Factors Influencing Gestation Length in Tropically Adapted Beef Cattle Breeds in Northern Australia

Records from artificial insemination programs and accurate birth date recording on 812 calves were used to compute gestation length (GL) for 3 tropically adapted beef breeds in northern Australia. Calves were a subset of those generated across 2 years and 2 locations as part of an ongoing beef genetics project in northern Australia. Analyses revealed few fixed effects were significantly influencing the trait, however calf sex was highly significant in all three breeds with males having 2.1, 3.0 and 3.7 days longer GL than females in Droughtmaster, Santa Gertrudis and Brahman, respectively. Large sire differences (up to 13 days) were also observed within each breed, and indicate a large degree of genetic control on the trait. These results have implications for breeding program design, parentage assignment and the development of a genetic evaluation for this trait, both within and across-breeds.