Small Grains: Problem Weeds and Herbicide Options

By Dwight Lingenfelter

Dense populations of winter annual weeds can compete with wheat, barley, and other small grains in late fall and early spring and slow the rate of crop development potentially reducing yield. If winter annual weeds like common chickweed, henbit, marestail, winter annual grasses, and others emerge with the small grain and are left unchecked, the potential impact on yield could be great. In these situations, it may make sense to kill these weeds in the fall rather than early spring. Harmony Extra (or generic equivalent) is still one of the most widely used broad-spectrum herbicides for broadleaf weed control. Fall applications have been more consistent than spring applications for weed control in regional trials. However, it has its weaknesses and other options, or tank-mixes should be considered. The table below summarizes some fall herbicide options for small grains. Check a current herbicide label for the latest use information. Here are some "best bets" for common problem weeds:

• Common chickweed: Harmony Extra is still effective on it in many parts of the state however, there are pockets of resistant populations starting to thrive. Tank-mix with Quelex or Starane Ultra or include Metricor (metribuzin) in your spray program if ALS-resistant chickweed is prevalent.
• Marestail: products such as 2,4-D, dicamba, Huskie, Quelex and Stinger applied POST have activity on this weed.
• Speedwell species: there are several speedwell (Veronica) species, including corn, common, ivy leaf, and others in our area. Depending on the species, their life cycle is either annual or perennial. Many of the commonly used small grain herbicides provide little or no control of speedwell. Harmony Extra + Quelex has provided control of ivy leaf speedwell (better than either herbicide applied alone). The same with Harmony Extra + Starane Ultra. PowerFlex has provided control/suppression of certain speedwell species in some university trials. Finesse has a few annual and perennial speedwells listed on its label and usually provides about 80% control of them. Be cautious of crop rotation intervals if using Finesse. If speedwell is small, Aim provides some control/suppression. Metricor also has activity on speedwell when its small.
• Downy brome: herbicide options include Anthem Flex/Zidua, Finesse, Osprey, and PowerFlex.
• Annual ryegrass: Anthem Flex/Zidua, Axiom, Axial, Finesse, Osprey, Prowl, and PowerFlex have activity on this weed.
• Annual bluegrass: active herbicides include, Axiom, Anthem Flex/Zidua, Metricor, Finesse, and Osprey.
• Roughstalk bluegrass: is a tall (>3 feet), cool-season perennial that is becoming more of a problem in small grain and forage fields across the state. As it matures, it typically turns reddish-tan and sets seed prior to summer and goes dormant until the fall and remains green during the winter months. Osprey, PowerFlex, and Axial provide control/suppression of it. Fall and early spring are the best times to apply effective herbicides. Osprey generally works better on this weed in the fall.

For any of these applications, make sure to include the necessary spray adjuvants. Remember that cool (less than 50°F) cloudy days can reduce herbicide activity. Also, if you plan to frost-seed or drill a companion crop such as red clover in early spring it may be best to make a fall herbicide application to avoid certain issues with herbicide residuals affecting their establishment. However, even if products such as PowerFlex, Osprey, Metricor, and others are applied in the fall, their re crop restrictions still prevent seeding of certain crops next spring. Burndown herbicides for no-till small grains include dicamba, Gramoxone, glyphosate, Harmony Extra, and Sharpen. Refer to the specific product label for more application information. The legitimate use of 2,4-D for burndown in wheat and other small grains is uncertain. None of the 2,4-D ester or amine labels specify application just prior to small grain seeding or emergence. Some research suggests a minimum delay of 7-10 days after application at rates up to 1 pint/A 2,4-D ester. Since 2,4-D burndown in small grains is ambiguous at best, if injury occurs liability rests with the consultant or applicator. Also, for those looking for herbicides that legally can be sprayed in cereal rye, especially for weedy grass control, the choices are limited. At Penn State, we have ongoing herbicide tolerance studies in rye to hopefully find some options for the future.

Common Fall Herbicide Options for Small Grains