Nutritional and Hormonal Factors Influencing Follicle Development in Cattle

Abstract

Ovarian antral follicular development in cattle proceeds through stages of follicular recruitment, selection and dominance. This is a reoccurring process during the estrous cycle when either one or two dominant anovulatory follicles develop prior to development of the ovulatory follicle. These changes in follicle development can be monitored by ultrasonography (Savio et al, 1988: Sirois and Fortune, 1988; Knopf et al, 1990). Apparently, dominant follicles control development of other follicles through the production of factors that act locally and/or systemically. This regulatory process is currently under intensive investigation. The process of dominance causes a single ovulation which is characteristic of cattle. We believe that growth of anovulatory dominant follicles during the estrous cycle is halted due to the negative feedback effect of progesterone from the CL on LH secretion. Insufficient LH secretion will not support the continued growth and function of the follicle. Consequently, the dominant follicle undergoes atresia and a new follicle is recruited and becomes the dominant follicle of the next wave. The importance of LH secretion was tested (Savio et al, 1990a) by giving PGF2 " (Lutalyse; Upjohn Co., Kalamazoo, MI) on day 8 of the estrous cycle and maintaining a weak progestin environment by the insertion of a norgestomet implant into the ear (Synchromate-B; Ceva Laboratories, Overland Park, KS). The implant was either removed on day 23 post estrus (Treatment 1) or replaced with a new implant on day 18 which also was removed on day 23 (Treatment 2). It is clear that the first wave follicle was sustained until the implant was removed on day 23 (Figure 1 ). Furthermore, growth of this single follicle wave was greater in cows of treatment 1 that retained the original implant. Indeed basal LH secretion was greater on days 10 (5 pulses per 8 hours) and 19 (7 pulses per 8 hours) postestrus in treatment 1 compared to day 10 of the estrous cycle ( control animals with an intact CL; .75 pulses per 8 hours). Insertion of a new implant on day 18 (treatment 2) reduced the pulse frequency of LH on day 19 (3 pulses per 8 hours). This experimental model demonstrated that alterations in basal LH secretion modulate follicular development during the estrous cycle. This has profound implications relative to the management of ovarian follicles for the synchronization of estrus and subsequent fertility at this estrus. Furthermore, this endocrine scenario may account for the occurrence of ovarian follicular cysts in the early postpartum period. Indeed the early postpartum period is characterized by early growth of ovarian follicles in the absence of a CL (low progesterone; Savio et al., 1990b ). Perhaps a sustained basal secretion of LH and the inability to have a preovulatory surge of LH (hypothalamic-pituitary dysfunction) leads to the development of dominant and estrogenic follicular cysts. Our basic understanding of ovarian follicular dynamics will lead to rationale systems of follicular management to improve reproductive efficiency.