The pregnancy rate in 321 Friesian dairy replacement heifers was not different following two inseminations on a fixed time basis when oestrus was synchronised with either a 12-day progesterone treatment using silastic coils or a double injection regimen of a synthetic analogue of prostaglandin f2alpha (chloprostenol). There was a significantly higher (P less than 0-01) pregnancy rate following insemination when 143 Hereford-cross beef suckler cows were treated with the 12-day progesterone treatment (55 per cent pregnant) in comparison to the pregnancy rate following insemination of 131 cows receiving the double injection of cloprostenol 12 days apart (32 per cent pregnant). The ovarian activity at the start of treatment affected pregnancy rate following the cloprostenol regimen but not following the progesterone regimen. In suckler cows in these trials where ovarian activity was classified at the start of treatment, 30 per cent had inactive ovaries, indicating the magnitude of the problems of synchronising oestrus in beef suckler cows.
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