Abstract:
Animal Production in Australia EFFECT OF STOCKING RATE OF STEERS ON NET AND TRUE GROWTH RATES OF PERENNIAL PASTURE J.W.D. CAYLEY*, P.R. BIRD*, M.J. WATSON* and J.F. CHIN* In an experiment at Hamilton, Victoria, yearling steers were stocked at a range of rates on a pasture of perennial ryegrass, subterranean clover, barley grass, fog grass and capeweed. Measurements of net pasture production (NP) during the first 2 full years of this study showed that NP was reduced as stocking rate (SR) increased (Bird et al. 1978). Our objective was to obtain more information on the effect of SR on more fundamental components of NP, viz. herbage decay (D) and true pasture production (TP) where NP = TP - D. Between 1975 and 1977 the stock grazed alternate half-plots for 2-week periods, and NP was determined from changes in yield at a number of fixed sites in the ungrazed half-plot. During 1978, each half-plot was grazed continuously, and NP was determined each 4 weeks using pasture cages. Dead pasture present was determined each 2 weeks between 1975 and 1977 and each 4 weeks during 1978. Instantaneous rates of decay of dead herbage (Di),were estimated by a method reported in these proceedings (Cayley et al. 1980), and the rate of D for corresponding rates of NP estimated by multiplyingDiby the mean amount of dead herbage present. The effect of SR on NP, D and TP are shown in Table 1. TABLE 1 Effect of SR+ on components of pasture production expressed as regression equations of the form y = a + bSR The reduction in pasture production with increasing SR was consistent between years. The regression slopes (b) and the.SR at which pasture growth is predicted to be zero (-a/b) show that the effe& of SR on TP is substantially greater than indicated by effects on NP alone. There is a reduction in animal production per head with increasing SR (Bird et al. 1978); the rate of this decrease depends on how soil type/pasture species respond to increases in treading and/or defoliation. BIRD, P.R., CAYLEY, J. W.D. and WATSON, M.J. (1978). Proc. Aust. Soc. Anim . Prod. == 12: 216. CAYLEY, J.W.D., BIRD, P.R. and CHIN, J.F. (1980). Proc. Aust. Soc. Anim . Prod . $2: 469. * Pastoral Research Institute, Hamilton, Vic. 3300. 468