Fall Herbicides Can Get a Jump on Winter Annual Weeds
By Kurt Vollmer, Ph.D., Extension Specialist, Weed Management
Farmers looking ahead to next spring may want to consider fall residual herbicide applications to control two troublesome weeds: Italian ryegrass (aka annual ryegrass) and marestail.
Fall residual herbicides can be especially beneficial in fields left fallow and are critical for small-grain weed management. Because early spring weather often can make timely postemergence herbicide applications difficult, reducing winter annual weeds in the fall can give growers a head start before spring planting.
Several factors should be considered when developing a fall herbicide program.
Match the herbicide
The first step is choosing an herbicide that fits both the target weed and the field situation.
Pyroxasulfone-containing products, such as Zidua SC and Anthem Flex, and S-metolachlor-containing products, such as Dual Magnum, remain good options for Italian ryegrass control. However, they provide little to no control of marestail.
Flumioxazin-containing products, such as Valor EZ, can control both Italian ryegrass and marestail. Metribuzin also can be used to control marestail but will not control ryegrass.
Timing is critical because these products must be applied before weeds emerge. Valor EZ can be applied seven days before planting no-till wheat. Zidua SC can be applied to wheat from the spiking through fourth-tiller growth stages.
Zidua SC, Valor EZ, metribuzin and Dual Magnum also can be applied as winter-fallow treatments before planting corn or soybeans.
Residual herbicides will not control weeds that have already emerged. Paraquat will be needed to control emerged plants. Newer products, such as Voraxor and Empera, also can help suppress emerged winter annual weeds.
Use the right rate
Herbicide rates also are important when developing a fall program. Higher rates often are needed to provide longer residual control, but growers should consult product labels for specific rates based on soil type.
For winter wheat, up to 4 fluid ounces per acre of Zidua SC and up to 2 fluid ounces per acre of Valor EZ can be applied.
For winter fallow, up to 6.5 fluid ounces per acre of Zidua SC, up to 2 fluid ounces per acre of Valor EZ, up to 1.67 pints per acre of Dual Magnum and up to 0.27 pound per acre of metribuzin can be applied.
Pendimethalin-containing products, such as Prowl H2O, may be considered for control of other weeds. However, the maximum labeled rate of 3 pints per acre is too low for effective Italian ryegrass control and will not control marestail.
Watch the weather
Weather plays an important role in the effectiveness and longevity of fall-applied herbicides.
Cooler fall temperatures can extend herbicide persistence by slowing microbial degradation. However, rainfall or irrigation is needed to activate these herbicides.
If soil conditions are dry, herbicides may not incorporate properly, regardless of how long they persist in the soil. At least one-third of an inch of rainfall is needed to activate Dual Magnum and metribuzin, while Zidua SC and Valor EZ both require one-half inch.
For maximum efficacy, a precipitation event generally should occur within three days of application.
Even when winter temperatures extend residual control, growers should remember that fall applications are unlikely to control weeds that emerge in spring. A 2022 study found that a fall application followed by a spring application was necessary to achieve an average of 81% control of Italian ryegrass.
Consider the cost
Cost also should be considered when choosing a fall herbicide program.
University of Maryland Extension field crop budgets list Anthem Flex at $4.17 per ounce and Valor EZ at $3.03 per ounce. Zidua SC costs about $4.09 per ounce.
Metribuzin followed by an S-metolachlor-containing product, such as Dual Magnum or Dual II Magnum, may provide a lower-cost option for fallow weed control. Metribuzin costs about 20 cents per ounce, while S-metolachlor-containing products cost 38 to 51 cents per ounce.
Taking the time to choose the right herbicide, rate and application timing can help set your fields up for success next spring. Here’s to safe and successful fall applications, and best of luck managing weeds in the year ahead!
This article appears in October 2026, Volume 17, Issue 7 of the Agronomy news.