Impacts of soil damage by grazing livestock on crop productivity

Soil damage by livestock on cropped land is a concern for mixed crop-livestock producers. The hypothesis that livestock-induced soil damage does not effect subsequent crop productivity was investigated in three ways; a review of existing literature; two experimental studies in southern Australia; and based on these, use of a farming system model to simulate likely consequences of changes to soil conditions on crop productivity under a variety of seasonal conditions. The literature suggests that treading by livestock increases soil strength and bulk density and reduces macro-porosity and infiltration rate. Yet, the effects are confined to the soil surface (0-0.1 m) and are short-lived due to amelioration through natural processes or tillage. Despite the potential for soil damage, few studies report yield penalties in following crops, possibly because effects are too small in magnitude or depth to influence plant growth significantly, or because of the highly season-dependent nature of crop response. Two recent field experiments in southern Australia also found no effect of sheep grazing on subsequent crop grow :h or yield, despite evidence of surface compaction and reduced infiltration rate. Using reviewed literature and this experimental evidence, we adjusted soil parameters in APSIM (Agricultural Production Systems Simulator) to reflect mild, moderate and severe reductions in surface root growth and surface water infiltration. The predicted sensitivity of wheat crop productivity to these simulated impacts was investigated at 6 locations in eastern Australia. Consistent with experimental evidence, predicted average grain yield was reduced by <10% (+3 to -10%) under mild and moderate scenarios typical of those reported experimentally. Crop yields were reduced by lower rainfall infiltration and fallow efficiency in locations with summer-dominant-rainfall, and by reduced root exploration at locations with winter-dominant-rainfall. Lower residue cover levels amplified reductions in rainfall infiltration, especially reducing the accumulation of soil water during summer fallow. Predicted grain yield was only reduced substantially (up to 43% on average) when severe changes in soil parameters were implemented (e.g 90% reduction in rate of root exploration in surface layers). Overall, most experiments support our hypothesis that grazing livestock have little effect on subsequent crop yields. Long-term simulations suggest that soil impacts generating large reductions in root growth and infiltration rate are required to reduce subsequent crop yields substantially. Such impacts are unlikely where current best-practice grazing management occurs, but would be possible on structurally degraded soils where surface cover is allowed to fall below critical levels. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.