Evaluation of calving interval and calving date as measures of reproductive performance in a beef herd

Abstract

Calving interval is currently used as the measure of reproductive performance in South Africa and many other countries, but some authors have claimed that calving interval is not a suitable measure of reproductive performance, when cows are mated during a restricted breeding season. Numerous authors have suggested that calving date is a suitable alternative measure of reproductive performance. This study was undertaken to investigate the factors influencing calving interval and calving date, in order to determine their suitability as measures of reproductive performance under conditions of restricted breeding. Two models were developed, each having the following independent effects: Year of birth, age of dam, previous sex of calf, previous calf birth weight, previous calf weaning weight and previous calving date. A number of interactions were included. In Model 2, gestation length was included as an additional independent effect. The dependent variables were: calving interval and calving date. Year of birth and previous calving date had a highly significant influence on calving interval and calving date whilst age of dam had a highly significant influence on calving interval and a significant influence on calving date. A 1 day delay in previous calving date, resulted in a 0.72 day decrease in calving interval, and a 0.27 day increase in calving date (Model 1). In Model 1, previous calving date was the variable having the greatest influence on calving interval and calving date with previous calving date having a much greater influence on calving interval than on calving date, as determined by the significantly larger mean square associated with calving interval than calving date. When calving interval was corrected for the large negative effect of previous calving date, calving interval and calving date became similar measures. It was concluded that calving interval was a more biased method of evaluating reproductive performance than calving date, when cows are mated during a restricted breeding season, due to the negative relationship between previous calving date and calving interval and that calving date could be a suitable alternative measure of reproductive performance.