UC Davis

California has approximately 200,000 ha of land in rice (Oryza sativa L.) production centered in the Sacramento and Northern San Joaquin Valleys. Herbicide-resistant weeds are a continuing challenge in California rice which has led to the reduction of weed control from available herbicides. Our greenhouse herbicide resistance screening showed that pendimethalin controlled herbicide-resistant grass populations. Currently, there are no group 3 mode of action herbicides used in water-seeded rice; therefore, we evaluated pendimethalin for use in water-seeded rice. Three pendimethalin formulations applied at three rates and three timings were evaluated for rice response and weed control in 2020 and 2021 field studies. Additionally, rice cultivar response was examined in greenhouse studies across five common California rice cultivars to two pendimethalin formulations at two rates and application timings. The results demonstrated rice injury was reduced when pendimethalin was applied at 10 days after seeding (DAS) (3-leaf stage rice) and 15 DAS (4-leaf stage rice) compared to a 5 DAS (1-leaf stage rice) application; however, weed control was reduced up to 34% at these later timings. The cultivars demonstrated 68% reduced stand establishment when pendimethalin was applied at 5 DAS. When treated at 10 DAS, all cultivars except ‘M-205’ were similar to the nontreated in stand establishment and dry biomass. Cultivars with increased seedling vigor were more tolerant to the pendimethalin post-emergence applications. The reduction in weed control from a 15 DAS application compared to earlier timings in the field study led to a further field study evaluating weed control and rice response from pendimethalin applied post-emergence alone and in herbicide mixtures at 1.1, 2.3 and 4.4 kg ai ha-1 in 2022 and 2023. When pendimethalin was applied in mixtures, grass weed control was 68% to 86% compared to pendimethalin applied alone which only control weeds 48% to 63%. All treatments resulted in less than 8% visual rice injury and tiller counts and grain yields resulted similar to the standard treatment of clomazone applied at day of rice seeding. Pendimethalin did not cause injury of concern on rice when applied at the 4- to 5-leaf stage water-seeded rice. In 2021, a study was carried out to determine pendimethalin behavior in flood water of a water-seeded rice field. In this study, three pendimethalin formulations were applied at three rates onto the water and water samples collected at 1, 3, 5, 10 and 15 days after treatment for analysis. The residue concentrations at 1 day after treatment ranged from 3.0 to 125.6 parts per billion and dissipated quickly over time. A first-order dissipation model fit the data and calculated half-lives were 2.3 to 3.5 days from the capsule suspension, 0.6 to 0.7 days from the emulsifiable concentrate and 3.5 to 6.9 days from the granule formulation. Pendimethalin did not result in residue concentrations of environmental concern. The results can assist in generating management tactics to ensure weed control activity and reduce off-target contamination. Overall, the results from this research provide supporting data to pursue registration of pendimethalin for water-seeded rice and add to the body of knowledge of rice herbicide tolerance and herbicide dissipation in water-seeded rice.