SEASONALLY ANESTRUS GOATS MATED BY SEXUALLY ACTIVE BUCKS CAN BECOME REFRACTORY TO THE PRESENCE OF BUCKS

Background. Anestrous goats can cycle and become pregnant after the introduction of sexually active bucks, but some of them become anestrus probably because they have been fertilized and early embryonic death occurred or because they have not been fertilized and become refractory to the stimulation of bucks. Objective. Determine whether goats that were mated by bucks become anestrus probably because of an early embryonic death or refractoriness to buck stimulation. Methodology. Vasectomized (n = 2) and intact (n = 2) bucks were rendered sexually active by exposure to artificially long days (16 h of light per day) followed by natural variations of photoperiod. Bucks were introduced into two groups of anestrous goats (n = 15 each) and remained with them for 53 days. Estrous behavior, ovulation and pregnancy rate were determined. Results. All goats came into estrus, ovulated and were mated by bucks at least once during the study. The proportions of females that displayed estrus with or without ovulation did not differ between the two groups within 9 days after the introduction of bucks. However, the proportion of goats that came into estrus with or without ovulation was greater in goats mated by vasectomized bucks from 20 to 28 and from 50 to 53 days after introducing the bucks (P < 0.05). Finally, the proportion of goats that ovulated after the introduction of the bucks, but later returned to anestrus did not differ in goats mated by vasectomized or intact bucks (14% vs 33%; P > 0.05). Implications. These results indicate that in nonpregnant goats, refractoriness to the continuous presence of the same bucks may explain at least in part, the return to anestrus of most goats that stop cycling after an initial sexual response to the introduction of bucks. Conclusion. Nonpregnant goats mated by sexually active bucks become in anestrus, probably because they become refractory to the presence of the same males.