L. 't MANNETJE and R.M. JONES

Division of Tropical Crops and Pastures, CSIRO, Brisbane, Queensland, Australia

Abstract

Results are presented from a long-term grazing trial on a red/yellow podzolic soil at Narayen, south-east Queensland. The trial studied changes in pasture yield, composition and quality, and also in liveweight gain of cattle, over a 17 year period (1969–86). The improved pastures, fertilized with superphosphate, were based on buffel grass (Cenchrus ciliaris), either with lucerne (Medicago sativa), or lucerne and Siratro (Macroptilium atropurpureum), or alone with or without N fertiliser. Areas of native pasture dominated by spear grass (Heteropogon contortus), either unfertilised or fertilized with superphosphate, were used as controls. Stocking rates and grazing method were varied over the 17 years.
Lucerne died out after 3 years and the former lucerne pastures were then oversown with Siratro. Buffel grass without a source of N gradually disappeared and was replaced by unsown species. In contrast, buffel grass pastures fertilised with N or sown with Siratro remained weed free and productive. During the first 10 years (1968–78), the Siratro percentage in the pastures often averaged 20% on a year round basis. Siratro density remained high (≥ 5 plants/m²) and the associated buffel grass had higher levels of N than buffel grass without Siratro or N fertiliser.
Liveweight gains of steers per hectare from Siratro-based and N fertilised buffel grass were 4.5 and 6.5 times higher than from unimproved native pastures over the first 10 years. This increase was a result of the higher stocking rate and the 40% higher liveweight gain/head on the improved pastures. Steers on native pasture and on buffel grass without a legume or N fertiliser lost weight in winter, whilst those on buffel grass with N fertilizer or Siratro did not. Continuous and rotational grazing on buffel grass-Siratro pastures gave similar liveweight gains. The overall response of N fertiliser on buffel grass pastures was 1.5–2.0 kg liveweight gain per kg of N.
During the last 7 years (1978 to 1986) there was a decline in the yield, plant density and soil seed reserves of Siratro. However, there was a residual effect of previous N input from Siratro on buffel grass persistence and productivity and also on liveweight gain.