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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org..

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The semen of 20 mature, evaluated bulls was split-sample diluted and contained 2.1 x 10(6) to 17.3 x 10(6) total spermatozoa per 0.25-ml French straw. The number of viable inseminated spermatozoa ranged from 1.1 x 10(6) to 11.8 x 10(6). Each bull had 2430 to 5330 first or second inseminations performed. The nonreturn rate at 56 d after AI was estimated for every dilution. The daily nonreturn rates to 180 d were used to estimate conception and calving rates at a given concentration. The relationship was determined between these estimations and the number of spermatozoa that were actually inseminated. The bulls differed significantly in their maximal nonreturn rate at high sperm numbers per AI and in the rate at which they approached this maximum. There was no correlation between the maximum nonreturn at high sperm numbers and the rate of approach, which implies that the ranking of the bulls for nonreturn rate 56 d after AI changes with the number of spermatozoa inseminated. Multiphasic analysis of reproductive efficiency revealed bull differences in estimated conception and calving rates. The estimated calving rate after conception was 82 to 90% and was independent of the number of spermatozoa that were inseminated. The sperm numbers needed to obtain 95% of the maximal conception rate ranged from 1 x 10(6) to 11 x 10(6).