Replacement heifer development is a critically important area for veterinarians to offer production medicine advice to their beef-producing clients. In order for replacement heifers to calve at approximately 24 months of age and to reach puberty the equivalent of three heat cycles before the start of the mature cow breeding season, heifers must become puberal by 11 to 13 months of age. Once puberty is attained, nutrition must be at a level that allows the heifer to continue cycling, ovulate a viable oocyte, and establish pregnancy. Nutritional demands of heifers during pregnancy exceed that of mature cows because the heifer is partitioning nutrients for her own growth as well as fetal growth and development. This increased demand for nutrients continues through early lactation, when the beef female has her highest nutritional requirements. Deficiency of energy or protein for extended periods of time during any production phase during the first two and one-half years of life will have a negative impact on: fetal development, calf viability, milk production, and/or rebreeding for the next pregnancy.

Birth to Weaning

During the early preweaning phase (first 90 days of life), the heifer calf's requirements are met primarily by her dam's milk production, but starting early in life, forage plays a role in supplying nutrients for the calf. By the time a calf is 60 days of age, she is consuming 1.5% of her bodyweight as forage drymatter. As the preweaning phase progresses, forage becomes an increasingly important nutrient source. As long as nutrient intake (milk and forage) is adequate for growth, no additional energy is needed in most heifer development systems. However, some investigators speculate that nutritional plane during the first two to three months of life influences the timing of puberty and the effectiveness of later dietary manipulations to affect age at puberty. Increased plane of nutrition early in life can be due to superior forage quantity and quality or high dam milk production. The use of creep feeding in replacement heifers should be avoided if the additional energy is used for fat deposition, most importantly into the udder parenchyma. Fat deposition in the udder of immature heifers has been shown to decrease lifetime milk production and offspring's weaning weights.

Puberty in the beef heifer is reached when she is able to express estrous behavior, ovulate a fertile oocyte, and obtain normal luteal function. The maturing of the neuroendocrine system that induces maturation and ovulation of the first oocyte as well as the hormonal changes that induce the first expression of behavioral estrus are the result of a gradual increase in gonadotropic (luteinizing hormone; LH, and follicle stimulating hormone; FSH) activity. This increased gonadotropic activity near the time of puberty is due to a decreased negative feedback of estradiol on the hypothalamic secretion of gonadotropin-releasing hormone (GnRH). As puberty approaches, the gradually increased frequency of LH pulses results in increased secretion of LH which enhances development of ovarian follicles that produce enough estradiol to induce behavioral estrus and a preovulatory surge of gonadotropins.¹¹ Wave-like patterns of follicular development can be detected as early as 2 weeks of age in heifer calves, and the duration of follicular waves increases and the maximum diameter of dominant follicles increases with age through puberty.

The onset of puberty is primarily influenced by age and weight within breed. Other factors can also have some influence on the onset of puberty and include: exposure to bulls, time of year, and exposure to progestogens. Age of puberty in other species such as humans and rats is influenced by percent body fat or by body fat distribution, however, in cattle, fatness is not the sole regulator of puberty, as puberty does not occur at a constant percentage of body fat, and age and breed appear to be important contributing factors.