Study Offers Guidance to Control Problematic Weed for Rice Farmers

A word of caution to rice growers: the herbicide fluridone has become a valuable tool in fighting Palmer pigweed, but it can cause injury to some rice cultivars, depending on when it is used.

Registered under the trade name Brake by SePRO Corporation, fluridone is a residual herbicide used to suppress grasses and broadleaf weeds before they emerge, also known as a preemergence herbicide. In 2023, fluridone was approved by the U.S. Environmental Protection Agency for use in rice at the three-leaf stage and onward. It had already been approved for use in cotton and peanuts.

Fluridone offers a new tool in the arsenal to fight herbicide resistance in Palmer pigweed, said Jason Norsworthy, Distinguished Professor of weed science in the Department of Crop, Soil and Environmental Sciences for the Arkansas Agricultural Experiment Station and the Dale Bumpers College of Agricultural, Food and Life Sciences. The experiment station is the research arm of the U of A System Division of Agriculture. 

"This is an herbicide that is very effective in controlling Palmer pigweed in rice," Norsworthy said. "That's a major weed for us now in rice, more so in row rice than in flooded rice, since the flood itself can help take it out."

Furrow-irrigated rice, also known as "row rice," has gained momentum with Arkansas rice farmers over the past decade in efforts to conserve water, retain flexibility for crop rotations with corn and soybean, maintain off-season cover crops and allow for limited tillage.

Although a row rice field isn't entirely flooded, the bottom one-third to one-quarter of the field often does retain water, Norsworthy said. Since fluridone is an aquatic herbicide — a herbicide that is highly active in moist environments — it can injure intolerant rice varieties located in the wet zones of row rice fields if not applied according to the label.

The goal of furrow-irrigated rice is to achieve increased profit margins by reducing input costs, according to the Arkansas Furrow-Irrigated Rice Handbook.

Tolerance test

In 2022, Norsworthy and a team of Division of Agriculture researchers began a two-year study on a dozen rice cultivars commonly grown in Arkansas to test the tolerance levels when sprayed before and after rice plants emerge.

In the 2022 and 2023 trials, they used Brake's label rate and twice the label rate. Both tests were done in flooded rice on silt loam soil, but still offer insights into fluridone's use in row rice, which calls for more frequent and timed irrigations. Traditional rice cultivation that uses levees and gates to manage water, also known as "flooded rice" or "paddy rice," allows the entire field to be flooded.