Calving date versus calving interval as a reproductive measure in beef cattle.

Calving date and calving interval records obtained from the Beef Cattle Division of Pioneer Hi-Bred International, Des Moines, Iowa, were studied to determine the reproductive measure of choice when a fixed breeding season is employed. Fixed model, least-squares procedures were used to analyze 2,411 records collected on purebred Red Angus, Angus and Hereford cows during the years 1969 through 1976. Calving interval decreased .86 d and calving date was delayed .11 d for each 1-d delay in previous calving date. A 1-d increase in gestation length was associated with a 1.17-d delay in calving date and 1.17-d increase in calving interval. The bias caused by the use of a fixed breeding season was determined to be much greater for calving interval than for calving date, as evidenced by the strong dependence of calving interval on previous calving date. Because calving date is likely to be more heritable and has clear economic significance, and because selection for earlier calving date should not lead to adverse response in other reproductive traits, calving date was preferred over calving interval as a reproductive measure.