Genetic studies on fertility in A.I. bulls. I. Age, season and genetic effects on semen characteristics in young bulls

Abstract

Genetic and environmental factors that influence semen quantity and quality were evaluated for 215 young bulls of the two main dual-purpose breeds in Sweden, Swedish Red and White (SRB) and Swedish Friesian breed (SLB). Semen was collected at three bull stations from September 1983 to August 1985, inclusive. The effects of bull's sire, bull, season and year of collection, and bull's age at collection were estimated for ejaculate volume, motility, total no. of sperm cells per ejaculate and no. of semen doses per collection. Heritabilities ( h 2), repeatabilities ( t) and phenotypic correlations were estimated for these traits. Motility and ejaculate volume were significantly affected by bull's sire ( P < 0.01). The effect of bull, season of collection, and bull's age at collection were highly significant ( P < 0.001) for most of the traits. The effect of year of collection was only significant for one semen characteristic. The summer months June, July and August were the best months for quantitative traits. Estimates of the heritability for semen characteristics ranged between 0.02 and 0.28, with the highest estimates found for ejaculate volume and semen doses per collection. The h 2-estimates differed somewhat between breeds. The repeatabilities ranged between 0.5 and 0.6 for all the traits studied. It was concluded that the h 2-estimates were sufficiently high to permit selection on individual bull level for some of the traits. At least 20 ejaculates have to be considered for total no. of sperm cells per ejaculate and post-freezing motility in order to achieve a selection accuracy ( r TI) of 0.3–0.5. One must take into account a bull's age and season of collection when evaluating semen quality.