A COMPARISON OF ONCE-YEARLY AND TWICE-YEARLY WEANING OF AN EXTENSIVE HERD IN NORTHERN AUSTRALIA .2. PROGENY GROWTH AND HEIFER PRODUCTIVITY

Continuous mating is common in northern Australian beef herds. The res ulting calves are born throughout the year making weaning and herd man agement difficult. Calves born late in the wet season do not reach sui table liveweight for weaning until late in the year. Low liveweight an d poor feed conditions make management of these animals for subsequent turnoff or breeding more difficult. We recorded the growth and surviv al of all progeny and reproductive performance of heifers from 2 group s of 250 Bas indicus-Bos taurus-cross cows grazing native pastures at Kidman Springs, Northern Territory, for 5 years from July 1985. Calves were weaned in June only (W1) or June and October (W2). Unweaned calv es remained with their dams until June the following year and heifers entered the breeding herd as 2 year olds. Calves weaned in June (WJ) w ere heaviest (157.1 kg; P<0.05) but gained least (84.6 kg; P<0.05) ove r the following year. Unweaned (UW) calves were lighter (51.1 kg; P<0. 05) at branding time, in June, than those weaned in October (WO; 66.8 kg). However, they gained more weight (144.5 v. 112.4 kg; P<0.05) over the following year than WO calves because of a superior wet season gr owth rate, so that WO calves were lightest (P<0.05) at the yearling mu ster. Mortality rates of all calf groups were low (2.5%). Liveweight a t branding and liveweight gain to yearling stage affected whether heif ers subsequently joined the breeding herd. More (P<0.05) heifers from WJ joined the herd than from WO and UW groups. Overall, 63.6% of heife rs were selected as replacement breeders (no difference between W1 and W2). Bull control was difficult and many heifers (48%) conceived befo re the planned start of mating. Cumulative conception rate was 91.3% b y 30 months post-branding. In the first year, 60% of heifers mated wer e confirmed pregnant, but reproductive losses of 25.6% from pregnancy diagnosis to branding contributed to a branding rate of 44.9%. The con ception rate of heifers when lactating was 18.6%. Mortality during the first year in the herd was 11.7%. In general, WJ heifers had lower mo rtalities, higher conception, calving and weaning rates, and higher co nception rates when lactating than WO or UW heifers. Animals weaned in October were identified as the most difficult group to manage because of lower liveweight as 2 year olds compared with WJ and UW calves. Th is affected the numbers of heifers suitable for joining. Cost-effectiv e management systems are needed to improve growth rates or delay onset of oestrus in these animals.