Efficacy of human chorionic gonadotrophin and gonadotrophin releasing hormone for hastening ovulation in thoroughbred mares.

Plasma progesterone levels were measured daily to determine the accuracy of diagnosing ovulation by rectal palpation carried out every other day; 81.5 per cent mares injected with human chorionic gonadotrophin showed increases of progesterone more than 1 ng/ml by 72 h after injection compared with 65 per cent of mares injected with gonadotrophin releasing hormone (GnRH) or saline. Mating at ovulation achieved a 74 per cent pregnancy rate in mares given hCG compared with 50 per cent given GnRH and 45 per cent controls. Diagnosis of ovulation per rectum on the basis of a pit in the ovarian surface or the presence of a soft friable structure was found to be accurate in 91 per cent of cases. Diagnosis based on the presence of a firm plum-like structure or the disappearance of a previously identified follicle at a given site was 60 per cent accurate. In cases of rectal diagnosis of non-ovulation 38 per cent had actually ovulated based on a rise in progesterone more than 1 ng/ml and a further 19 per cent showed a rise in progesterone by 24 h after the diagnosis. The use of hCG may improve conception rates by synchronisation of ovulation and mating and where rectal palpation of the ovaries remains the only basis for timed mating.