Beef heifers normally have their first estrous cycle between 12 and 15 months of age. The major factors delaying onset of puberty are poor nutrition or a genetic predisposition for late maturity. In order for heifers to calve at 24 months they must conceive by 15 months of age.
As heifers mature, increasing amounts of the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are released from the brain. The increase in LH and FSH stimulates ovarian follicular development. Ovarian follicles produce estrogen and contain the developing ovum (egg). In prepuberal heifers these follicles grow and regress without ovulating. Puberty occurs when the heifer responds to the estrogen from the growing follicle by expressing estrus (heat) and producing an LH surge. The LH surge causes ovulation, which begins the heifer’s first cycle.
Heifers must exhibit two to three cycles for fertility to be maximized. Conception rates are 20 percent greater in heifers that are bred on the third rather than puberal (first) estrus (Byerley et al., 1987). The increase in fertility is a result of enhanced uterine acceptance of the embryo or maternal recognition of pregnancy. Exposure to low levels of progesterone for seven to ten days can induce puberty in heifers. The mechanism appears to be related to a hastening of the normal changes in hormone receptors and hormone release by the brain (Kinder et al., 1996). Progestins can reduce age at puberty by about 1 month. Therefore, synchronization systems that “jump start” heifers and allow more heifers to have several cycles before breeding improve pregnancy rates as well as the percentage of heifers bred early in the breeding season.
Source: John B. Hall, Department of Animal and Poultry Sciences, Virginia Tech, Amanda Liles, Department of Animal and Poultry Sciences, Virginia Tech, W. Dee Whittier, Department of Large Animal Medicine, Virginia Tech