Time of Artificial Insemination and Fertility in Dairy Cattle

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Abstract
Data were from 44,707 cows and first-calf heifers in a commercial artificial insemination program. The percentage of heifers first seen in estrus in the morning was 73.2, not different from 72.6 for cows. Unfrozen semen was used for insemination. The 150-180 day % nonreturns for cows inseminated before noon the same day (67.1) was not different from those inseminated between noon and 1800 h (69.9) and after 1800 h (68.9) the same day. Insemination the following morning was too late for best results (62.7%). Cows 1st seen in estrus in the evening should be inseminated by noon the next day. The nonreturn rate for cows inseminated after 1400 h the next day resulted in 63.8% 150-180-day nonreturns. A single mid-morning insemination for all cows 1st seen in estrus the night before or the same morning should give near maximum conception.