Water-Seeded Rice Response to Pendimethalin Applied at Different Rates and Timings

Currently, there is a limited number of herbicides available in California water-seeded rice, with widespread resistance to most of these herbicides. Because no resistant grasses showed resistance to pendimethalin, a series of studies were conducted to evaluate water-seeded rice response to pendimethalin. In a field study conducted at the Rice Experiment Station at Biggs, California, in 2020 and 2021, three pendimethalin formulations, a granule (GR), emulsifiable concentrate (EC), and capsule suspension (CS), were applied at 1.1, 2.3, and 3.4 kg ai ha-1 rates, and at 5, 10, and 15 days after seeding onto water-seeded rice. In addition, a greenhouse study was conducted to examine the response of five common California rice cultivars to GR and CS formulation applications. Echinochloa control levels were reduced at the 15 days after seeding timing after use of EC and CS formulations compared to earlier timings. In both years, rice grain yields were increased by 3,014 kg ha-1 after application of pendimethalin at 3.4 kg ai ha-1 when applied at 15 days after seeding compared to 5 and 10 days after seeding, and similar to 1.1 kg ai ha-1 applications. The GR and CS were safer formulations based on a reduction in injury and an increase in grain yields compared to the EC. Differences in seedling vigor across cultivars appeared to incur an advantage after a pendimethalin application. However, most cultivars evaluated for stand reduction and dry biomass demonstrated tolerance to GR and CS formulation applications only after rice reached the 3-leaf stage. In contrast, an application at 1-leaf stage rice reduced stand up to 68%. Application rate, timing, and formulation are important factors to consider if the use of pendimethalin in water-seeded rice is to be pursued.