[Genetic determinism of sperm production in bulls]

In this study, the relationships between semen production and semen quality of Normande bulls in performance test station and in two IA centers were evaluated and the genetic characteristics and evolution of semen production of young bulls in station were calculated. When analysing the relationship between semen production of young and adult Normande bulls data from 2,677 bulls between 12 and 15 months of age exhibited large difference between 5 groups corresponding to an overall classification given by a technician. Correlations between corrected data collected in station and bull effect estimates in AZ studs were quite high for volume and concentration, and moderate for motility and number of doses per ejaculate or per month. The correlations calculated are underestimates of the true ones, as bulls with undesirable semen characteristics were culled before entering AI. Nevertheless, among the remaining bulls, important differences (40%) in semen output were observed between extreme groups, as classified by the technician. Genetic parameters evaluated from a subset of 1957 young Normande bulls confirms earlier results reported in the literature. The heritability of the average volume of the ejaculate is very high (h2 = 0.65 +/- 0.09). Sperm quality traits are moderately heritable (h2 = 0.23 +/- 0.08 for motility score to 0.37 +/- 0.09 for concentration) and strongly correlated. Total percentage of abnormal spermatozoa estimated from a selected subset seems less heritable (h2 = 0.19 +/- 0.07). A Best Linear Unbiased Prediction was then carried out on the records from 2,387 bulls using these estimates. No clear genetic trend could be detected in the population, although the phenotypic selection performed should be efficient. This apparent contradiction results from the increasing influence in the breed of one family of heavily used bull sires with a poor genetic merit on sperm production traits.