By Scout Nelson
Fall herbicide applications give farmers an important opportunity to control weeds before the next growing season. Treating weeds in the fall can reduce problems in spring, especially when fields contain perennial, winter annual, or biennial weeds.
North Dakota State University explained that fall treatments work well because many cool-season weeds are still active late in the season. Herbicides applied during this period can move through the plant and improve control of weeds that survive freezing temperatures.
Cool-season perennial weeds are major targets for fall applications. Winter annual weeds, including field pennycress, shepherd’s-purse, horseweed, and Japanese brome, are also good targets because they emerge in fall and continue growing in spring.
Foxtail barley control
Foxtail barley is an important target for fall herbicide applications. Glyphosate can be applied at 1.5 pounds acid equivalent per acre with ammonium sulfate, or AMS, at 17 pounds per 100 gallons of spray mixture.
Timing is important. Glyphosate should be applied after the first freeze or later, but several days before the soil freezes. Applying too early can reduce control because the plant may not move enough herbicide to its perennial parts.
If the weather remains warmer than normal, applications should begin by mid-October. Glyphosate generally works better when daytime temperatures are above 45 °F, mornings remain above freezing, and conditions are sunny. Warmer weather can provide faster and more complete foxtail barley control.
Farmers should watch the weather forecast and allow enough time for treatment before the ground freezes. Warm temperatures and heavy rainfall after application may cause another flush of foxtail barley to emerge.
Horseweed and dandelion control
Adding 2,4-D ester at 0.75 to 1.0 pound acid equivalent per acre can improve control of dandelion and glyphosate-resistant horseweed. However, 2,4-D ester can reduce glyphosate activity on grasses, especially foxtail barley.
When 2,4-D, dicamba, or another contact herbicide is used with glyphosate where foxtail barley or quackgrass is present, the glyphosate rate should increase to 2.0 pounds acid equivalent per acre. AMS should remain part of the spray mixture at 17 pounds per 100 gallons.
A high-surfactant methylated seed oil adjuvant at 1.5 pints per acre can improve the activity of 2,4-D ester and contact herbicides. In many cases, fall treatment is more useful than waiting until spring, even when temperatures are not ideal.
Wild oat and kochia management
Pyroxasulfone products, including Zidua, can improve spring control of wild oat and kochia when applied in the fall at 0.13 pound active ingredient per acre or more. Residual herbicides should be applied only after soil temperatures stay below 50 degrees Fahrenheit.
Pyroxasulfone may also help control early waterhemp and suppress very early Japanese brome, downy brome, and horseweed. It can be used ahead of chickpea, corn, field pea, lentil, potato, safflower, soybean, sunflower, and wheat.
The product can be broadcast over undisturbed soil or incorporated with a field cultivator to a depth of 2 inches. Very shallow spring tillage does not greatly reduce wild oat control, although it may reduce control of some summer annual weeds.
Flumioxazin considerations
Flumioxazin products, such as Valor, have been used to control spring-emerging horseweed and kochia. However, increased kochia resistance to flumioxazin and other PPO-inhibiting herbicides has reduced their usefulness for fall applications.
Fields with foxtail barley that are planned for small grains in spring can benefit from fall-applied glyphosate because POST herbicides for small grains do not completely control foxtail barley.
FAQs
- Why should farmers apply herbicides in the fall?
Fall applications control weeds early and reduce weed pressure in the following spring.
- Which weeds are good targets for fall herbicides?
Perennial and winter annual weeds such as foxtail barley, dandelion, horseweed, and Japanese brome are key targets.
- When should glyphosate be applied for foxtail barley?
Glyphosate works best after the first freeze but several days before the soil freezes.
- Can 2,4-D be mixed with glyphosate?
Yes, but 2,4-D can reduce glyphosate control of grass weeds, especially foxtail barley.
- What does pyroxasulfone help control?
Pyroxasulfone can improve spring control of wild oat and kochia and suppress several other weeds.
Photo Credit: pixabay-franck-barske
Categories: North Dakota, Crops, Sustainable Agriculture