NITROGEN RATE AND STOCKING RATE EFFECTS ON STEER GAINS FROM GRAZED IRRIGATED PANGOLA GRASS IN THE ORD VALLEY, WESTERN AUSTRALIA

Annual liveweight gain of steers grazing flood-irrigated, fertilised pangola grass in the Ord Valley, Western Australia, was modelled as a function of nitrogen rate (90-350 kg/ha) and stocking rate (5.2-12.6 steers/ha). Steer gains decreased linearly with increasing stocking rate (SR) at each nitrogen rate (NR). Increasing NR resulted in higher steer gains, particularly at the low levels of SR. The model predicted an increase in the slope (b values) of the linear relations between steer gains and SR with an increase in NR. As a consequence, SR for maximum gain/ha.year only varied between 6 and 7 steers/ha within the NR used. Responses to SR and NR for gains/ha were both quadratic, and the model predicted a maximum gain of 1266 kg/ha when SR was 6.2 steers/ha and NR was 420 kg/ha.year. Within the experimental range, maximum gain was 1233 kg/ha when SR was 6.6 steers/ha and NR was 350 kg/ha.year. Steer gain varied seasonally, even in this tropical, irrigated environment. Gains were highest in January-March and lowest in June-July. Increasing stocking rate had the greatest depressing effect on liveweight gain during the cool season with b values, for the relation between steer gain and stocking rate at the 220 kg N rate, varying from-1 g/day in November to -94 g/day per unit of SR in July.