Progestational and Estrogenic Compounds Estrous Cycle Synchronization and Fertility of Gilts Fed

Eighty-five Yorkshire and Berkshire × Yorkshire gilts were used in three experiments to determine the effect of the addition of a progesterone or estrogen derivative to the feed on the estrous cycle and on subsequent fertility. In experiment 1 the addition to the ration of 0.66 or 1.1 mg. of AMP per kg. of body weight for a 15-day period was effective in inhibiting estrus in all gilts during the feeding period. Twenty-one of 22 treated gilts showed signs of estrus within 1 week after hormone withdrawal. Some of the gilts in each group slaughtered 2 or 4 days following artificial insemination had a high incidence of cystic ovarian follicles, especially gilts on the 0.66 mg. level. The estrous cycle was synchronized to some degree at the second cycle following hormone withdrawal (21- to 33-day cycles). Conception at this estrus using natural service was 83.3%. Average litter size for the six gilts that farrowed in each of the control, 0.66-mg. and 1.1-mg. groups was 8.7, 10.2 and 7.8 live pigs, respectively. In experiments 2 and 3 the feeding of 16 mg. of MEE per gilt daily for 9 days, followed by feeding 1.32 mg. of MAP per kg. of body weight daily for 9 days, inhibited estrus in 29 of 31 gilts. Treated gilts showed typical signs of estrus, including swollen vulvas and behavioral changes, 3 to 8 days after hormone withdrawal in experiment 2 and 4 to 10 days in experiment 3. However, 2 of 11 treated gilts in experiment 2 and 5 of 20 in experiment 3 refused to accept the boar. First service conception of mated gilts was 81.8% for controls and 77.8% for treated gilts in experiment 2 and 100% for controls and 93.3% for treated gilts in experiment 3. Corresponding average litter sizes (live+stillborn pigs) were 8.1 and 8.9 in experiment 2 and 10.4 and 11.7 in experiment 3.