A MODEL FOR REPRODUCTIVE EFFICIENCY OF DAIRY BULLS

Reproductive efficiency of bulls is usually measured by nonreturn rate, which is commonly defined as the proportion of cows that were inseminated and did not return for another service within a specified number of days. The AI organizations use nonreturn rate to evaluate fertility of a bull or performance of a technician. Measures derived from nonreturn rate, such as conception rate and calving rate, might be more reliable for evaluation than nonreturn rate itself. Estimated conception rate is a better early measure of efficiency than nonreturn rate, because conception rate depends on the population of spermatozoa at insemination and not on developmental potential of the conceptus after insemination. A mathematical function is presented to model reproductive efficiency of bulls by estimation of the probability of conception at time of insemination (conception rate) and the probability of completing gestation after insemination (calving rate) through the relationship of nonreturn rate to the concentration of spermatozoa at insemination and the time after insemination. The model is illustrated with three bulls, using nonreturn rates by 28, 56, and 84 d after insemination.