Calving date affects feed costs, calf weaning weights, marketing dates, labor availability, and, potentially, calf health. Employing recent estimates of calf weaning weights and surveys of producers, feedlot operators, and veterinarians, we model expected profits as a function of calving date. January-calving herds had the highest annualized net returns, a result driven by older weaning dates and heavier weaning weights. However, survey results show March and April were the most common calving months. This may be due to both weather and labor demand concerns. If weaning ages are restricted to 205 days, January calving was the least profitable because of higher feed costs.

Calving date (CD) records of 68 896 polled Hereford dams of 120 682 calves were coded within herd-year-season with date 1 being the date the first calf was born. Factors affecting CD and its relationship with production traits were investigated. Early calving was found to be associated with high performance, conceivably because dams calving early were able to utilize the season-dependent range pastures to their advantage thus providing adequate milk supply to their calves. Weight and age of dam, weight of calf, initial CD, index of milk production potential, and previous CD were found to have significant influence on current CD. Extremely small cows at previous weaning had a tendency to calve late. Cows calving in the spring produced calves with higher overall weight per day of age than did cows calving in other seasons. Initial CD was found to affect CD in subsequent parities. Repeatability of CD was estimated at 0·23. Heritability was estimated at 0·16 (s.e. 0·05) and indicated that additive genetic variance constitutes a moderate proportion of the variation in CD. Genetic correlation of CD with birth weight (−0·30, s.e. 0·10), post-weaning rate of gain (·0·64, s.e. 0·19) and yearling weight (−0·60, s.e. 0·11) indicated that ‘plus’ additive genes for CD influenced these traits favourably. However, the corresponding correlations with pre-weaning rate of gain (−0·03, s.e. 0·08) and weaning weight (−0·05, s.e. 0·03) were not significant.

The records of 1397 cows calving in the spring were collected in the years 1979 to 1984. They were from 638 Angus and 759 Hereford × Friesian cows, and were used to examine the dependencies of post-partum anoestrous interval (ppai), length of oestrous cycle after calving, and conception rate to each oestrus after calving.

Two-year-old cows had longer ppai (P < 0·05) than older cows (76 v. 72 days), and Hereford × Friesian 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-parametric regressions showed that the dependence between ppai and calving date was linear between plus or minus 20 days of the mean ppai in both early- and late-calving herds. Outside these limits variability of ppai increased and there was no relationship with calving date.

The length of the first oestrous cycle after calving was 22 days in 2-year-old cows and 24 days in older cows. Second and third oestrous cycles after calving were 20 days for all breeds and age classes. There was a correlation of −0·41 between the ppai and the length of the first oestrous cycle after anoestrum. There was no significant deviation from normality of ppai or the succeeding post-partum oestrous cycle lengths.

The 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 regressions showing a 0·005 to 0·01 proportional improvement in conception 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.