Genetic and environmental factors affecting semen quality of young Holstein bulls.

Semen traits were measured from 294 young Holstein bulls between the ages of 10 and 18 mo. Bulls were sampled between the years 1987 and 1989 and the first four suitable ejaculates from each bull were retained for study. The season of the year, the year and the sire of each bull were used as factors and the age of the bulls and the inbreeding coefficient were used as co-factors for the analyses of variance and covariance. The sire effect was declared significant for the volume (P < 0.001), concentration (P < 0.05) and the number of motile spermatozoa per ejaculate (P < 0.001). Intraclass regression studies indicated that each increase of 1 d of age in young bulls increased the volume of the ejaculate by 0.004 mt, the concentration and the motility of the ejaculate by 0.024 x 10(8) spermatozoa mL(-1) and 0.034% and the number of motile spermatozoa by 0.091 x 10(8) per ejaculate. A 1% increase in the inbreeding coefficient (F) decreased the motility of spermatozoa by 0.30% and the concentration of the ejaculate by 0.14 x 10(8) mt, resulting in a decreased total number of inseminations possible by 13.83 doses. It also increased the number of rejected samples by 0.25 and delayed sexual maturity by 3 d. Heritability estimates of semen traits varied from 0.25 and 0.79. The genetic and phenotipic correlations were respectively -0.72 and -0.47 for the correlations between volume and concentration, 0.140 and 0.07 between volume and motility and -0.22 and -0.01 between motility and concentration.