BACKGROUND Ever since the beginning of agriculture, yields have been threatened by weeds. Chemical weed control is far more effective and economical than other methods. The frequent use of herbicides has led to environmental and human health concerns, resulting in the banning of several herbicides and challenges for the future of important active compounds such as glyphosate. RESULTS The herbicidal activity of sustainable alternatives based on certain esters of fatty acids (FA), the action of which is unrelated to the free acid, on common weeds is assessed and reported. The 13 derivatives of FA showed better physicochemical properties than pelargonic acid‐based herbicides. All the reported compounds have phytotoxic activity, the highest efficacy being displayed by the methyl end‐capped polyethylene glycol (mPEG) ester of pelargonic acid having 6EO (ethylene oxide). This mPEG ester showed equal or better phytotoxicity than the pelargonic acid benchmark at reduced application rate and spray volume. The active compound is a liquid at ambient temperatures, has no bad smell and is not volatile, in contrast to pelargonic acid. Notably, this active compound can be the final product, can be sprayed without adjuvants and is relatively easy to co‐formulate. CONCLUSION A new lead substance is presented that is a sustainable alternative to current contact herbicides. In particular, it has potential application on railways, in precision agriculture and as a harvest aid. Its good performance and technical properties suggest this mPEG ester group may also overcome the volatility‐related problems of other organic acids such as auxins. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Only a limited number of contact herbicides exist in agricultural production. Although systemic herbicides are also more efficient for suboptimum spray coverage with long-lasting weed control, contact herbicides provide several advantages. For example, there is no translocation to fruits, roots, and other crop parts and neither have residual activity nor minor risk for spray drift damage. Synthetic products in their use often imply toxicological or other drawbacks. Whereas, natural fatty acids, particularly pelargonic acid (PA), have contact activity and are safer for home and garden use. We recently described a methyl capped polyethylene glycol ester of pelargonic acid (PA-MPEG) as a novel contact herbicide. Both PA-MPEG and PA are applied at high rates per hectare to achieve excellent weed control. We herein report about potential additives to increase PA-MPEG efficacy. Herbicidal active, 1-decanol, and the non-phytotoxic alkylated seed oil-based adjuvant, HastenTM, improved performance by achieving better results than the PA benchmark. Both PA-MPEG and PA appear to mainly act by the disintegration of bio-membranes besides having effects on transpiration. The main suggested effect is desiccation due to cutting the water continuum at the site of evaporation in the intercellular spaces. The synergistic action of the adjuvant HastenTM and its practical uses are also discussed.

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