The Brigalow Catchment Study: III.* Productivity changes on brigalow land cleared for long-term cropping and for grazing
B. J. Radford A D , C. M. Thornton A , B. A. Cowie B and M. L. Stephens CA Queensland Department of Natural Resources and Water, LMB 1, Biloela, Qld 4715, Australia.
B Queensland Department of Natural Resources and Water, PO Box 1762, Rockhampton, Qld 4700, Australia.
C CSIRO Livestock Industries, PO Box 5545, Rockhampton, Qld 4702, Australia.
D Corresponding author. Email: bruce.radford@nrw.qld.gov.au
Australian Journal of Soil Research 45(7) 512-523 https://doi.org/10.1071/SR07062
Submitted: 21 May 2007 Accepted: 5 October 2007 Published: 12 November 2007
Abstract
Productivity of grain crops and grazed pastures inevitably declines without soil nutrient replacement and may eventually make these enterprises unprofitable. We monitored these declines in north-eastern Australia during 23 years after clearing 2 of 3 adjacent brigalow catchments, in order to define the productivity levels of developed brigalow land over time. One catchment (11.7 ha) was used for grain production and another (12.7 ha) for beef production from a sown buffel grass pasture. There was no upward or downward trend in annual rainfall amounts throughout the study period. In the cropped catchment, grain yield from 14 winter crops without added nutrients declined significantly in 20 years from 2.9 to 1.1 t/ha.year on the upper-slope clay soil (92 kg/ha.year) and from 2.4 to 0.6 t/ha.year on the Sodosol (88 kg/ha.year). Crop production per year declined by 20% between 2 successive 10-year periods. Wheat grain protein content also declined with time, falling below the critical value for adequate soil N supply (11.5%) 12 years after clearing on the Sodosol and 16 years after clearing on the clay soil. Such declines in grain quantity and quality without applied fertiliser reduce profitability. The initial pasture dry matter on offer of 8 t/ha had halved 3 years after clearing, and a decline in cattle liveweight gain of 4 kg/ha.year was observed over an 8-year period with constant stocking of 0.59 head/ha. Due to fluctuating stocking rate levels of 0.3–0.7 head/ha over the trial period, liveweight productivity trends are attributed to the multiple effects of stocking rate changes and fertility decline. The amount of nitrogen exported from the cleared catchments was 36.1 kg/ha.year in grain but only 1.6 kg/ha.year in cattle (as liveweight gain). Total soil N at 0–0.3 m declined by 84 kg/ha.year under cropping but there was no significant decline under grazing. The soil nutrients removed during grain and beef production need to be replaced in order to avert productivity decline post-clearing.
Additional keywords: beef cattle, brigalow, fertility decline, grain yield, grain protein, soil organic carbon, soil N.
Acknowledgments
We thank the managers and staff of Brigalow Research Station from 1965 to 2006 for providing the study area and assisting with the planting, tillage, spraying, and harvesting operations and for measuring the cattle weights. We thank the staff of the Chemistry Centre of the Department of Natural Resources and Water for the soil and grain analyses. A complete list of names of people who helped generate these results is given in paper I of this series (Cowie et al. 2007, this series).
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