| The objectives of this research were to evaluate yield and yield components of 'CL161', 'Wells', and 'XL8' rice under various levels of barnyardgrass control and rice plant densities and in a weed-free environment, and to elucidate the fate of cyhalofop-butyl in barnyardgrass when applied alone or in combinations with other rice herbicides using High Performance Liquid Chromatography (HPLC). Results indicated that rice density did not affect panicle density, and panicle density increased 14 panicles m-2 for each 10% increase in barnyardgrass control. As rice density increased, biomass production, panicle weight, and harvest index decreased for all cultivars. As barnyardgrass control increased, biomass production, panicle weight, and harvest index increased. Rice density had no affect on rice yield, and rice yield increased 750 kg ha-1 with each 10% increase in barnyardgrass control for each cultivar. In a weed-free environment, canopy closure increased linearly for each cultivar at a rate of 0.4% for each heat unit accumulated and 3% for each 100 plants m-2 increase in rice density. Wells had a higher weight per panicle and harvest index than CL161. Wells and XL8 yield was similar, and greater than CL161 across all seeding rates. Seeding rates between 52 and 416 seeds m-2 did not affect yield, indicating that optimum yield may be possible with lower seeding rates than presently recommended. Based on these results, a 50% reduction in currently recommended seeding rates is suggested. HPLC analysis indicated that absorption and hydrolysis of cyhalofop-butyl in barnyardgrass to its phytotoxic metabolite, cyhalofop-acid, was rapid, and halosulfuron and triclopyr had no effect. Due to a possible interaction of propanil with an apoplast esterase enzyme, increased levels of cyhalofop-butyl and cyhalofop-acid were detected in barnyardgrass tissue, indicating that cyhalofop-butyl metabolism was hindered by propanil. |
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