Abstract

PIP: Results of ovulation studies in the pig from 1935-1975 are reviewed. The pig studies set out to determine the age at which the ovaries might respond to gonadotropic hormones and when ovulation could be produced. It was found that a response may be obtained once the foll icle reaches the vesicular stage. The fertility of ovulations produced in the luteal and follicular phase of the estrous cycle was tested and low luteal phase egg fertility noted. Early postpartum estrus suggested the possibility of increasing the numbers of litters/year. However, estrus proved to be without ovulation. The effects of feed level on ovulation rate has shown increased ovulations with high energy feed. Possible causes of differences in ovulation rate due to feed intake and breed are: 1) differences in level of gonadotropic hormones; and 2) differences in ovarian sensitivity. In experiments measuring follicular development in gilts receiving 100 mg progesterone or progesterone plus 1150 I.U. pregnant mare serum gonadotropins, the breed differences in ovulation rate could be based on the number of sensitive follicles present. An experimental herd composed on non-inbred descendents (both purebred and crossbred) was used to determine if variability in ovulation rate could be attributed to breed and to feed levels and if other traits of the reproductive mechanism were covariates of ovulation rate. Developmental differences between the breeds showed insignificant differences in ovary weight at birth but differences appearing by Day 42 and continuing to maturity. Only insignificant differences in percent of follicular atresia was found between the different feed levels. Follicle size was studied. Maternal effects had negative associations with additive genetic effects upon mature follicle number and may be a h emeostatic mechanism to protect the female from litters of large size.