Effect of stocking rate, season and pasture characteristics on liveweight gain of beef steers grazing perennial pastures

Abstract

In each of 4 years (1975-78) yearling steers at stocking rates (SR) of 1.1-3.6 steers/ha continuously grazed pastures sown in 1971 with Trifolium subterraneum cv. Mt. Barker, Phalaris aquatica cv. Australian and Lolium perenne cv. Victorian at Hamilton from Mar. to Jan. Data on liveweight gain (LWG), fat depth and pasture attributes were collected over the 4 years. Body composition was estimated periodically over the first 2 years, using tritiated water. Relationships between LWG and SR were established and the overall annual effect was LWG (kg)=237-39.7 SR. However, most or all of the gains made at intermediate or higher SR were achieved in the spring and summer. Compensatory gains in these 5 months were greatest at 3 steers/ha but varied from 6.5 kg (in 1976) to 34 kg (in 1978) more than achieved at the lowest SR. The heaviest steers had more subcutaneous fat and steers at higher SR were leanest, but in 1 year of prolonged spring growth (1978) there was no difference in fat depth at any SR despite a 68 kg difference in liveweight (W) between lowest and highest SR. Seasonal LWG was modelled from W and/or pasture mass (green and dead fractions). Use of green pasture mass rather than total mass substantially improved precision in all seasons. A smaller but significant further improvement was obtained using green mass, dead mass and the interaction in the regressions. Severe penalties to LWG occurred in grazed pastures with a large component of dead material.