Eric P. Webster and Wei Zhang

Ricestar (fenoxyprop), a relatively new selective herbicide, is used for postemergence control of grasses in rice. It provides good to excellent control of major grasses such as barnyardgrass, broadleaf signalgrass and sprangletop. Because Ricestar does not have activity on broadleaf weeds or sedges, it is likely that other herbicides with broadleaf or sedge activity will be needed in a weed control program containing Ricestar.

It would be beneficial to rice producers to apply Ricestar plus a herbicide with broadleaf or sedge activity in a mixture, which would save time and money. Often, however, broadleaf or sedge herbicides in a mixture with herbicides with grass activity, such as Ricestar, may antagonize or reduce the activity of the herbicide on grass weeds. This is a common problem observed with Ricestar when mixed with herbicides with broadleaf or sedge activity including those potential mixture herbicides mentioned on the Ricestar label. Thus, identifying the compatibility of potential tank-mix herbicides with Ricestar is important to rice producers. Such information will help rice producers select herbicides to be tank-mixed with Ricestar for optimum grass and broadleaf weed control.

Two-year Field Study
A field study was conducted (2001 to 2003) at the LSU AgCenter Rice Research Station near Crowley, La., to evaluate the compatibility of five herbicides used in rice, with activity on broadleaf weeds and sedges, with Ricestar for barnyardgrass control. Ricestar was applied at zero, 15, and 17 ounces per acre. The broadleaf or sedge herbicides evaluated were Aim at 1 ounce per acre, Basagran at 1.5 pints per acre, Grandstand at 0.67 pints per acre, Londax at 1 ounce per acre, and Permit at 1 ounce per acre.

The herbicide mixtures were applied postemergence to three- to four-leaf rice with a backpack sprayer. Barnyardgrass had two to four leaves at the time of herbicide application. Barnyardgrass control was visually estimated, on a scale of zero to 100, at 10, 20 and 30 days after treatment (DAT). If the visually estimated, or observed, control was significantly lower than the expected control, an antagonistic effect occurred. The expected control is determined by a mathematical equation.

Reduced Barnyardgrass Control
Barnyardgrass control with Ricestar at 15 ounces per acre was antagonized by Aim, Londax and Permit at 10 days after treatment (DAT) and by Aim, Grandstand and Permit at 30 DAT. At 20 days after treatment, control of barnyardgrass by Ricestar at 15 ounces per acre was reduced when mixed with Aim, Grandstand, Londax and Permit (Figure 1).

Basagran did not antagonize the activity of Ricestar at 15 ounces per acre on barnyardgrass at any evaluation date. It is generally believed that the antagonistic effect on a grass herbicide by other broadleaf herbicides can be overcome by increasing the rate of the grass herbicide. However, in this study an increase in the Ricestar rate from 15 to 17 ounces per acre did not reduce the antagonistic effect from the mixture herbicides.

Compatibility Ranking
The compatibility analysis indicates that among the six herbicides evaluated, Basagran is the most compatible with Ricestar. Barnyardgrass control was 91 percent with Ricestar in combination with Basagran. Londax ranked second in compatibility with Ricestar with 86 percent control of barnyardgrass. Permit and Grandstand had an antagonistic effect on the activity of Ricestar on barnyardgrass.

These results indicate that Basagran and Londax are safe to be used as tank-mix partners with Ricestar for barnyardgrass control. But Grandstand and Permit can potentially antagonize the activity of Ricestar on barnyardgrass. Thus, these herbicides should be used with caution when applied in a mixture with Ricestar.

Eric P. Webster, Professor, and Wei Zhang, Postdoctoral Researcher, Department of Agronomy & Environmental Management, LSU AgCenter, Baton Rouge, La.

(This article appeared in the summer 2005 issue of Louisiana Agriculture.)