Conception rates in 535 commercial lactating Friesian dairy cows on two farms were compared between treated animals after two injections of dinoprost at an 11-day interval, and untreated contemporary controls bred by conventional artificial insemination. The conception rate for a single insemination 75 to 80 hours after the second dinoprost injection was 46 per cent, for two inseminations 72 and 96 hours after the second injection was 47 per cent, and for untreated controls was 50 per cent. The differences are not statistically significant. Both dinoprost treated groups had a mean calving interval of 366 days compared with 378 for controls. The time advantage of 12 days in calving interval was principally due to the shorter calving-to-first-service interval of treated cows. This 10-day advantage in calving-to-first-service was increased to a 12-day advantage in calving-to-conception in treated cows, and applied also to cows which failed to conceive to the induced oestrus. The accuracy of pregnancy diagnosis was confirmed by calving data; pregnancy diagnosis by rectal palpation was 94 per cent and by milk progesterone assay 81 per cent accurate, overall. Oestrus occurred in 3.8 per cent of pregnant cows, on the basis of stockmen's observations. The relevance of the information to herd sterility control is discussed.
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