In a trial using 2530 recorded services of 1619 animals on 19 commercial dairy farms, alternate cows were injected intramuscularly with 10 micrograms 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 520 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 treatment 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 for comparison with the control, control group. For 136 pairs 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 third or later service were 23.9 per cent and 52.2 per cent, respectively (P < 0.001). For 40 pairs of control, control and treated, treated cows the pregnancy rates to the third or later service were 15.0 per cent and 45.0 per cent, respectively (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)
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