Bull selection and use in northern Australia. Part 2. Semen traits.

Detailed semen evaluations were carried out on approximately 363 Santa Gertrudis, 5/8 Brahman and Brahman bulls on 12 different properties across northern Australia, as part of systematic breeding soundness examinations. A subset of bulls (n=245) were subsequently mated in groups, to cows and heifers at bull:female ratios of 2.5-6.0%, with the paternity of resulting calves being determined by microsatellite DNA testing. Motility traits of semen and spermatozoa were moderately repeatable and correlated with each other, but were unrelated to calf output. The percentage of morphologically normal spermatozoa in ejaculates was moderately to highly repeatable (e.g. r=0.10-0.64). The most common morphological abnormalities seen were mid-piece abnormalities, in particular, distal mid-piece reflex associated with a cytoplasmic droplet. Semen quality, particularly percent normal spermatozoa, was consistently related to calf output. In general, bulls with <50% normal spermatozoa sired few calves while bulls with the highest calf outputs had >70% normal spermatozoa. The presence or absence of heparin binding proteins in semen did not influence calf output. Semen from 93% of tested bulls was positive for heparin binding proteins. These results confirm that examination of semen, in particular, evaluation of percent morphologically normal spermatozoa, should be included in the breeding soundness examination of bulls.