Zero-Tillage Induces Reduced Bio-Efficacy Against Weed Species Amaranthus retroflexus L. Dependent on Atrazine Formulation

Zero-tillage (ZT) is a conservation soil management approach which relies more heavily on herbicide application for weed control than in ploughed soil. Changes in soil management can influence the structure and organisation of pore space in soil, which drives changes in the transport of particulates and dissolved substances. Formulation of pesticides can be used to change the delivery of active ingredients to soil: however, it is currently unknown how changing the formulation of an herbicide can influence the transport properties between ZT vs. ploughing. We investigated the bioefficacy of two formulations of the herbicide atrazine, a pre- and post-emergence herbicide that inhibits photosystem II. Bioefficacy was assessed using physical measures and survival analysis of an early photosynthesis-dependent weed species, Amaranthus retroflexus L., over time, and soil pore network structure was assessed by analysing three-dimensional images produced by X-ray Computed Tomography. Increasing the herbicide application rate generally improved bioefficacy, though it was reduced in soils managed under ZT. Under herbicide-treated ZT samples, survival time was higher, ranging from 13.4 to 18.2 days compared with 12.6 to 15.4 days in ploughed samples, the mean dry plant mass was higher, ranging from 0.5 to 2.5 mg compared with 0.05 to 0.68 mg in ploughed samples, and the mean total plant length was higher, ranging from 1.73 to 12.1 mm compared with 0.2 to 5.45 mm in ploughed samples. Changes in the soil pore network previously demonstrated to be indicators of preferential transport were correlated with measures of bioefficacy, including pore thickness and connectivity density. Reduced atrazine efficacy under ZT is problematic considering the inherent reliance on chemical methods for weed control, we suggest that pursuing formulation strategies to alleviate potential risks of loss via preferential transport may be fruitful.