THE RELATIONSHIP BETWEEN THE NUMBER OF SPERMATOZOA INSEMINATED AND THE REPRODUCTIVE EFFICIENCY OF INDIVIDUAL DAIRY BULLS

The semen of 20 mature, evaluated bulls was split-sample diluted and c ontained 2.1x 10(6) to 17.3 x 10(6) total spermatozoa per 0.25-ml Fren ch 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 i nseminations performed. The nonreturn rate at 56 d after Al was estima ted for every dilution. The daily nonreturn rates to 180 d were used t o estimate conception and calving rates at a given concentration. The relationship was determined between these estimations and the number o f spermatozoa that were actually inseminated. The bulls differed signi ficantly in their maximal nonreturn rate at high sperm numbers per AI and in the rate at which they approached this maximum. There was no co rrelation between the maximum nonreturn at high sperm numbers and the rate of approach, which implies that the ranking of the bulls for nonr eturn rate 56 d after AI changes with the number of spermatozoa insemi nated. Multiphasic analysis of reproductive efficiency revealed bull d ifferences in estimated conception and calving rates. The estimated ca lving rate after conception was 82 to 90% and was independent of the n umber of spermatozoa that were inseminated. The sperm numbers needed t o obtain 95% of the maximal conception rate ranged from 1 x 10(6) to 1 1 x 10(6).