EFFECT OF BIRTH DATE ON GROWTH-PERFORMANCE OF CALVES OF SEASONALLY-BRED RANGE CATTLE

This study was undertaken to investigate the relationship between date of birth and growth performance of calves born to seasonally-bred ran ge beef cows. Late-born calves exhibited an apparent superiority in pr eweaning growth over that of their early-born counterparts. This was a ttributed to a positive residual effect of maternal environment arisin g from the fact that calves born late were those whose dams had a subs tantial proportion of tl e critical last trimester of the gestation pe riod in a favourable season. This advantage soon disappeared and early -born calves gained more weight and at a faster rate between 90 days a nd weaning. A combination of the effect of weaning stress and unfavour able nutritional environment in the immediate postweaning period had a check on calf growth but late born calves suffered most. It was concl uded that, in this environment, nutrition was most limiting to calf gr owth between weaning and 12 months of age and it was during this perio d that a clear advantage of early birth of calves appeared; A 1 day de lay in BD was associated with a 9.1 kg (P < .01) decrease in net weigh t change in this period. However, due to compensatory growth, the corr esponding figure for the 12 months to 18 months period was an increase of 11.6 kg (P < .01) in weight change. In terms of actual postweaning weights (not age-adjusted), early BD was superior to late BD at all a ges except at 18 month. A 1 day delay in BD was associated with a.7 kg reduction (P <.01) in liveweight between 90 days and weaning, a 6.7 k g reduction (P < .01) in actual weaning weight, a 6.8 kg reduction (P < .01) in actual 12 months liveweight, and a non significant (P > .05) 0.7 kg reduction in actual 18 months liveweight. Under seasonal breed ing and group management of calves, all animals from a calf crop are e xpected to go to market at the same time, thereby rendering age-adjust ment of liveweights irrelevant. However, the sale of late calves could be delayed a few months to take advantage of the rainy season and to exploit compensatory growth. The benefit of this practice should, howe ver, be weighed against both the direct Cost (of feeding veterinary ca re, labour and so on) and the indirect cost - the competition factor - introduced on the next calf crop.