EFFECTS OF GONADOTROPIN-RELEASING-HORMONE ADMINISTERED 11 DAYS AFTER INSEMINATION ON THE PREGNANCY RATES OF CATTLE TO THE 1ST AND LATER SERVICES

In a trial using 2530 recorded services of 1619 animals on 19 commerci al dairy farms, alternate cows were injected intramuscularly with 10 m ug buserelin, an analogue of gonadotrophin releasing hormone (GnRH), ( Receptal; Hoechst Animal Health) 11 days after insemination. Pregnancy was diagnosed manually six to 12 weeks after the last service in all but three herds, for which calving data were collected. The pregnancy rates were calculated for control and treated cows, paired for calving to first service interval, parity and week of service. For 521) pairs of control and treated cows the pregnancy rates to first service were 50.6 per cent and 60 per cent, respectively (P<0.01). For the second and subsequent services each cow was grouped according to its treatmen t or control status in the immediately preceding dioestrus period, in addition to its status on day 11 after service. As a result there were three treated groups: treated, treated; treated, control, and control . treated lor comparison with the control, control group. For 136 pair s of control, control and control. treated cows the pregnancy rates to second service were 41.2 per cent and 54.4 per cent, respectively (P< 0.05), and for 67 similar pairs of cows the pregnancy rates to the thi rd or later service were 23.9 per cent and 52.2 per cent, respectively (P<0.001). For 41) pairs of control, control and treated, treated cow s the pregnancy rates to the third or later service were 15-0 per cent and 45.0 per cent, respectively (P<0.01). The pregnancy rates of cows that were treated, treated at the second service or treated, control at the second or later services, were not significantly different from the pregnancy rate of control, control cows. Inter-oestrus intervals of 11 to 17 days occurred in 7 per cent and 2 per cent of the control and treated cows, respectively (P<0.01), suggesting that the increased pregnancy rates after treatment were associated with a reduction in t he number of cows with inter-oestrus intervals of less than 18 days.