MORTALITY, WASTAGE, AND LIFETIME PRODUCTIVITY OF BOS-INDICUS COWS UNDER EXTENSIVE GRAZING IN NORTHERN AUSTRALIA .2. CONTINUOUS MATING IN THE SEMIARID TROPICS

Mortality and wastage rates and lifetime productivity were studied ove r 4800 cow years from 1981 to 1990 with continuous mating and minimal management in the semi-arid tropics at Kidman Springs, Northern Territ ory. The proportional hazards model was used to estimate age-specific mortality and wastage rates, adjusted for cohort effects. Annual breed er mortality rate averaged 11.5%, with similar rates in the range 9.0- 12.0% for 2-9-year olds, increasing to 15.5% for 10-year-olds, 17.9% f or 11-year-olds, and >20% for older cows. A further 20.8% of the cows that calved failed to rear the calf to branding. Wastage included mort alities and physical culls but minimal discretionary culls based on re productive performance or maximum age. The wastage rate averaged 16.7% , ranged from 11.1 to 14.4% for 2-9-year-olds, increased to 21.1% for 10-year-olds and 34.3% for 11-year-olds, and exceeded 50% for older co ws. The heifer replacement rate to maintain a stable herd size was 16. 3%. Typical lifetime productivity up to 10 years of age in this harsh north-western environment was 3.1 calves reared over a lifetime of 6.5 years in the breeding herd, at a rate of 44.5% calves per year. Timin g of first pregnancy was identified as a useful predictor of lifetime productivity when overall productivity was low. Cows that were lactati ng as 2-year-olds reared 4.0 calves in their lifetime up to 10 years o f age, cows pregnant at this age reared 3.2 calves, and nonpregnant co ws reared only 2.6 calves.