An interaction between season of calving and nutrition on the resumption of ovarian cycles in post-partum beef cattle

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Abstract
Angus cows were first mated at .apprx. 27 mo. of age in 2 herds, calving July 21-Sept. 15 (Group E) or Sept. 9-Oct. 30 (Group L). The cows were fed a high (H) or medium (M) plane of nutrition for 55 days before and 40 days after calving. There was a mean live difference of 35 kg between cows in Groups EH + LH and Groups EM + LM immediately after calving and at 40 days after calving. Immediately after calving cows in Groups EH + EM were 11 kg heavier than cows in Groups LH + LM, but there was no difference at 40 days after calving. There was a significant interaction between calving time and nutrition in the return of cyclic ovarian function assessed from both interval to 1st estrus and 1st elevated progesterone concentration. Mean intervals from calving to 1st estrus were 66.7, 82.7, 56.7 and 62.3 days in Groups EH, EM, LH and LM, respectively. The season of calving evidently influences resumption of ovarian cycles even at a constant high plane of nutrition and interacts with nutrition such that season effects are more likely to be expressed under conditions of low nutrition.