A COMPARISON OF ONCE-YEARLY AND TWICE-YEARLY WEANING OF AN EXTENSIVE HERD IN NORTHERN AUSTRALIA .1. COW LIVEWEIGHTS, MORTALITIES AND FERTILITY

In northern Australia, many cows conceive only after their calves are weaned. The subsequent calves are born in the late wet or early dry se ason. Where producers wean once a year these late calves stay with the cows with resultant increased risk of cow mortality. A second weaning may reduce this risk. From June 1985 to June 1990 we recorded cow liv eweights, mortalities and reproductive rates from a herd of 500 Bos in dicus-Bos taunts-cross cows at Kidman Springs, Northern Territory. The herd grazed native pastures, and was continuously mated and unsupplem ented. Calves were weaned if they weighed 100 kg or more at muster in June (W1) or in either of June and October (W2). In both groups (W1 an d W2), 75% of calves were weaned at the June muster, and a further 14% were weaned at the October muster (W2 group). There was no difference in liveweight change, mortality or branding rates between the 2 group s. Overall productivity was low, probably due to poor nutrition during both dry and wet seasons. For example, average branding and mortality rates were 51.8 and 11.8%. Forty-seven percent of all cows were pregn ant at the June muster but a quarter of these failed to rear a calf. T he low proportion of calves suitable for weaning in October indicated that the weaning times were too close together, and did not allow enou gh late calves to reach the target liveweight for weaning. Consequentl y, the benefit of the second weaning was reduced. Weaning times for co ntinuously mated herds should be determined by the time taken for calv es born late in the wet season to reach target liveweights for weaning at the second weaning round. Effectively, this means the first weanin g should be earlier, perhaps in April, since high temperatures and the increased likelihood of rain make mustering after October difficult.