GENETIC-PARAMETERS FOR TROPICAL BEEF-CATTLE IN NORTHERN AUSTRALIA - A REVIEW

This paper reviews published estimates of genetic parameters for traits of growth, reproduction and resistance to environmental stresses for Bos indicus and Zebu derived breeds in northern Australia. Most published estimates of heritabilities for growth and reproduction traits were higher for tropically adapted breeds in northern Australia than for Bos taurus breeds in temperate Australia. Weighted mean estimates of heritabilities for the direct component of weaning weight were 39% for the Brahman breed and 30% for Zebu-derived breeds in northern Australia compared with 13% for Bos taurus breeds in temperate areas of Australia. Mean estimates for the maternal component of weaning weight were 5, 24 and 13% respectively. Mean heritabilities for yearling and 550 day weights for Zebu derived breeds in northern Australia (24 and 25%) were similar to those for Bos taurus breeds in temperate areas, though estimates for Brahmans were higher (39 and 39%). Published estimates of heritabilities of later weights (700 and 900 days), which are most relevant to northern Australian production systems, were rare but averaged between 32 and 45% for Zebu-derived breeds and Brahmans. Weighted mean heritability for female calving success was 14% and for realised bull fertility was 5%. Published estimates of heritablities of scrotal circumference averaged 31%, and testosterone response to GNRH stimulation was 52%. Heritabilities of resistance to various environmental stresses were all moderate with weighted means between 20 and 34%. Genetic correlations between growth, reproduction and resistance to environmental stresses are also reviewed. There appears to be predictable variation in estimates of parameters between breeds in different environments which is related to level of resistance to environmental stresses, and this is likely to affect the prediction of breeding values for cattle in northern Australia.