Factors affecting the relationship between calving interval of cows and weaning weights of calves.

Characters related to production and reproduction were observed on 744 straightbred and F2 calves and their dams produced in a five-breed diallel with Angus, Brahman, Hereford, Holstein and Jersey breeds. These data were analyzed to estimate the effects of these characters on the relationship between weaning weight and calving interval. The model used for analysis of postpartum conception and calving interval included breedtype, season of conception, parity and management of the dam as main effects; condition of dam nested within parity; and age, weight and weight change of dam and weaning weight of previous calf as covariates. All effects and covariates were significant sources of variation in conception and calving interval. Brahman cows exhibited the longest conception and calving intervals, whereas Hereford X Jersey F1 cows had the shortest intervals. Heterotic effects for these intervals were low and nonsignificant. Weaning weight of the previous calf was positively correlated with postpartum conception and calving interval, but to determine the magnitude of the relationship the need to adjust for differences in breedtype, condition, parity, early management, age, weight and weight change of the dam was apparent.