Observations on the length of the post‐partum oestrous cycles and their relationship to other reproductive parameters in mature angus cows calving in the spring of two consecutive years

Abstract The lengths of the first and second oestrous cycles following calving were observed in 87 mature Angus cows in 1976, and in 121 Angus cows in 1977. Other reproductive data collected were the length of the post‐partum anoestrous period and the conception rate at each expression of oestrus. The length of the first oestrous cycle after calving averaged 21 ± 1.3 days in 1976 and 26 ± 1.3 days in 1977. This variable was observed to be independent of pre‐calving liveweight change, age of the cow, calving date, probability of conception at the succeeding oestrus, and other oestrous cycle lengths on the same cow (within‐year and between‐year).The length of the first oestrous cycle after calving was observed to be influenced by the liveweight change of the cow over the first 20 days after calving (1976: r = −0.69, P < 0.01; 1977: r = −0.27, P < 0.05), liveweight gain being associated with shorter oestrous cycle lengths. A relationship between the length of the post‐partum anoestrous period of the cow and the length of her first oestrous cycle after calving was not consistent between years. A correlation of −0.68, ( P < 0.01) was measured in 1977 but there was no such correlation in 1976. There was no apparentreason for such adifference in result In both years the conception rate to the first post‐partum oestrus was low (45%). Conception at the second and third oestrus rose to between 67 and 78% in both years.