AGE-RELATED-CHANGES IN THE SEXUAL PERFORMANCE OF HEREFORD BULLS

Twelve polled Hereford bulls were subjected to annual sexual performan ce evaluations starting at 12-13 months of age and continuing to 5 yea rs. Significant year effects (P < 0.001) were obtained for ejaculation frequency, frequency of mounts without ejaculation and the proportion of mounts that culminated in an ejaculation. As yearlings, the subjec ts exhibited more mounts, fewer ejaculations and a lower ejaculation-t o-mount ratio than in years 2 through 5. Increases in mating efficienc y between years 2 through 5 were not significant. Some individual year -to-year variation in sexual performance was noted until the bulls rea ched 4 years of age. Ejaculation and mount frequencies in year 4 were correlated with performance in year 5 but between-year correlations at younger ages were not significant. While average sexual performance o f a population of bulls may remain relatively stable from years 2 thro ugh 5, individual year-to-year variation in sexual performance may be sufficient up to 3 years of age to preclude long-term assessment of th e mating potential of individual males.