Abstract:
Proc. Aust. Soc. Anim. Prod. (1978) 12: 216 EFFECT OF STOCKING RATE ON ANNUAL PASTURE PRODUCTION AND STEER GROWTH P.R. BIRD*, J.W.D. CAYLEY* and M.J. WATSON* The effect of stocking rate (SR) of cattle on annual productivity of temperate pasture based on subterranean clover and perennial ryegrass has received little attention. Nor has the effect of SR on annual liveweight gain (LWG) per head been adequately documented. In June 1974, and thereafter in February of 1975 and 1976, yearling Hereford steers were allocated to 10 plots at SR of 1.2, 1.8, 2.4, 3.0 and 3.27 animals/ha. One replicate was on high ground and the other on lower slopes. Each plot was equally divided and the steers grazed alternate half-plots for 2-week periods. Apparent pasture growth was estimated on the ungrazed half-plots from the difference in yield at a number of fixed sites over the 2-week periods. Anthelmintics were given each 42 d. Annual max. LWG was based on 24 h fasted weights of 9 steers in each plot. Fat depth was measured over the 12013th rib with a scanogram in mid-December. The annual production data for steers and pastures were subjected to regression analyses, SR being the independent variable in the following table where y = a + b SR + c SR2. The initial live weight in 1975 was 253 + 1.07 kg (mean ,+ SE) and the max. weights attained varied due to SR from 315-406 kg. In 1976 these values were 227 ,+ 0.78 kg and 336-428 kg, respectively. LWG differed between years, as did the pattern of pasture growth and climatic conditions, but annual pasture production was similar. In both years pasture production was depressed as SR increased, as noted with sheep by Carter and Day (1970). LWG/head was also denressed as SR increased, as proposed by Jones and Sandland (1974). Connolly (1976) warns of the danger of assuming linearity when estimating the 'optimal SR'; our data also indicate that errors will arise when the SR range does not straddle the optimum. From the linear model our 'optimum SR' in 1975 and 1976 was 2.68 and 3.04 steers/ha. 'These values 'represent max. yield and are not necessarily economic optima since at these SR the animals are too lean for subjective livestock marketing and would incur a price penalty. CARTER, E.D. and DAY, H.R. (1970). Aust, J, agric, Res. 21:473. CONNOLLY, J. (1976). J. agric. Sci. Camb. 86:103. JONES, R.J. and SANDLAND, R.L. (1974). J. agric. Sci, Camb. 83:1335. * Pastoral Research Institute, P.O. Box 180, Hamilton, Vic. 3300. 216.