Pasture production, pasture quality and their relationships with steer gains on irrigated, N-fertilised pangola grass at a range of stocking rates in the Ord Valley, Western Australia

Pasture presentation yield, composition and quality attributes were measured on grazed, irrigated pangola grass pastures fertilised at a range of nitrogen rates (NR; 90-350 kg/ha/yr) and continuously stocked at a range of stocking rates (SR; 5.2-12.6 steers/ha) over 2 years on the Ord Irrigation Area at Kununurra, Western Australia. A central composite design was used to model the responses obtained. Pastures remained pangola-dominant throughout under all treatments. Mean presentation yields of dry matter (DM), digestible dry matter (DDM), nitrogen (N), phosphorus (P) and sulphur (S) all declined with increasing SR at each NR. NR increased yields with responses inversely related to SR. The %N of the pastures increased linearly with SR from a mean of 0.7%N at 5.2 steers/ha to 1.5%N at 12.6 steers/ha. By contrast, the effect of NR on pasture N% was small at any given SR. The %P of the pastures also increased with increasing SR but declined with increasing NR, especially at high SR. The predicted highest %P of 0.26% occurred at 50 kg N/ha/yr at a SR of 12.6 steers/ha. Sulphur concentration followed a similar trend to that of P, with highest %S of 0.14% also at 50 kg N/ha/yr with 12.6 steers/ha, and the lowest S concentration of 0.07% with 350 kg N/ha/yr and 5.2 steers/ha. In vitro dry matter digestibility (DMD) of total pasture was generally low (< 55%) and varied seasonally, with lower values in the cool dry season and higher values in spring and summer. Mean values tended to increase with SR, especially at low NRs. Annual steer gains/head were positively correlated with yields of DM, DDM, N, P and S (r >= 0.92***) but negatively related to the quality attributes of %N, %P, %S and %DMD. Both DM yield and LWG/head were closely related to pasture height (r >= 0.95***). Moderate stocking rates of around 5 steers/ ha enabled pastures to produce well and remain stable, giving high steer gains/head and also a satisfactory economic outcome (Izac et al. 1990).