Effect of breeder body condition and weight on pregnancy and calf growth in three age groups of composite-bred beef cattle

Abstract

The reproductive performance of 4 herds of composite-bred (50% Bos indicus/Sanga, 50% Bos taurus) cattle grazing native pasture in the Barkly Tableland region (Northern Territory) was studied for 5 years. Predictive models of reproductive performance were developed from 6210 breeder records and 2006 calf records, based on the easily measurable indicators, breeder body condition (BCS; using a 9-point scale) and weight for heifers, second calf heifers and cows. Breeder BCS (mean = 6) and weight (mean = 504 kg) were higher than typically reported for northern Australia, as were pregnancy rates and calf growth rates from birth to weaning. Pregnancy rates for heifers (90%) were lower (P<0.05) than second calf heifers (93%) and cows (95%). Calf growth for progeny of heifers (853 g/d) was lower (P<0.05) than for second calf heifers (900 g/d) and cows (903 g/d). The exceptional reproductive performance of these herds is likely to be a consequence of above average nutrition, breed effects and management. Increasing pregnancy rates and earlier conception dates (as measured by rectal palpation) were associated with increasing breeder weight. Increasing pregnancy rates were also associated with increasing BCS. There was an inverse relationship between BCS and calf growth for progeny of breeders of BCS 4 and above. This study has established new reproductive performance benchmarks for this region of northern Australia, and has shown that breed composition of up to 62.5% Bos taurus in the tick-free regions can be highly productive. The results of this work support findings of previous research indicating that breeder BCS of approximately 5 (moderate condition) is optimum.