Mortality, wastage, and lifetime productivity of Bos indicus cows under extensive grazing in northern Australia. 1. Seasonal mating in the speargrass region

Abstract

Mortality and wastage rates and lifetime productivity for seasonally mated, Bos indicus cows were studied over 8000 cow year records from 1972 to 1992 at Swan's Lagoon in the subcoastal speargrass region of North Queensland. The proportional hazards model was used to estimate age-specific mortality and wastage rates, adjusted for cohort and genotype effects. The 1970-72 cohorts had an average mortality of 1.7% (range 0-4.2%), with this low overall rate tending to increase with age. The average mortality rate for the 1973-87 cohorts was 1.2% (range 0.5-2.6%), with no pattern with age. For 1970-72 cohorts, the average wastage rate was 8.9% and ranged from 2.1 to 18.3% with no clear age pattern. However, there was a clear age pattern for the 1973-87 cohorts, with an average wastage rate of 9.2% comprising very high rates (27.5%) for 2-year-olds and low rates (2.8-8.9%) for 3-7-year-olds, increasing to 11.9% for 8-year-olds and 14.2% for 9-year-olds. The heifer replacement rates to maintain a stable herd size were 17.5 and 19.2% for the 2 herds. Lifetime productivity was very low, with 1970-72 cohorts rearing 3.3 calves over 4.7 years at a rate of 57.5% calves per year, and 1973-87 cohorts rearing 3.1 calves over 4.9 years at a rate of 45.0% calves per year. Total weaner weights reared up to 10 years of age were 578 kg for 1970-72 cohorts and 315 kg for 1973-87 cohorts. However, variability between individual cows was high, indicating scope for selection based on productivity, provided that better performing cows can be identified from predictors early in life.