Thursday, May 27, 2010

Livestock and Soil Compaction Quantified for Tropical Regions

Most agronomists would have been aware of earlier NZ studies on the influence of livestock on soil compaction and subsequent reductions in pasture or crop yields. New Zealand conditions are wet, and soils are quite different to northern Australia.

Yet, there is this perception that livestock cause severe soil compaction. More recent Australian studies would appear to refute these earlier claims, at least for our own conditions.

The following material was in the GRDC Magazine Ground Cover, no 86 in May 2010, and reproduced with permission.
Livestock compaction quantified

The compaction effects livestock have on soil may not be as problematic as previously thought, a Queensland-based research project is finding Livestock compaction may be less of a problem than many producers think, research by CSIRO’s Dr Lindsay Bell, who leads a GRDC-supported Southern Queensland Farming Systems (SQFS) project*, has found.

By examining numerous existing studies Dr Bell found:
• stock apply similar pressures on the soil to unloaded vehicles;
• treading by livestock can reduce soil porosity and infiltration rate, and increase soil bulk density and soil strength, although these consequences are mainly in the soil surface (the top five to 10 centimetres);
• despite these effects, reductions in crop performance have rarely been measured, possibly because effects are too small in magnitude or depth to influence plant growth significantly;
• crop simulations with reduced root growth and surface conductivity suggest that even in the most severe case a 10 to 15 per cent reduction in yield on average could occur;
• the risk of compaction can be reduced by removing stock during wet conditions and maintaining soil organic matter; and
• because compaction from livestock is shallow it is not long-lasting and is rectified by natural processes or tillage.

For example, one study Dr Bell looked at was Bruce Radford’s (Department of Environment and Resource Management, Biloela) two-year study on grey vertosol soils in Central Queensland.

It found that when cattle grazed sorghum stubble when the soil surface was dry there was no impact on subsequent crop growth or grain yield. But grain yield was reduced by 15 per cent when cattle grazed stubble when the soil was wet.

Similarly, a Western Australian study found there was no effect on grain yield from grazing a pasture the previous year. However, reduced wheat plant density did occur when sown no-till into areas that were continuously grazed the previous year.

As well as examining numerous studies, Dr Bell also undertook a modelling study using APSIM (Agricultural Production Systems Simulator) to investigate how sensitive simulated wheat yield is to livestock’s surface compaction effects. The study was modelled over a 50-year period (1956 to 2006) and explored the two main effects – reduced root growth and reduced surface water conductivity – independently and when combined. The model did not account for the possible effects of diseases or waterlogging.

Averaged over half a century, the results showed that mild surface soil compaction from livestock would result in reductions in grain yield of less than 10 per cent. These mild compaction effects are similar to most documented changes in soil conditions after treading by livestock, implying that in most cases the effects of compaction by livestock on crop performance are small. This is supported by the few studies that have investigated this experimentally, Dr Bell says.

Crop losses could be higher if more severe soil compaction occurred, especially if surface infiltration was greatly reduced and ground cover levels were low. Yield losses in the severe scenarios averaged 10 to 15 per cent, but might be as high as 30 per cent compared to the control paddock. However, these severe reductions in root growth and surface infiltration that were modelled are far more severe than most observed effects in the field.

The modelling also found that reductions in crop growth and yield were more affected by lower surface conductivity and rainfall infiltration rate rather than by reduced root growth in the surface layers. Hence, maintaining surface conditions such as stubble cover, which improves infiltration, decreased these effects.

Although this study used computer models to explore livestock compaction effects, there is a need for more experimental data to investigate how crops respond to changes in soil surface condition from livestock grazing.

* Dr Lindsay Bell leads the SQFS project ‘Short-term pastures in grain systems’.

Common sense says avoid too many stock on areas that are very wet and boggy.........but that effects are generally modest, and may not last too long. Interestingly, adequate soil organic matter mitigated compaction problems.

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