March 30, 2007

Much of western Nebraska was blessed with good winter precipitation this year. This moisture was very welcome by winter wheat growers; however, in addition to helping the winter wheat crop, it may have also promoted winter annual weed germination and growth. Winter annual weeds are very competitive with winter wheat because they compete with the crop throughout much or all of the growing season. Additionally, some of the winter annual grass weeds also result in dockage, foreign material discounts or both when contaminated grain is delivered to the elevator. In order to minimize losses, growers must control these weeds in a timely manner.

Broadleaf Weeds

Common broadleaf winter annual weeds in winter wheat include blue mustard, tansy mustard, tumble mustard, field pennycress and shepherd's-purse. Unfortunately, many growers are unaware of these weeds in their fields until they start to bloom in the spring. By this time, control is difficult and most of the crop damage has already occurred. To be effective, winter annual broadleaf weeds need to be controlled in the late winter or very early spring, before the plants begin to bolt, or stems elongate.

Blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts early. To be effective, herbicides typically need to be applied to blue mustard in late February or early March. Early April applications of 2,4-D usually provide excellent control of tansy mustard and the other winter annual broadleaf weeds, but provide only fair control of blue mustard. If timed correctly, 2,4-D (8 oz/acre of LV4 ester or 16 oz/acre of 4 lb/gal amine) provides low-cost and effective control of these weeds. Wheat should have at least four tillers before applying 2,4-D or serious crop injury may occur. Adding a sulfonylurea herbicide, such as Ally Extra® or Amber®, to 2,4-D may improve control, particularly after these plants have bolted. If the sulfonylurea herbicide is used after bolting, but prior to seed production, it may be useful to reduce the amount of seed produced, but such late control may not help wheat yield much.

Grass Weeds

Only in the last few years has it been possible to selectively control some of the winter annual grass weeds, such as downy brome, jointed goatgrass or feral rye, in winter wheat. Although control of these weeds is often best when herbicides are applied in the fall, some spring control is possible. Maverick® and Olympus™ herbicides provide selective control of downy brome and other Bromus species in winter wheat. Although both products provide similar control of downy brome when applied in the fall, Olympus may provide slightly better control than Maverick of downy brome in the spring.

Maverick should be applied at a rate of 2/3 oz product per acre in 5 to 20 gallons of water per acre. A non-ionic surfactant should be added at 0.5% on a volume basis. Spring applications to downy brome have been more inconsistent than fall applications, with an occasional control rating as high as 85%; more typically, spring control is in the 35-70% range. Surviving downy brome plants are usually significantly stunted, but will produce some viable seed. Precipitation following application appears to be important for improved herbicide activity. Growers should be aware of the rotation restrictions with this product.

Olympus herbicide should be applied at a rate of 0.9 oz product per acre for downy brome control. A non-ionic surfactant should be added at a rate of 0.25-0.5% on a volume basis. Olympus herbicide may be applied in the spring, but downy brome control in the spring is more inconsistent than fall applications and may not provide the level of control desired. Both Olympus and Maverick herbicides also provide control of many winter annual broadleaf weeds.

Fields seeded to Clearfield wheat varieties, for example Infinity CL or Above, can be treated with Beyond™ herbicide this spring to control downy brome, jointed goatgrass, and certain broadleaf weeds. Although spring applications of Beyond have typically provided poor control of feral rye, spring applications have typically provided good to excellent control of jointed goatgrass and downy brome when treated with 4 oz of product per acre. Herbicide applications should be made as soon as active spring crop growth begins. Postemergence applications require the addition of a surfactant at 0.25% and a nitrogen fertilizer solution at 2.5% on a volume basis. One percent on a volume basis is one gallon in 100 gallons of spray solution.

If winter annual weeds are a regular problem in fields, changing the crop rotation is recommended. Including a spring-seeded crop such as corn, sorghum, oat, proso millet, or sunflower in the rotation with winter wheat and summer fallow provides an additional year in which to prevent seed production and allows the soil seed bank to gradually decrease. Additional information on weed control in winter wheat can be found by visiting the Wheat Production Systems Handbook at: archive/-/asset_publisher/VHeSpfv0Agju/content/wheatbook.unl.edu.

Drew Lyon
Extension Dryland Cropping Systems Specialist
Panhandle REC, Scottsbluff