Managing Phytotoxicity of Soil-borne Herbicide Residues in Grain Cropping Systems

Win Win Pyone

Herbicides residues persisting in soil from applications used to control weeds in previous crops can inhibit non-target crop production but there is a shortage of information on critical threshold levels in different soil types for a range of herbicides and crop species. Moreover, the toxicity thresholds of herbicides vary with soil properties that alter their bioavailability. Thus, this glasshouse study was conducted with these objectives: (1) to determine robust phytotoxicity thresholds of diuron on three common crop species, (2) to evaluate how loamy sand soil can change the toxicity thresholds relative to an inert sand, (3) to define toxicity thresholds levels of four priority soil residual herbicides on six common winter grain crops, (4) to evaluate the phytotoxicity of mixtures of herbicides compared to single herbicides on wheat and canola growth, and (5) to assess the influence of key soil physicochemical properties on the degree of diuron phytotoxicity on canola growth. The phytotoxicity of diuron was evaluated by exposing canola, chickpea, and wheat to a wide range of concentrations applied to inert sand and loamy sand. The log-logistic nonlinear regression model was used to estimate effective dose (ED50) of diuron required to cause a 50% reduction in crop growth based on dose-response curves. Canola was the most sensitive to diuron in sand followed by wheat and chickpea. ED50 values of diuron for canola shoot biomass reduction in sand and loamy sand (0.03 and 0.07 mg kg-1) and ED50’s for root biomass (0.01 and 0.06 mg kg-1) indicates much higher phytotoxicity in sand than loamy sand. The ED50’s for wheat shoot biomass (0.11 and 0.24 mg kg-1) in sand and loamy sand, respectively, and for root growth (0.14 and 0.19 mg kg-1) were lower than label rates and previously estimated average and maximum diuron residue loads (0.17 and 0.29 mg kg-1) in Western Australia paddocks. The larger ED50 values of diuron in loamy sand can be attributed to higher soil organic matter and cation exchange capacity that reduced bio-available diuron levels. A dose response experiment was conducted to measure shoot and root responses in six common crops (canola, chickpea, fieldpea, lentil, lupin and wheat) to soil rates of four common herbicides (clopyralid, pyroxasulfone, propyzamide and trifluralin). Lentil emergence was highly sensitive to clopyralid (29 μg kg-1 for a 50% reduction, ED50) while wheat was sensitive to propyzamide and trifluralin, with complete inhibition at 100 μg kg-1 and 375 μg kg-1, respectively. Shoot and root parameters of the legumes, except lupin, were significantly reduced by clopyralid, with ED50 values ranging between 3-27 μg kg-1. Canola was sensitive to pyroxasulfone, with ED50’s for shoot and root biomass at 21 and 8 μg kg-1, respectively. Pyroxasulfone also severely reduced root length of all tested crops (ED50 values 6-53 μg kg-1). Root and shoot growth in wheat was most susceptible to propyzamide followed by trifluralin. The phytotoxicity of diuron, diflufenican, and trifluralin alone and in combinations on wheat and canola was investigated. Increasing herbicides doses lead to reduced plant growth and chlorophyll content in crops, with diuron exhibiting higher toxicity on canola than wheat. Diuron doses (0.03 and 0.15 mg kg-1) significantly decreased canola shoot biomass (73-91%), root length (69-90%), and chlorophyll content (62-87%), while diflufenican had similar effects on shoot growth and chlorophyll inhibition in both crops. Trifluralin significantly suppressed wheat growth more than canola. Most combinations of herbicides exerted greater impacts on canola, except for trifluralin (1.25 mg kg-1) mixed with all diuron doses which affected both species similarly. Most mixtures exhibited additive effects, except for an antagonistic interaction between diuron (0.15 mg kg-1) and trifluralin (0.25 mg kg-1) on shoot growth, while synergism was evident between this trifluralin dose and all diuron doses on wheat root length. Synergistic or antagonistic effects varied between combinations of diuron with diflufenican or trifluralin on canola root and shoot growth and chlorophyll inhibition based on concentrations. Soil organic carbon (OC) (3.8 to 26 g kg-1), clay content (40 to 195 g kg-1) and soil pH levels (4.4-5.1, 5.3, 6.3, and 7.3) in WA field soils were investigated for their effects on phytotoxicity of diuron for canola. The toxicity thresholds of diuron required to reduce shoot growth by 10, 20, and 50% (ED10, ED20, and ED50) were strongly and positively correlated with soil OC, cation exchange capacity (CEC), and exchangeable calcium (Exc. Ca), whereas clay content and soil pH did not significantly affect the degree of phytotoxicity in the ranges evaluated. Approximately 7 times more diuron was required to produce ED20 at 26 g OC kg-1 than at 4.2 g OC kg-1. Except for the highest OC content soil, ED20 concentrations were 1.8– 3.7 times lower than typical application rates. Among linear and non-linear regression models of simple and multivariate analysis with OC, CEC, and Exc.Ca, non-linear regression with OC alone as an independent variable predicted phytotoxicity best. In conclusion, some herbicides investigated in this study showed considerable phytotoxic effects on crops at doses below label rates and at concentration less than those found in a recent pre-sowing field survey, indicating a risk of early crop damage and reduced yield of non-target crops from residual herbicides in soils. The soil OC content is the main factor that determined the bioavailability of diuron herbicide. The herbicide mixtures in this study showed additive, antagonistic, and synergistic effects depending on the mode of actions of herbicides, concentrations, and tested crops.

Managing Phytotoxicity of Soil-borne Herbicide Residues in Grain Cropping Systems

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