A total of 368 ovulations from 92 lactations in 69 cows were analysed to determine the effects of timing of ovulation and signs of oestrus on the fertility of the cows. During the study period of 10 to 13 weeks postpartum, cows ranged from having reached their first ovulation to having reached their fifth. Before ovulation, the cows exhibiting standing oestrus or mounting activity with other oestrous signs were considered to be in oestrus. The total number of anoestrous ovulations was 136 (37 per cent). From the second to fifth ovulations, the percentage of cows expressing ST increased from 10 to 74 per cent, while the percentage of cows with ovulations accompanied by mounting oestrus increased from 1 per cent at the first ovulation to 19 to 24 per cent at subsequent ovulations. Seventeen relapses to anoestrous ovulation occurred in 14 cows that had resumed an oestrous cycle, and sixteen of these relapses were observed at the second to fourth ovulations. Cows exhibiting first oestrus at the third ovulation had a similar number of days open as at the second ovulation, but a significantly fewer number of days open than at the first, fourth or fifth ovulations. These results can be valuable for studying and improving the reproductive performance of high-yielding dairy cows.
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