BREED AND OTHER EFFECTS ON REPRODUCTIVE TRAITS AND BREEDING SOUNDNESSCATEGORIZATION IN YOUNG BEEF BULLS IN FLORIDA

Yearling, grass-fed, beef bulls at the USDA Subtropical Agricultural R esearch Station, Brooksville, Florida, were assessed for physical and semen traits in January, April, July and October of 1991 (Trial 1) and 1992 (Trial 2). Bulls were given a breeding soundness evaluation (BSE ) using revised semen and scrotal circumference (SC) criteria. In Tria l 1, the bulls consisted of Angus (n=15), Brahman (n=14), Hereford (n= 15) and Senepol (n=14). In Trial 2, the breeds were Angus (n=15), Brah man (n=16), Romosinuano (n=13) and Nellore x Brahman (n=9). Trial bull s generally showed delayed growth compared with grain-fed bulls in tem perate environments. Breed influenced semen traits (percentage sperm m otility, normal spermatozoa and those with primary abnormalities) in b oth trials. Temperate Bos taurus breeds (Angus, Hereford) were general ly superior to Bos indicus breeds (Brahman, Nellore x Brahman). Tropic ally-adapted Bos taurus breeds (Senepol, Romosinuano) were intermediat e for those traits tested. In general, tropically-adapted Bos taurus b reeds were more similar in reproductive development to temperate Bos t aurus than to Bos indicus breeds. Breed by test period interactions oc curred and were mainly influenced by delayed sexual maturity of Bos in dicus bulls. Qualitative semen traits increased with bull age, particu larly from 12 to 18 mo. Scrotal circumference development was slower i n the Bos indicus breeds. Bulls of satisfactory BSE status at 18.1 to 22 mo of age were 73.9% in Trial 1 and 58.5% in Trial 2. Brahman bulls had the least satisfactory BSE scores in both years(Trial 1, 44.4%; T rial 2, 22.2%). Most bulls failed to achieve satisfactory BSE status d ue to a small SC relative to age (Trial 1, 66%; Trial 2, 72%). The mos t efficacious use of the BSE was greater than or equal to 15 mo in Bos taurus bulls and >18 mo for Bos indicus bulls. Although the BSE has p roven to be useful for the assessment of young, pasture-raised bulls i n semi-tropical environments, use of SC thresholds linked more with gr owth traits than with calendar age would improve comparisons of relati ve reproductive development in such bulls, particularly those of Bos i ndicus derivation.