RELATIONSHIPS AMONG POST-CALVING ANESTROUS INTERVAL, ESTROUS CYCLES, CONCEPTION RATES AND CALVING DATE IN ANGUS AND HEREFORD X FRIESIAN COWS CALVING IN 6 SUCCESSIVE YEARS

The records of 1397 cows calving in the spring were collected in the y ears 1979 to 1984. They were from 638 Angus and 759 Hereford X Friesia n cows, and were used to examine the dependencies of post-partum anoes trous interval (ppai), length of oestrous cycle after calving, and con ception rate to each oestrus after calving. Two-year-old cows had long er ppai (P < 0.05) than older cows (76 v. 72 days), and Hereford X Fri esian cows had shorter (P < 0.05) ppai than Angus cows (68 v. 72 days) . Cows due to begin calving on 8 August (early calving) had longer (P < 0.01) ppai than cows due to begin calving on 19 September (71 v. 55 days).The linear regression between ppai and calving date (-0.60) did not differ between years, nor between early and late calving. Non-para metric regressions showed that the dependence between ppai and calving date was linear between plus or minus 20 days of the mean ppai in bot h early- and late-calving herds. Outside these limits variability of p pai increased and there was no relationship with calving date. The len gth of the first oestrous cycle after calving was 22 days in 2-year-ol d cows and 24 days in older cows. Second and third oestrous cycles aft er calving were 20 days for all breeds and age classes. There was a co rrelation of -0.41 between the ppai and the length of the first oestro us cycle after anoestrum. There was no significant deviation from norm ality of ppai or the succeeding post-partum oestrous cycle lengths. Th e conception rate to the first oestrus after calving was significantly lower (P < 0.01) than for later oestrous periods. The conception rate to first post-partum oestrus showed dependence on the ppai, logit reg ressions showing a 0.005 to 0.01 proportional improvement in conceptio n at first oestrus per day increase in ppai. The significance of these results to the construction of mathematical models of reproduction in the beef cow is discussed.