight months of age, and remain relatively constant thereafter [28]. Growing and antral follicles increase in number during the first three to four months of age, which corresponds to the transient increase in circulating LH concentrations observed during that time period [29].
Height of the luminal epithelium of the tubular reproductive tract is stimulated at birth but regresses by one or two months of age. Thereafter, increases in height of epithelia are most rapid after six months. From this information it is concluded, at least for Holstein heifers, that rapid peripubertal growth of the reproductive tract commences during the seventh month and is largely terminated by 10 months of age [28].
Factors that Influence Age of Puberty
It is generally accepted that puberty in cattle occurs at around 9 or 10 months of age. However, there are reports of puberty occurring any time between 6 and 24 months [1], with anecdotal reports of heifers calving at 13 months indicating that puberty can occur as early as 4 months of age. Recently, the author assisted an induced delivery of a near‐term calf in a 14‐month‐old Shorthorn heifer, indicating that puberty in this heifer must have been around 5 months of age. Age at puberty is influenced by body weight and composition, breed, nutrition, genetics, and season of the year. Any adverse factor that decreases prepubertal growth, such as protein‐energy malnutrition, scours, pneumonia, parasitism, or harsh weather conditions, results in delay of the onset of puberty.
Weight and Body Composition
An early study by Sorensen [30] showed that attainment of puberty in heifers was more influenced by weight than age. He found that heifers on a higher plane of nutrition reached puberty at an earlier age than similar heifers with lower average daily gain [30]. However, heifers may have similar body weights but vary in frame size, indicating they have differing body composition. For any given frame size, heifers that are heavier reach puberty at an earlier age. Body weight at puberty has a heritability coefficient of 0.40 [31]. Overall, heifers reared on higher planes of nutrition are heavier but younger than nutritionally restricted animals at puberty [32]. However, puberty does not occur at similar body composition or metabolic status in all heifers [33] but is positively correlated with body fat percentage and negatively correlated with carcass moisture percentage [33]. The precise mechanisms involved in the relationship of body composition and puberty are not clearly defined. However, it is known that somatotropin and the insulin‐like growth factor (IGF)‐I system are involved [34]. This fact was demonstrated by Armstrong et al. [35] who immunized heifers at three or six months of age against growth hormone releasing factor. By 14 months of age 75% of control heifers but only 22% of immunized heifers had reached puberty and had significantly fewer follicles >7 mm by six months of age.
Breed
Age and weight at puberty are affected by several factors, including breed. Generally, breeds of a larger size at maturity are older and heavier upon reaching puberty [36]. A classic example of the effect of breed on puberty is illustrated in the study by Laster et al. [37]. Their group found that female progeny of a Charolais bull were 50 days older and 120 kg heavier at puberty than progeny of a Jersey bull when all dams were Angus cows. Although the Charolais × Angus heifers grew faster than the Jersey × Angus heifers, they did not reach puberty at as young an age as the Jersey × Angus heifers due to breed effect, which, in this case, overrode the influence of rate of gain. Generally, European breeds reach puberty younger but at slightly heavier weights than Hereford or Angus heifers [37]. In a study on the effects of heterosis on age at puberty, Wiltbank et al. [38] found half to three‐fourths of the heterosis effect on age at puberty was independent of heterosis effects on average daily gain. Individual sire within a specific breed also has a significant effect on age of puberty of female offspring. The heritability coefficient for age at puberty is 0.41 [31].
Plane of Nutrition
Body energy reserves and metabolic state are relevant modifiers of puberty onset and fertility. Heifers in a peripubertal state may be induced to ovulate by abruptly increasing the plane of nutrition [39, 40]. A study by Chelikani et al. [41] involving Holstein heifers illustrates this point. Heifers were fed to gain 1.1, 0.8, or 0.5 kg/day from 100 kg liveweight. Age at puberty for the three groups was 9, 11, and 16 months, respectively. Precocious puberty (<300 days of age) can be induced in beef heifers by early weaning and feeding a high‐concentrate diet [42]. The mechanism by which early weaning and feeding a high‐concentrate diet results in precocious puberty in heifers is associated with advancement of the reduction of estradiol negative feedback on secretion of LH. The mechanisms linking nutrition and metabolism to puberty are not clearly understood, but it appears several neuropeptides operating in a reciprocal manner (orexigenic and anorexigenic) are involved.
Kisspeptin is a primary candidate for modulating the effects of nutrition on reproduction. Kisspeptin acts in the role as a final common pathway to integrate stimulatory and inhibitory inputs to influence GnRH synthesis and release. Much of the work with this hormone has been in a rodent model, but one may speculate that the same mechanisms are operational in cattle. Kisspeptin signaling in the hypothalamus has been suggested as the primary positive regulator of the GnRH pulse generator [43]. In the mid‐1990s, the adipose hormone leptin was shown to be an essential signal for transmitting metabolic information to the centers governing puberty and reproduction and kisspeptins have emerged as conduits for the metabolic regulation of reproduction and putative effectors of leptin actions on GnRH neurons [44]. Leptin is produced by adipocytes and regulated by long‐term and recent nutritional status [45]. Circulating leptin concentrations increase as puberty approaches but do not change appreciably in response to dietary change when percentage of total carcass body fat is above a minimum threshold, indicating that a certain minimum body condition is required for puberty [46]. Ghrelin and other metabolic effector hormones known to modulate the hypothalamic–pituitary–gonadal axis, such as neuropeptide Y, melanocortins, and melanocyte‐concentrating hormone, are additional putative regulators of the hypothalamic/kisspeptin system [44].
Genetic Markers for Age at Puberty
Two of the major factors that influence reproductive efficiency and hence profitability due to greater