Pregnancy rate of cows given synthetic gonadotrophin‐releasing hormone at the time of service

A trial involving 25 herds was conducted in the Maffra area of Victoria to study pregnancy rates after injection of synthetic Gn-RH at service. Of 3502 cows having a first service, 674 cows were injected by the farmer at first service with a single intramuscular injection of 250 micrograms of synthetic Gn-RH, while the remaining 2828 cows acted as untreated controls. First service pregnancy rates were 58.8% in the group treated with Gn-RH and 54.1% in the untreated group, a 4.7% increase or an odds ratio of 1.21 (P less than 0.05). However, adjustment for the potentially confounding variables calving to first service interval and herd reduced the odds ratio to 1.07 (P less than 0.4). Treatment with Gn-RH at second or third service did not alter pregnancy rate to that service compared to untreated services. There was no interaction of treatment with Gn-RH at first service with calving to service interval. It is concluded that injection of 250 micrograms of synthetic Gn-RH at the time of service does not increase pregnancy rate to that service.