Weeds and rice response to post-emergence applications of pendimethalin alone and in herbicide mixtures in water-seeded rice

Herbicides are an important tool for weed management in water-seeded rice (Oryza sativa L.); however, the reduced efficacy from available herbicides and the lack of new herbicides have encouraged research on new use of older herbicides for this production system. This research evaluated weed control and water-seeded rice response to pendimethalin applied post-emergence in a field trial. Pendimethalin was applied alone and in herbicide mixtures at 1.1, 2.3 and 4.4 kg ai ha−1 with three graminicide or broad-spectrum foliar herbicides to rice at the 4- to 5-leaf stage. A greenhouse study was conducted to evaluate rice response to pendimethalin applied at 1.1 and 2.3 kg ai ha−1 at the 4- to 5-leaf stage grown under 5-cm and 10-cm flood depth conditions. Grass weed control at 14 days after treatment was 68%–86% when pendimethalin was applied in herbicide mixtures compared to 48%–63% when applied alone. The mixtures with bispyribac-sodium and propanil provided broad spectrum control of grass, sedge, and broadleaf weeds unlike the mixture with cyhalofop-butyl, a graminicide herbicide. All treatments resulted up to 8% of visual rice injury. Rice tiller counts and grain yield were not affected by pendimethalin. The 5-cm and 10-cm flood depth, in the greenhouse study, affected shoot length, root length, and root biomass but not shoot biomass averaged over pendimethalin applications; however, rice was normal by 14 or 21 days after treatment. Only shoot length was reduced by 12% at 21 days after treatment at 3.4 kg ai ha−1 of pendimethalin. The results from these studies demonstrate pendimethalin can be a potential herbicide for water-seeded rice and does not cause injury of concern on rice when applied at the 4- to 5-leaf stage rice.