Human chorionic gonadotropin (hCG): the effect of dose on ovulation and pregnancy rate in Thoroughbred mares experiencing their first ovulation of the breeding season

AIM: To determine the effect of hCG dose on ovulation and pregnancy rate in Thoroughbred mares experiencing their first ovulation of the breeding season. METHODS: Over 3 successive breeding seasons, a total of 101 mares were randomly assigned to 1 of 4 treatment groups (intravenous injection of either saline, 1500, 3000, or 6000 IU hCG), as they approached their first ovulation of the breeding season. Mares were bred 1 day post-injection to 1 of 11 stallions, and every other day until ovulation occurred. Data were analysed using multivariable logistic regression with correction for over-dispersion due to clustering. RESULTS: Mares treated with hCG were more likely to ovulate within 72 h of treatment than mares treated with saline (p<0.001); there was no significant difference between doses of hCG on risk of ovulation (p>0.15). Farm also had a significant impact on the risk of ovulation (p=0.027). Mares treated with hCG were more likely to be diagnosed pregnant 14 days post ovulation than saline-treated mares (p=0.081, p=0.029 and p=0.026 for the 1500, 3000 and 6000 IU doses, respectively); there was no significant difference between doses of hCG on risk of pregnancy (p>0.45). CONCLUSIONS: A single injection of hCG (1500-6000 IU) is effective at inducing ovulation in late transitional mares and increases the likelihood of pregnancy at 14 days post ovulation. This paper supports the use of hCG as an integral part of optimal broodmare management.