Physiology and Endocrinology of Puberty in Heifers
Most beef producers expect their heifers to be able to calve for the first time at two years of age. Frequently, the breeding season for heifers begins two to three weeks prior to the beginning of the breeding season for cows. Furthermore, the fertility associated with the first pubertal cycle is reportedly lower than subsequent cycles. Therefore, having heifers that reach puberty six to nine weeks before the start of the breeding season can have a positive impact on conception rates and ultimately profit for producers. Additionally, heifers that conceive earlier in the breeding season, calve earlier in the calving season, and have a better chance of conceiving the following breeding season. Early calving heifers produce more calves in their lifetime and remain in production longer than later calving heifers. Understanding the physiology and endocrine maturation leading to onset of puberty is critical to maximizing heifer development. Puberty, defined as the first ovulatory estrus, is preceded by progressive growth and development of the uterus, ovaries, and hypothalamic-pituitary-ovarian axis. Follicle waves endure longer, dominant follicles become larger, and oocyte competence improves as heifers approach puberty. The CL formed after the first ovulation and luteinization is short lived due to a premature release of PGF2α. This short exposure of progesterone followed by a rise in estradiol is important in establishing the appropriate timing of PGF2α release and can be mimicked using a progestin in estrous synchronization protocols. Estradiol shifts from having a strong negative feedback on the hypothalamus and pituitary to a reduced negative feedback and finally a positive feedback causing the gonadotropin surge and ovulation (gonadostat hypothesis). The switch in the hypothalamic response to estadiol may be due to a drop in estradiol receptors coupled with increased concentrations of kisspeptin and increased sensitivity to existing kisspeptin molecules by GnRH neurons. Producers can use reproductive tract scoring (RTS), a subjective measurement of the uterus and ovaries, to assess the sexual maturity of heifers prior to the breeding season (4 to 6 weeks ideally). This gives producers enough time to make management decisions based on the RTS assessment. In summary, this paper/presentation provides a review of the research into the physiological and endocrine maturation leading up to puberty in beef heifers.
Keywords: Beef, Heifer, Puberty