Bull selection and use in northern Australia

Summary

The project was conducted on 3 research stations and 8 co-operator properties in Queensland and 1 research farm in the Northern Territory. There were about 1000 bulls, mainly 2-to 4-year-old Santa Gertrudis, 5/8 Brahman, Brahman, Belmont Red and Belmont Red cross, that were subjected to physical and reproductive examinations prior to mating. The bulls came from properties where there was already in place a selection policy on physical and reproductive traits. Thus, many of the extremes in physical conformation, scrotal size, excessive pendulous sheaths and poor temperament had been culled from the population.

The key findings of the project were:

(i) The systematic physical examination of a bull is the foundation of a breeding soundness examination. The moderate to high repeatabilities for key physical traits (eg scrotal circumference) demonstrated that bulls may be selected accurately on the basis of an annual pre-breeding examination.

(ii) Semen examination, including assessment of sperm morphology, should be incorporated into bull selection programs and premating examinations to ensure that bulls have at least 50% normal sperm. Fertility recommendations should not be based solely on semen or sperm motility. Assessments of semen heparin-binding protein profiles are unlikely to enhance the evaluation of bull fertility.

(iii) Brahman and Santa Gertrudis bulls are capable of performing in a serving capacity test although the degree of expressions of sexual behaviour, particularly serves, is less than in Bos taurus. The main value of the serving capacity test in these breeds is a means of identifying whether a bull is capable of serving and not as a predictor of calf output when mated in multiple-sire herds.

(iv) Behaviour as expressed through social dominance has a significant influence on calf output of bulls in multiple-sire mated herds under extensive management systems. Bulls are very territorial, a behaviour which is presumably related to social dominance; bulls expressing this behaviour to the highest degree tended to sire more calves.

(v) DNA typing is an accurate method of identifying paternity of calves resulting from multiplesire matings. There was no one single physical or reproductive trait that was consistently related to calf output in multiple-sire herds. Even so, selecting on combinations of traits did not explain all of the variation in calf output. Bulls should be selected for physical and reproductive traits that lie above threshold values.

(vi) Multiple-sire mating Brahman and Bos indicus derived bulls which are reproductively-sound at a rate of 2.5% of cycling females will not jeopardise herd fertility under most conditions in extensive parts of northern Australia.