The effect of HCG on duration of oestrus, ovulation time and fertility in mares.

Two experiments were conducted to determine the effects of HCG on duration of oestrus, dioestrus, the length of the oestrous cycle, the time of ovulation and fertility in non-lactating mares. In the first experiment, the injection of HCG was repeated for three successive cycles. Mares injected with 2000 i.u. HCG on Day 2 of oestrus during their first cycle had a shorter oestrus and ovulated sooner than untreated control mares, but in the third cycle, treated mares had a longer oestrus and ovulated longer after the onset of oestrus than controls. In the second experiment, one intramuscular injection of 3300 i.u. HCG was given 24 hr before the first insemination in the first cycle. Oestrus was shortened by 3-1 days, but there was no difference in pregnancy rate between treated and control mares (61-1 versus 63-9%). During the two cycles following injection, the conception rate in the treated mares was higher. The number of inseminations/cycle to effect fertility was significantly less over three cycles in the treated mares.