Key Takeaways:
- Newly published WSSA research underscores fall-applied residual herbicides as key to glyphosate-resistant Italian ryegrass (GR ryegrass) control.
- Mississippi State University field trials tested multiple herbicide mixtures and sequential applications.
- Dimethenamid-P (Outlook®), pyroxasulfone (Zidua®), and S-metolachlor (Dual Magnum®) achieved over 94% control 21 days after treatment.
- Sequential studies showed Outlook® outperformed acetochlor (Warrant®) in most cases.
- Findings reinforce fall-applied programs as necessary for sustainable rice production in Mississippi.
WSSA Research on GR Italian Ryegrass
The Weed Science Society of America (WSSA) has published new findings in its Weed Technology journal indicating that fall-applied residual herbicides are among the most effective tools for managing glyphosate-resistant Italian ryegrass in Mississippi rice production.
Jason A. Bond, Extension/Research Professor at Mississippi State University’s Delta Research and Extension Center, said: “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.”
Field Studies in Mississippi
Bond and his team conducted two field studies between 2021 and 2023 in Stoneville, Mississippi:
- Herbicide Mixture Study: Outlook®, Zidua®, and Dual Magnum® alone each provided more than 94% control of GR ryegrass 21 days after treatment. Adding flumioxazin (Valor® EZ) to these herbicides did not improve long-term control.
- Sequential Herbicide Application Study: Programs with fall-applied Outlook® consistently performed better than those with Warrant®, except when Warrant® was followed by clethodim (Select Max®) plus Dual Magnum®.
Implications for Rice Production
Rice growers in Mississippi rely on fall or early-spring herbicide applications to reduce weed competition before planting. Bond noted that without fall-applied herbicides, control programs suffer: “In the absence of a fall-applied residual herbicide, S-metolachlor (Dual Magnum®) can reduce efficacy of clethodim (Select Max®) targeting GR Italian ryegrass. Also, little benefit was realized from adding a residual herbicide to a sequential clethodim (Select Max®) application.”
These findings underscore the importance of integrating fall-applied herbicides into management strategies to protect yields and maintain profitability in rice production.
Read the full letter here.
