Evaluation of Reproductive Efficiency of Dairy Cattle with Emphasis on Timing of Breeding

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Abstract
Approximately 2600 inseminations in Holstein cows and heifers were performed to evaluate the effect of time differences between first observation of estrus and insemination on conception and to assess the effects of management and environment on fertility. Factors that significantly affected conception rate included: lactation number, service number, sire, site of semen deposition in the reproductive tract, lactation number by service number interaction, and maximum temperature the day after insemination. Days postpartum to first estrus were 36.8, to first service were 77.8, and to second service were 115.4. Mean time for onset of estrus was 0815 h (n = 2420) under our management system. About33% of heifers were in estrus 18 to 19 d following breeding compared to about 9% of cows. Forty-four percent of older cows returned to estrus 23 to 24 postbreeding compared with 15.8% heifers. There was no correlation between time of onset of estrus for early postpartum heats and breeding heats. Fifty-three percent of problem cows responded to prostaglandin F2.alpha. injection and had a normal conception rate. Cows that were inseminated at 80 h postprostaglandin had a conception rate of 20.7%.