124 Factors influencing follicle development in gilts and sows and management strategies used to regulate growth for control of estrus and ovulation.

Abstract

Factors that affect follicle health and growth can influence estrus, ovulation, conception and litter size. Breeding herd schedules are determined by synchronized follicle growth following litter weaning in sows. Insemination over only 3 days facilitates farrowing over fewer days and helps improve the uniformity of pigs at weaning. Breeding synchrony is frequently reduced from disturbances in the follicle phase that result in delayed estrus. The failure of >15 follicles to uniformly progress beyond the 6.0 mm size within 6 days during the follicular phase is associated with failures in breeding, gestation and farrowing. In sows, the follicle phase is initiated at weaning by removal of the suckling inhibition, while in cycling gilts, luteolysis and clearance of progesterone begins the process. The timing and patterns of FSH and LH stimulation to the ovary determine follicle health and selection for ovulation. Interestingly, abnormal wean to estrus intervals in sows and deviations from a 19–22 day estrous cycle in gilts, are associated with reduced fertility. However, in both cases, it is not entirely clear whether the abnormal intervals are a direct result of problems occurring prior to or only during the follicular phase. Although the mechanisms are not entirely, factors such as boar exposure, transportation, mixing, and increased dietary energy, have been shown to induce follicle development in pigs. In contrast, inhibitors to follicle growth have been associated with heat stress, photoperiod, negative energy balance, poor body condition, parity, nursing problems, and short lactation length. Hormonal aids for reproductive management have been approved to control the follicle phase in pigs. These hormones include orally administered progestagen to delay and exogenous gonadotropins to induce the follicle phase and estrus. GnRH agonists are also being used in protocols designed to advance and synchronize ovulation for application of a single timed insemination for weaned sows.