Two injections each of 500 μg Cloprostenol (ICI-80,996), a synthetic analogue of PGF-2α, 11 or 12 days apart or pretreatment for 7 days with progesterone from an intravaginal silastic coil and one injection of 500 μg Cloprostenol were both effective in synchronizing oestrus in heifers or nursing beef cows. After two inseminations at 72 and 96 hr after the end of treatment, the calving rate for cows observed in oestrus after both treatments or for cows that had oestrous-like mucus after the progesterone+Cloprostenol treatment did not differ from that of control cows, but was significantly (P<0·025) lower for cows diagnosed as ready for insemination by the characteristics of mucus after the two injections of Cloprostenol. Treated cows diagnosed per rectum as having inactive ovaries had a significantly (P<0·005) lower calving rate than those diagnosed as having active ovaries at the start of treatment. Significantly (P<0·005) more treated than control cows were inseminated and became pregnant in the first 15 days of the treatment period, but the overall level of reproductive efficiency was low.
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