Permanent pastures on a brigalow soil: The effect of nitrogen fertiliser and stocking rate on pastures and liveweight gain

This paper reports changes in pasture and animal productivity during the second phase of a grazing experiment established in 1968 on a fertile brigalow soil in subtropical south-east Queensland. The objective was to see how fertiliser application and stocking rate affected the gradual decline in productivity, or run-down, of pastures established on a cultivated brigalow soil. The sown pastures were a mixture of green panic (Panicum maximum var. trichoglume cv. Petrie) and rhodes grass (Chloris gayana cv. Pioneer). The treatments applied were: with and without superphosphate (187 kg/ha/yr); with and without N fertiliser (100 kg/ha/yr N); and 5 stocking rates ranging from 1.0-2.2 yearling steers or heifers/ha. The complete experiment ran from 1973-1985 and a restricted subset of treatments from 1985-1993, although no more N fertiliser was applied after 1985. Superphosphate application had no effect on pasture or animal production. For the first 3 years, all pastures were dominated by green panic and there was no effect of N fertiliser on liveweight gain. Over the next 3 years, there was a marked decline in the proportion of green panic in pastures receiving no N fertiliser and stocked at above 1 hd/ha. Concurrently, liveweight gain declined with increasing stocking rate on the unfertilised pastures and carcase quality was higher on the fertilised pastures. N application on pastures stocked at 2 hd/ha produced 1.1 kg liveweight gain per kg N. The highest stocking rate of 2.2 hd/ha was discontinued in 1982, as it could not be maintained in dry years, even with N fertiliser. In 1985, green panic remained dominant in the N-fertilised pastures at all stocking rates from 1.0-2.0 hd/ha, but only at the lightest stocking rate in the unfertilised pastures, The effect of stocking rate on liveweight gain pet head from N-fertilised pastures remained similar to that recorded at the start of the experiment in 1973. In contrast, there was a 3-fold greater decline per unit increase in stocking rate on the unfertilised pastures, Even after N fertilisation ceased in 1985, green panic dominated the lightly grazed and previously N-fertilised treatments until 1992, and liveweight gain was significantly higher in the previously fertilised pastures in 4 of the 8 years. Thus, on this fertile soil, run-down could be alleviated by using a low stocking rate, which enabled pasture composition and animal production to be maintained for more than 20 years following pasture establishment. At higher stocking rates, run-down could be averted by application of N fertiliser.