Weed control in an intermittent irrigation system of rice production and environmental effects on efficacy with cyhalofop-butyl

Field experiments were conducted to evaluate the effects of an intermittent irrigation system for rice production on grass weed control, water usage reduction, rice grain yield, and yield components. Broadleaf signalgrass and propanil-susceptible barnyardgrass control was generally greater than 82% for all herbicide treatments 70 days after emergence (DAE). Propanil-resistant barnyardgrass control ranged from 53 to 100% 70 DAE. The lowest moisture regime using a volumetric water content (&thetas;v) of 20% reduced cumulative water usage by 22 to 28%. Rice grain yield was not adversely affected by intermittent irrigation using a &thetas;v of 34 to 48% compared to the current season-long flooding method. The reductions in yield by intermittent irrigation using a lower &thetas;v of 20 and 27% was explained by reductions in total florets, fertile floret grain ratio, and floret weight.;The effects of temperature and growth stage on efficacy of cyhalofop-butyl, fenoxaprop, and fenoxaprop + isoxadifen applied at three different rates were evaluated in a growth chamber experiment. When temperature was reduced from a day/night temperature of 32/22 C to 16/14 C there was a reduction in broadleaf signalgrass and propanil-susceptible and -resistant barnyardgrass control when treated at the four- to five-leaf growth stage, but not at the two- to three-leaf stage. Cyhalofop-butyl activity was generally affected by reductions in temperature more than the other two herbicides.;A second glasshouse experiment evaluated the effects of soil moisture content on absorption and translocation of 14C-cyhalofop-butyl in broadleaf signalgrass and propanil-resistant barnyardgrass. Reducing soil moisture content from 36% gravimetric water content (&thetas; _w) to 9 or 18% &thetas;_w reduced absorption of 14 C-cyhalofop-butyl by 14 to 25% into broadleaf signalgrass during the 6- and 24-hour exposures. After 72- and 168 hour exposure, no significant differences in absorption were observed at the various soil moisture contents. Decreasing soil-moisture content to 9 or 18% &thetas;_w reduced total translocation of 14C-cyhalofop-butyl throughout broadleaf signalgrass. 14C-cyhalofop-butyl absorption into propanil-resistant barnyardgrass was reduced only by the 9% &thetas;_w at the 72- and 168-hour exposures. Soil moisture content did not affect translocation of 14C-cyhalofop-butyl in propanil-resistant barnyardgrass.