Assessing clethodim spot spraying applications for control of problematic weedy rice and other grasses in California rice fields

Spot spraying applications offer the opportunity to target specific weeds in a field, while simultaneously reducing herbicide usage and increasing the long-term efficacy of chemical control options. The study is focused on controlling California weedy rice accessions (Oryza spp.) and problematic grass weeds with a spot spray application of clethodim in a flooded rice system. The efficacy of incorporating nonionic surfactant to clethodim applications was also assessed. Dose-response experiments were carried out in a greenhouse on five weedy rice accessions, common grass rice weeds, and cultivated rice varieties L207, M105, M206, M209, M211, and S102 to determine the dose needed to affect these populations. Clethodim was applied in a field setting to assess spot spraying efficacy, the possibility of herbicide dispersion in the water, and crop injury. Clethodim successfully controlled weedy rice and grasses in the greenhouse. The effective rates to control 90% of the five test populations (ED₉₀) were between 51 and 74 g ai ha⁻¹ clethodim for weedy rice accessions. Adding nonionic surfactant to clethodim increased its efficacy by 1.6- to 1.9-fold. Cultivated rice varieties did not exhibit any tolerance to clethodim, however, spot spraying applications at 150 g ai ha⁻¹ clethodim did not cause any dispersion in the field. Clethodim spot spray application was effective both at the three- to four-leaf growth stage and tillering growth stage for weedy rice.