HERBAGE AND ANIMAL PRODUCTION FROM NATIVE PASTURES AND PASTURES OVERSOWN WITH STYLOSANTHES-HAMATA .1. FERTILIZER AND STOCKING RATE EFFECTS

Native pastures dominated by Heteropogon contortus (speargrass) were s own to Stylosanthes hamata cv. Verano in 1972, and herbage production and steer growth rates were compared with those of native pastures fro m 1973 to 1985. The native pastures also contained naturalised Stylosa nthes humilis, but its contribution to pasture yield diminished rapidl y, after infection by Colletotrichum gloeosporioides (anthracnose) in 1973. The experiment was a factorial design of 2 pasture types (native , native plus Verano) x 2 stocking rates (0.6, 1.2 steers/ha) x 2 supe rphosphate treatments (nil, 300 kg/ba.year) x 2 replicates. Fertiliser application decreased the proportion of legume but had no significant effect on herbage or animal production on this comparatively fertile site (extractable P, 18 mg/kg). Annual liveweight gains at the high an d low stocking rates, respectively, on the native pasture averaged 100 and 120 kg/steer. Sowing to Verano did not affect herbage yields but increased annual liveweight gains by 28 and 36 kg/steer at low and hi- h stocking rates. The high stocking rate of 1.2 steers/ha was sustaina ble for the first 9 years of the experiment, when above-average rainfa ll was received. However, in the following 3 below-average years, ther e was a shift to less-desirable species, and a decline in pasture prod uctivity. Relative to the low stocking rate, herbage production on the native pasture in the final season was reduced by 60% and on the Vera no pasture by 26%. The highest annual herbage utilisation rate that ap peared sustainable in the long term was about 45%, which corresponded to a utilisation rate of 30% during the growing season (about November -May). When oversown with Verano, speargrass tended to decline in favo ur of annual grasses, weeds, and the introduced grass Urochloa mosambi censis, which had been sown on an adjacent experiment. Urochloa appear ed to be a more suitable companion species than speargrass for Verano.