Superphosphate and stocking rate effects on a native pasture oversown with Stylosanthes humilis in central coastal Queensland. 2. Animal production

Abstract

Liveweight changes of beef cattle were measured over eight years in a grazing experiment on a native pasture (Heteropogon contortus dominant) oversown with S. humilis (T.S.). The pastures were subjected to three levels of molybdenized superphosphate (F0 = nil; F1 = 125 kg ha-2 annually; F2 = 250 kg ha-1 annually plus an extra 250 kg ha-1 initially) in factorial combination with four stocking rates and two sowing methods for T.S. (ground sowing into spaced cultivated strips, or aerial sowing). Stocking rates were gradually increased during the experiment and for the last three years overlapping ranges were used for the three fertilizer levels; the overall range was then from 0.55 to 1.65 beasts ha-1. Superphosphate increased liveweight gain per head in all years. The main effect was in the period of weight gain and the difference between rates of gain on fertilized and unfertilized pastures increased over years. The mean for F, and F, ranged from 370 to 71 0 g head-1 day-1 compared with 270 to 470 g head-1 day-1 for F0. Except for the last year, there were liveweight losses in all three fertilizer treatments in winter; treatment differences then were mostly small, with significant effects in only three of the eight years. F, gave higher annual liveweight gains than F1 in three of the first six years, but by years seven and eight these two treatments were equal. Comparisons with pasture data described in a previous paper showed highly significant linear relations between annual gains per head and T.S. yield (log10 kg ha-1), and it is concluded that this is the main factor in the liveweight response to fertilizer. Increasing stocking rate depressed gain per head in F, in all years (mean of 35 kg head-1 for an increase of 0.5 beasts ha-1). A similar effect occurred in F1 and F2 in early years, but there was virtually no stocking rate depression in F2 in the last two years, and a significant increase for F1 in the final year. This effect is ascribed to the increase in T.S. yield in fertilized pastures at high stocking rates. Sowing method had no effect on animal production.