A comparison of once- and twice-yearly weaning of an extensive herd in northern Australia 2. Progeny growth and heifer productivity

Abstract

Continuous mating is common in northern Australian beef herds. The resulting calves are born throughout the year making weaning and herd management difficult. Calves born late in the wet season do not reach suitable liveweight for weaning until late in the year. Low liveweight and poor feed conditions make management of these animals for subsequent turnoff or breeding more difficult.
We recorded the growth and survival of all progeny and reproductive performance of heifers from 2 groups of 250 Bos 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 calves remained with their dams until June the following year and heifers entered the breeding herd as 2 year olds.

Calves weaned in June (WJ) were heaviest (157.1 kg; P<0.05) but gained least (84.6 kg; P<0.05) over 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 growth rate, so that WO calves were lightest (P<0.05) at the yearling muster. Mortality rates of all calf groups were low (2.5%).

Liveweight at branding and liveweight gain to yearling stage affected whether heifers 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 heifers were selected as replacement breeders (no difference between W1 and W2). Bull control was difficult and many heifers (48%) conceived before the planned start of mating. Cumulative conception rate was 91.3% by 30 months post-branding. In the first year, 60% of heifers mated were confirmed pregnant, but reproductive losses of 25.6% from pregnancy diagnosis to branding contributed to a branding rate of 44.9%. The conception 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 mortalities, higher conception, calving and weaning rates, and higher conception 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. This affected the numbers of heifers suitable for joining. Cost-effective management systems are needed to improve growth rates or delay onset of oestrus in these animals.