Pre-mating injection of an analogue of gonadotrophin-releasing hormone (GnRH) and pregnancy rates to first insemination.

Two hundred and ninety two dairy cows received a subcutaneous injection of a 2.5 ml solution containing 10 microg GnRH-analogue (Receptal, Hoechst A.G.) 0-6 hours before insemination, while 284 cows acting as controls were injected with 2.5 ml of sterile pyrogen-free water at the same time. The two groups, the treated and the control, were formed by matching (pairing) each oestrous cow with another on the basis of interval from calving to first mating, condition score, and age on the day of first mating after calving. The cows were in 3 factory supply dairy herds, which were visited daily during the first 3 weeks of the mating season. All the cows presented for mating each day were inseminated by the same technician using 0.5 ml of ambient-temperature semen containing 2 million spermatozoa per insemination. The semen used on any particular day originated from one ejaculate of one bull (same batch number). The cows were manually examined for pregnancy 2 to 3 months after their first insemination. A similar response was seen in all three herds irrespective of the interval from calving to mating. First mating pregnancy rates were 9.3% higher in the Receptal-treated cows than in the control group (P = 0.025). It is suggested that Receptal may have a beneficial effect on the pregnancy rate of cows that have recently ovulated as well as on those having delayed ovulation.