Discovery and development of a novel sustainable contact herbicide based on natural fatty acids

Resumen

In crop production, a particular focus is given to the extensive use of pesticides and, in particular herbicides. In the last years, many herbicides have been withdrawn from the market due to the search for less toxic products for the environment, human health, and non-target organisms, and with lower persistence, increasing the investment for research and development of more sustainable herbicide products. However, today, there are few commercial options that can effectively replace traditional herbicides. The agricultural community has positioned pelargonic acid (PA) as a good alternative to conventional contact herbicides and even for glyphosate in specific situations. PA is a non-selective contact herbicide that achieves good weed control efficacy without harming the environment. However, its use is limited in agriculture due to some drawbacks, such as its formulation problems, the rancid and unpleasant odour after its application and mainly the high spray doses at which it must be applied to be effective, having a high cost, compared to other herbicides available on the market. Therefore, the objectives of this thesis were first to develop a non-toxic compound with herbicidal activity, which could be an alternative to conventional herbicides. Next, the optimal application conditions of the new substance were studied: application rates, spray bar settings and climatic conditions were defined for a higher performance of the new substance. Similarly, adjuvants were used to enhance the activity of the new herbicide. The results of this doctoral thesis report the herbicidal activity of thirteen sustainable alternative compounds based on fatty acid (FA) esters, which could be sustainable alternatives for weed control. Its herbicidal effectiveness in weed control was evaluated on Digitaria sanguinalis (L.) Scop. and Solanum nigrum L. All the components showed an interesting contact herbicidal activity that is not related to the free acid of fatty acids, in addition to good physicochemical properties. The highest efficacy was obtained with methylated polyethylene glycol pelargonic acid ester (PA-MPEG), with six ethylene oxides. PA-MPEG is a liquid compound, soluble in water, non-volatile and consequently without rancid smell, unlike PA. Furthermore, it has excellent wetting properties, being able to be diluted directly in water and applied without further formulation efforts in contrast to PA. Additionally, it was suggested that PA-MPEG and other FA herbicides could cause desiccation symptoms due to the break of the water continuum at the site of evaporation in the intercellular spaces. PA-MPEG applied at different spray volumes provided equal or better weed control than commercial PA herbicides. The application of PA-MPEG at a rate of 12.8 kg of PA acid equivalent per hectare in a spray volume of 500 L per hectare was the most suitable rate for achieving excellent weed control (above 90 %) on medium-sized weeds (18-22 cm height; BBCH 18-22). These product doses and spray volume are relatively lower than those recommended on the label of a commercial PA formulation (273.4 g L-1). Since PA-MPEG and PA have contact herbicidal activity and their weed control efficacy depends on the spray coverage, the study reported and confirmed that application performed with spray volumes below 200 L ha-1 resulted in poor D. sanguinalis and S. nigrum control due to insufficient weed coverage and spray penetration into the weed canopy. The efficacy of PA-MPEG at a suboptimal use rate was improved when the application was performed at a lower boom height, which reduced the distance to the weed canopy. Warm weather conditions (T: 33 °C; 30 % r.h.) during the application and the following days also increased weed control efficacy with PA-MPEG. Both application aspects could reduce PA-MPEG use rates while keeping its effectiveness. The use of adjuvants added into the spray tank also caused an enhancement of PA-MPEG performance. Although several tank