RESUMPTION OF OVARIAN ACTIVITY AND ESTRUS POSTPARTUM IN DAIRY COWS MAINTAINED INDOORS YEAR-ROUND

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Abstract
Sequential milk progesterone concentrations from semi-weekly samples were plotted for 310 calvings by cows housed indoors year-round. Resumption of ovarian activity (first estrus), number of complete estrous cycles within 91 days postpartum, lengths of estrous cycles and peak progesterone levels were assessed considering diet, parity, genetic line of cow and season of calving. Patterns of ovarian activity in postpartum cows maintained indoors year-round were similar to those previously reported for cows housed in less intensive systems. On the average, the first estrus occurred 24 days postpartum. The mean number of estrous cycles was 2.6. Estrous cycle lengths averaged 20 ± 6, 22 ± 5 and 22 ± 5 days (mean ± SD) in the first three estrous cycles. Peak progesterone levels were 19 ± 10, 20 ± 10 and 22 ± 12 ng/mL (mean ± SD). Most of the dairy cows housed in the intensive system had normal estrous cycles, had ovulated within 55 days postpartum and had two periods of estrus within 91 days postpartum and should have been capable of conception at such a time to have a calving interval of 1 yr. Seasonal influences on ovarian activity were more important than diet, genetic line or parity. Of 48 interaction effects studied, only interactions of season with genetic group, parity or diet were significant, as was genetic group with parity. Key words: Ovarian activity, estrous cycle, peak progesterone

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