Fall-applied Residuals Repel Resistant Italian Ryegrass

17 September 2025, Colorado: A recently published Weed Science Society of America (WSSA) research article indicates that fall-applied residual herbicides are among the best available options to use for glyphosate-resistant (GR) Italian ryegrass control. The article is among several newly published articles found online in the WSSA  Weed Technology journal.

“GR Italian ryegrass is one of the most troublesome weeds for rice production in Mississippi,” says Jason A. Bond, Extension/Research Professor, Mississippi State University, Delta Research and Extension Center, and WSSA member-scientist. “The most effective and economical management strategy to control GR Italian ryegrass is with preemergence (PRE) followed by postemergence (POST) herbicide programs. This research indicates that fall-applied residual herbicides such as dimethenamid-P (an active ingredient in Outlook^®) should be utilized to control GR-resistant Italian ryegrass in Mississippi.”

For their research, Bond and a team at Mississippi State University’s Delta Research and Extension Center conducted two separate field studies during 2021 to 2022 and 2022 to 2023 in Stoneville, Mississippi. They evaluated GR Italian ryegrass control with fall-applied residual herbicide mixtures (Herbicide Mixture Study) and sequential PRE followed by POST herbicide programs with multiple applications of residual herbicides (Sequential Herbicide Application Study).

In Mississippi, rice production typically requires a fall- or early-spring herbicide application because reducing undesirable vegetation at the time of seeding is key for establishing an adequate rice stand, explains Bond. To accomplish this, preplant herbicide applications (burndown) must be timely and are most often applied during February or March in Mississippi, he adds.

“In the Herbicide Mixture Study, dimethenamid-P (Outlook^®), pyroxasulfone (Zidua^®), and S-metolachlor (Dual Magnum^®) alone provided greater than 94% control of GR Italian ryegrass 21 days after treatment (DAT),” reports Bond. “The addition of flumioxazin (Valor^® EZ)to dimethenamid-P (Outlook^®), pyroxasulfone (Zidua^®), and S-metolachlor (Dual Magnum^®) did not improve control 130 DAT. In the Sequential Herbicide Application Study, treatments with fall-applied dimethenamid-P (Outlook^®) performed better than those with acetochlor (Warrant^®)except when fall-applied acetochlor was followed by a sequential application of clethodim (Select Max^®) plus S-metolachlor (Dual Magnum).

He adds that the study confirms fall-applied residual herbicides are a necessary component for GR Italian ryegrass control programs in Mississippi. “In the absence of a fall-applied residual herbicide, S-metolachlor (Dual Magnum^®) can reduce efficacy of clethodim (Select Max^®) targeting GR Italian ryegrass,” he points out. “Also, little benefit was realized from adding a residual herbicide to a sequential clethodim (Select Max^®) application targeting GR Italian ryegrass.”

More information about glyphosate-resistant Italian ryegrass weed control in rice production can be found in the Weed Technology article, “Glyphosate-resistant Italian ryegrass (Lolium perenne ssp. multiflorum) control in preemergence and postemergence programs containing mixtures of residual herbicides.” Weed Technology is a WSSA journal, published online by Cambridge University Press.

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