Breeding soundness and sex drive by breed and age in beef bulls used for natural mating.

A total of 92 range beef bulls (Hereford = 60; Angus = 32) were given a breeding soundness examination (BSE) and two assessments for sex drive prior to their use in 23 breeding trials employing estrous synchronized females. Bulls were in three age groups: yearlings (n=29), two year olds (n=36), and three year olds and older (n=27). All yearling bulls were virgins, but the majority of the older bulls had previous mating experience. Angus bulls were superior (P<0.01) to Herefords in spermatozoal morphology and BSE score. Scrotal circumference increased with age beyond two years in Angus bulls but not in Herefords. Spermatozoal abnormalities generally decreased with age. BSE scores did not differ significantly among age groups. Apart from number of mounts, measures of sex drive did not differ with age or breed of bulls. This represents qualified justification for the current practice of using the same sex-drive assessment procedures for Bos taurus bulls of various ages and breeds.