Evaluation of single and double artificial insemination regimes as methods of shortening calving intervals in dairy cows treated with dinoprost

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Abstract
Conception rates in 535 commercial lactating Friesian dairy cows on 2 farms were compared between treated animals after 2 injections of dinoprost at an 11-day interval and untreated contemporary controls bred by conventional artifical insemination. The conception rate for a single insemination 75-80 h after the 2nd dinoprost injection was 46%, for 2 inseminations 72-96 h after the 2nd injection was 47% and for untreated controls was 50%. 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-1st-service interval of treated cows. This 10-day advantage in calving-to-1st-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 estrus. The accuracy of pregnancy diagnosis was confirmed by calving data; pregnancy diagnosis by rectal palpation was 94% and by milk progesterone assay 81% accurate, overall. Estrus occurred in 3.8% of pregnant cows, on the basis of stockmen''s observations. The relevance of the information to herd fertility control is discussed.