The phytotoxicity of soil-applied herbicides is enhanced in the first-year post strategic deep tillage

Abstract

The sandplain soils of WA are inherently fragile with surface layers that are very low in organic matter and clay content. The advent of minimum- and no-till farming has seen the increase in frequency and intensity of cropping on these soils. However, a combination of soil physio-chemical constraints and agronomic issues remain a challenge to the sustainability of cropping systems on them. These constraints include sub-soil compaction, soil water repellence, sub soil acidity and herbicide resistant weeds. Strategic deep tillage such, as soil inversion and deep soil mixing, have been shown to ameliorate these multiple constraints and dramatically increase crop production. For WA soils, an increase in herbicide usage is correlated with a decrease in regular tillage, and how the two interact is imperfectly understood. As a result, current herbicide strategies and rates are designed to perform optimally in a minimum tillage environment. Two field experiments were established to compare crop damage from a range of commonly used pre-emergent herbicides when grown in soil that remained under minimum tillage, was deep mixed or inverted. These trials demonstrated that both strategic tillage methods significantly changed the soil surface composition that would be expected to directly affect the bioavailability of some herbicides. Two commonly used herbicides, Metribuzin and Diuron, detrimentally impacted crop performance following tillage in both trials. These same treatments reduced yield by a greater extent on both the soil inversion and deep mixing treatments (p < 0.001). No other herbicides, when applied at either label or triple label rates, significantly impacted yield on any of the soil treatments. There was a substantial crop production benefit from strategic deep tillage at Esperance but not at Geraldton. These results reflect that the influence of deep tillage on the toxicity of herbicides is highly dependent upon soil properties and rainfall.