Split herbicide application: A practical approach to enhancing water quality

Herbicide contamination of surface and groundwater poses a risk to water quality, particularly in regions where agricultural practices rely on herbicides to help maintain crop yields. Herbicides such as 2-methyl-4-chlorophenoxyacetic acid (MCPA; phenoxy-carboxylic acid) and clopyralid (Pyridine-carboxylic acid) are commonly used to control broadleaf weeds, and consequently they are frequently detected in European water bodies. This presents significant challenges to the achievement of “good” water quality status by European Union (EU) member states. This study evaluated the effectiveness of a novel approach to mitigate MCPA and clopyralid losses while maintaining agricultural efficacy, namely split herbicide application. Split herbicide applications involve multiple applications of herbicides across a season that cumulatively equal the same amount applied in one maximum single dose. The experimental approaches employed included controlled rainfall simulations and laboratory soil column trials to determine MCPA and clopyralid fate due to runoff and leaching, respectively, in agricultural soils. The results demonstrated a substantial reduction in total herbicide loss in both surface runoff and leachate, compared to conventional single-dose applications. Similarly, leaching experiments confirmed reduced herbicide mobility, particularly in organomineral soils, which exhibited enhanced adsorption capacity compared to mineral soils. Microbial community analysis indicated a potential biological contribution to the reduction in total herbicide loss with a split application approach. Soils treated with split herbicide applications exhibited increased diversification of classes of the bacterial tfdA gene, which is involved in the degradation of MCPA. This adaptive microbial response highlights the potential for split applications to enhance natural attenuation processes, thereby reducing the environmental persistence and transport of herbicides. The findings align with the objectives of the EU pesticide regulations, including the Water Framework Directive and Sustainable Use Directive, which aim to balance agricultural productivity with environmental sustainability. Split herbicide application offers a pragmatic solution, addressing key limitations of existing mitigation strategies such as buffer zones and precision agriculture, which often face implementation barriers due to cost or site-specific constraints. Despite its potential, the adoption of split applications requires an in-situ study to validate its efficacy in real life situations. Further research is needed to assess long term efficacy across diverse agroecological contexts, including variations in climate, crop type, and farming practices. Additionally, integrating split herbicide applications with complementary strategies, such as biochar amendments or constructed wetlands, could be investigated to evaluate their combined effectiveness. This thesis contributes to the scientific understanding of herbicide management by providing robust experimental evidence for the efficacy of split applications. The results support the adoption of this approach as a viable and sustainable practice for reducing herbicide contamination in surface and groundwater. By mitigating environmental risks while maintaining weed control efficiency, split herbicide applications represent a critical advancement in the pursuit of sustainable agriculture and water resource protection.

Publisher

University of Galway

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CC BY-NC-ND

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University of Galway Theses (Research Masters Theses)