THE PERFORMANCE OF BRAHMAN-SHORTHORN AND SAHIWAL-SHORTHORN BEEF-CATTLE IN THE DRY TROPICS OF NORTHERN QUEENSLAND .3. BIRTH WEIGHTS AND GROWTH TO WEANING

The birth weights and weaning weights of Bos indicus cross calves from 4 inter se mated herd groups in a dry tropical environment were studi ed. The calving season commenced in late October, about 2 weeks before the start of the pasture-growing season. Second and subsequent genera tions of 1/2 Brahman and 1/2 Sahiwal crosses and first backcross, and later generations of 3/4 Brahman and 3/4 Sahiwal crosses, were represe nted. The Bos taurus component of the crosses was primarily Beef Short horn. In general, Brahman crosses were heavier at birth and grew faste r to weaning than Sahiwal crosses (P<0.05), with consequent advantages in weaning weights of 11 and 6 kg in first backcross and late generat ion crosses, respectively. There was a birth weight advantage (P<0.05) of 1/2 crosses over 3/4 crosses, which was eroded by the time of wean ing. Superior growth rates of 3/4 crosses were primarily expressed in years with greater nutritional stress and appeared to be due to better adaptation to environmental stresses by the calf. The only difference between filial generations was the growth rate advantage (P<0.05) of F2 1/2 crosses over F3 1/2 crosses, apparently due to suckling of F1 a nd F2 dams, respectively. Males exhibited an 8% average advantage (P<0 .05) over females for all weight and growth traits in late generation calves. Calves with mature dams were 1.8 kg heavier (P<0.05) at birth. This, along with their higher (P<0.05) preweaning growth rate (19-49 g/day), resulted in calves with mature dams being 4-10 kg heavier (P<0 .05) at weaning than calves of young cows. Differences between years i n birth weights and preweaning growth rates were less-than-or-equal-to 25% (P<0.05). Birth weight was affected by nutrition of the dam in la te pregnancy (early calves were generally about 1 kg lighter than late calves), and growth of calves was higher in years when the pasture gr owing season commenced earlier.