Evaluation of calving date and calving interval as measures of reproductive efficiency in beef cows

Five hundred and thirty-five calving records were analysed to determine the relationship and factors influencing calving date (CD) and calving interval (CI) so as to determine which of the two is the best measure of reproductive efficiency in beef cows mated during a restricted breeding season. Effects included in the model for the analysis of effects of CD and CI were: age of dam, year, previous sex of calf (PSex), previous calving date (PCD), previous calf birthmass (PBM) and previous calf weaning mass (PWM) and the interactions PBM x PSex and PWM x PSex. PCD and year had a highly significant influence on CD and CI. PCD had a significantly larger influence on calving interval than on calving date as evidenced by the larger mean square associated with calving interval. It was found that cows that calved earliest during the calving season had the longest calving intervals, whilst cows that calved latest had the shortest calving intervals. Calving interval was found to be reduced by 0.61 +/- 0.05 days for each one day delay in PCD, whilst CD was delayed by 0.09 +/- 0.07 days for each one day delay in PCD. It was concluded, since calving date was influenced to a smaller extent than calving interval by previous calving date, that calving date is a more reliable reproductive measure where restricted mating is practised.