Fluridone widens Palmer pigweed control options for rice growers, but stick to the label

(Press-News.org) FAYETTEVILLE, Ark. — 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 University of Arkansas 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.

Rice cultivars tested included CLL15, CLL16, Diamond, DG263L, Jupiter, Lynx, Titan, PVL02, RT7321 FP, RT7521 FP, RTv7231 MA and XP753.

The study, titled “Rice cultivar tolerance to preemergence- and postemergence-applied fluridone,” was published in Weed Technology by lead author Maria Souza, a Bumpers College graduate research assistant in the department of crop, soil and environmental sciences. Co-authors included Norsworthy; Pâmela Carvalho-Moore, Bumpers College graduate research assistant; Amar Godar, post-doctoral fellow with the experiment station; Samuel B. Fernandes, assistant professor with the experiment station and Bumpers College; and Tommy Butts, formerly with the Division of Agriculture and now assistant professor and extension weed scientist for the department of botany and plant pathology at Purdue University.

Based on their findings, the authors noted that fluridone tolerance is cultivar-dependent, and applications of fluridone in rice before the three-leaf stage should be avoided as required by the label.

Fluridone applied in rice at the three-leaf stage did not cause a yield penalty to any cultivar in 2022, but there was a yield loss for eight cultivars in 2023 at twice the label rate.

The field trials showed that when applied before the three-leaf phase in rice, fluridone negatively affected shoot density, groundcover, chlorophyll content and days to 50 percent heading in most cultivars tested.

“When you apply Brake as a preemergence herbicide and you flood it, the rice has a tendency to go backwards, as noted in multiple trials,” Norsworthy said.

Visible injury varied between years in all experiments, likely due to different environmental conditions, Norsworthy said. In 2022, injury following preemergence applications of fluridone was below 25 percent across cultivars. In contrast, in 2023, more than 30 percent injury occurred to five cultivars, with a maximum of 58 percent observed for one.

Cut soils is also a factor when using fluridone, Norsworthy added. Their research has shown an increased risk for injury to irrigated rice on soil where the topsoil had been removed for elevation changes, also known as “cut ground.”

Tank mixing recommendation As Norsworthy noted, fluridone is only registered to control weeds before they spring up and when the rice plant has reached the three-leaf rice. So, what’s a row rice farmer to do if Palmer pigweed has already emerged by the three-leaf stage? Essentially, Norsworthy said, it depends on how tall the pigweeds are.

“What we found in our work is that the most effective option is to put Brake out with 8 ounces of Loyant,” Norsworthy said. “A lot of growers are reluctant to apply Loyant by airplane because of the risk of off-target movement to nearby soybeans. But 8 ounces of Loyant by ground in furrow-irrigated rice is very effective on Palmer pigweed and poses less risk of drifting.”

Norsworthy said he and his team have tested tank-mixing fluridone with propanil, but it was only effective on 1 inch or smaller Palmer pigweed. However, Loyant was able to kill 3- to 4-inch pigweed, and Brake’s residual herbicide action suppressed other pigweed from emerging, he explained.

“Depending on the growth stage of the rice, 2,4-D would also possibly be an option,” Norsworthy said.

The study was supported in part by the Arkansas Rice Research and Promotion Board and SePRO Corporation.

Mention of a product name does not imply endorsement by the University of Arkansas System Division of Agriculture.

To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website. Follow us on X at @ArkAgResearch, subscribe to the Food, Farms and Forests podcast and sign up for our monthly newsletter, the Arkansas Agricultural Research Report. To learn more about the Division of Agriculture, visit uada.edu. Follow us on X at @AgInArk. To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit uaex.uada.edu.

About the Division of Agriculture The University of Arkansas System Division of Agriculture’s mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation’s historic land grant education system.

The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on three system campuses.

The University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.

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