Imazethapyr: red rice control and resistance, and environmental fate
Imazethapyr was recently approved for use in rice, but limited information is available
regarding its efficacy, environmental fate or potential red rice resistance. Therefore,
experiments were conducted to 1) determine the effect of flooding time, and stage of
imazethapyr application in red rice control, 2) assess the acetolactate synthase resistance
to imazethapyr on red rice ecotypes, 3) determine the relative photolysis of imazethapyr,
and 4) determine the effect of soil and moisture on imazethapyr adsorption and
When imazethapyr was applied in sequential application of PRE followed by a POST
application, to achieve >95% red rice control, flood needed to be established within 14
DAT when imazethapyr was applied EPOST, and 7 DAT when imazethapyr was applied
LPOST. Delaying the flood up to 21 DAT reduced rice grain yield for both EPOST and
LPOST application timings.
Based on enzymatic activity, the mean I50 values were 1.5, 1.1, 1.5, 1.6, 20.8, and
590.6 mM of imazethapyr, respectively, for LA 5, MS 5, TX 4, ??Cypress??, ??CL-121??, and
??CL-161??. CL-161 was 32 times more resistant than CL-121, and at least 420 times
more resistant than the average of the red rice ecotypes and ??Cypress??. Results from the
ALS assay showed that red rice ecotypes and Cypress had high susceptibility to
imazethapyr when compared with the tolerant CL-121 and the resistant CL-161.
Measurable enzymatic tolerance to ALS-inhibiting herbicides has not yet developed in
these red rice ecotypes. Imazethapyr quantum yield (fI ) was 0.023 ?? 0.002 while the hydroxyl radical rate
constant ( I
OH k?? ) was 2.8 ?? 0.44 x 1013 M-1 h-1. These results show that imazethapyr is
susceptible to both direct and indirect photolysis. The results also show that
imazethapyr photolysis in paddy water will be affected by turbidity due to its impact on
the availability of sunlight to drive direct and indirect photolysis reactions.
Imazethapyr was more available and more concentrated in sandy soil. With higher
amounts of water in soil there was greater amount of imazethapyr in soil solution and a
lower concentration of herbicide due to dilution. The double centrifuge method
provided a better estimate of plant available herbicide.
Advisor:Senseman, Scott A.; Chandler, James M.; Donnelly, Kirby C.; McCauley, Garry N.
School:Texas A&M University
School Location:USA - Texas
Source Type:Master's Thesis
Keywords:water management als activity tolerance photolysis red rice oryza sativa adsorption availability
Date of Publication:08/01/2005