Conception rate at first insemination in beef cattle: effects of season, age and previous reproductive performance.

The effects of season, age at previous parturition, and whether the female was retained because she failed to conceive or be detected in estrus during the previous breeding season were analyzed for first-service conception rate in beef cattle. Twelve years of reproductive data on females that were at least 50% Simmental were obtained from a single farm. All females detected in estrus were artificially inseminated during 42-d breeding seasons (52 d for heifers), starting either in June or November. The data, consisting of 9,071 first-service records, were described by a logit model and parameters were estimated using weighted least squares procedures. Age and previous reproductive performance (whether the female had failed to conceive or be detected in estrus during the previous breeding season) significantly affected first-service conception rate, as did the interaction between the two factors. Season of insemination, when considered across all other effects, did not influence first-service conception rate, but the interaction of season and previous reproductive performance was significant, as was the three-way interaction between season, age and previous reproductive performance. Except for those calving at 2.5 yr of age, females that were inseminated in the winter subsequent to parturition in the fall had the highest first-service conception rates. Those that did not conceive during the breeding season subsequent to parturition and were carried over to the following season had lower first-service conception rates, with the lowest being for females carried over to the winter breeding season.