Effects of Grazing Sorghum Stubble on Soil Physical Properties and Subsequent Crop Performance

Two grazing trials were conducted on a Vertosol in central Queensland to assess the effects of stubble grazing by cattle on soil properties and subsequent crop performance. Two adjacent contour bays were selected for two treatments (grazed and ungrazed) in each trial. Both trials were conducted following a grain sorghum crop. In trial 1 (during 1996) the surface soil was dry throughout grazing. In trial 2 (during 1998) the surface soil became saturated during grazing after 125 mm of rain. Soil physical properties including bulk density, shear strength, cone index and hydraulic conductivity were measured pre-and post-grazing in both trials and the response of wheat crops sown after grazing was assessed. The area was then double-cropped to sorghum to measure any further residual impacts. After grazing on dry soil, hoof marks were barely evident and no significant change was found in any soil physical property or the establishment or yield of a subsequent wheat crop. After grazing on saturated soil, there were visible hoof marks 49 mm deep and increases in soil shear strength, cone index and drawbar power requirement. The following wheat crop had reduced dry matter at 32 days and reduced grain yield. There was no immediate change in ground cover and no effect on wheat establishment following direct-drilling 37 days after the cattle were removed. In a sorghum crop following the wheat crop, there were no carryover effects on any soil physical property or on crop yield. We conclude that under a similar regime to that of trial 1 (dry soil and no rainfall during grazing), adverse effects of stubble grazing are unlikely. It follows that if grazing can be restricted to times when the surface soil is dry enough to minimise compaction by animals, there is little risk of adverse effects on subsequent crop performance.