Determination of antagonism between cyhalofop-butyl and other rice (Oryza sativa) herbicides in barnyardgrass (Echinochloa crus-galli).

Herbicide antagonism is defined as the reduction of control of certain weeds as the result of applying mixtures of two or more herbicides. Cyhalofop-butyl, a graminicide used for postemergence grass weed control in rice, is antagonized by some rice herbicides when applied simultaneously. The result of this type of antagonism usually results in decreased control of grass weeds. Research has shown that herbicide antagonism between graminicides and other herbicides may be caused by different mechanisms as the result of activity of the tank-mix partner. Using HPLC, the objective of this experiment was to analyze the fate of cyhalofop-butyl in barnyardgrass tissue when applied alone and in combination with halosulfuron, propanil, or triclopyr. Results indicated that absorption of cyhalofop-butyl and hydrolysis to its phytotoxic metabolite, cyhalofop-acid, was rapid and that halosulfuron and triclopyr had no effect. Because of a likely interaction of propanil with an apoplastic esterase enzyme, increased levels of cyhalofop-butyl and cyhalofop-acid were detected in barnyardgrass tissue, indicating that cyhalofop-butyl metabolism was hindered by propanil.