If heifers are fed and managed well, they are large enough to breed at fifteen months and calve at twenty-four months of age. Dairy farmers often focus on nutrition as the primary factor in heifer management, but they should also understand reproductive function and manage carefully the breeding and calving of their heifers. At the onset of puberty, protein hormones affecting the ovaries are secreted by the pituitary gland at an accelerated rate. Simultaneously, the ovaries become capable of responding to these hormones, called gonadotropins, and produce their own hormones, estrogen and progesterone. These steroid hormones are responsible for normal follicular development and regulation of the estrous cycle.
Puberty is more closely related to body weight than to age.
Dairy heifers reach puberty when body weight is 30 percent or 40 percent that of average adult weight. If growth is slowed down by underfeeding, disease, or para- sites, puberty is delayed. Heifers should be bred at fifteen months of age without delay. If they have had proper nutrition they are large enough to preclude any increase in calving problems. Occasionally, the first few estrous cycles occur without behavioral signs of heat.