August 10, 2007

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 winter annual grass weeds also result in grain discounts 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 shepherdspurse. 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 by late winter or very early spring, before the plants begin to bolt or stems elongate.

If winter annual broadleaf weeds are present in the fall, they can be controlled with any number of sulfonylurea herbicides including Ally, Amber, Finesse or Peak. Some wheat varieties are more sensitive than others to fall application, but we have not screened the current varieties for this sensitivity. Although the addition of 2,4-D can improve the level of weed control, wheat should have at least four tillers before applying 2,4-D or serious crop injury may occur. This injury may not be noticeable until the following spring when wheat heads become trapped in the boot as they try to emerge from the stem.

Figure 1. Blue mustard in winter wheat.

Blue mustard. In the spring, blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts very 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 provides 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. As previously stated, wheat should have at least four tillers before applying 2,4-D or serious crop injury may occur. The addition of a sulfonylurea herbicide, such as Ally or Amber, to 2,4-D may improve control, particularly after these plants have bolted.

Grass Weeds

Only in the last few years has it been possible to selectively control some winter annual grass weeds, such as downy brome, jointed goatgrass or feral rye, in winter wheat. Control of these weeds is often best when herbicides are applied in the fall. Although good control can be achieved with spring applications, the consistency of performance is significantly less.

Maverick and Olympus herbicides provide selective control of downy brome, and other Bromus species, in winter wheat. Both products provide similar control of downy brome when applied in the fall, but Olympus has provided slightly better control than Maverick when both products are applied in the spring. Growers should be aware of the rotation restrictions with these products.

Maverick should be applied at a rate of 2/3 ounce of 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. Precipitation following application appears to be important for improved herbicide activity.

Olympus herbicide should be applied at a rate of 0.9 ounce of product per acre for downy brome control. A non-ionic surfactant should be added at a rate of 0.25% - 0.50% on a volume basis.

Clearfield. Fields seeded to Clearfield wheat varieties, for example Infinity CL or Above, can be treated with Beyond herbicide to control downy brome, feral rye and jointed goatgrass. Winter wheat cultivars that do not contain the tolerance gene are seriously injured or killed when treated with this herbicide. Fall and spring applications of Beyond have typically provided good to excellent control of jointed goatgrass and downy brome when treated with 4 ounces of product per acre. 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.

Controlling Feral Rye. For the best control of feral rye, apply 5 ounces of product per acre in the fall before the first rye tiller has emerged. Grower experience with Beyond for feral rye control has been inconsistent. Some growers have been very pleased, some have been very disappointed, and others have both experiences. Until we have a better understanding of what is causing the inconsistent results, growers are advised to use conservative estimates for feral rye control when deciding whether to use the Clearfield system in their operation.

Another option for controlling rye in winter wheat is to apply glyphosate with a rope-wick applicator. The rye should be 10 to 12 inches taller than the wheat for best results. In heavy stands of rye, travel in both directions. Remember that if you contact the wheat with the rope-wick or drip the herbicide on the crop it will cause injury. Glyphosate should be mixed at a 33 percent concentration or 1 gallon of herbicide with 2 gallons of water. Surfactants may be needed for some glyphosate formulations.

Cultural Control Practices

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, 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 the control of winter annual weeds in winter wheat is available online in UNL's Wheat Production Systems Handbook.

Drew Lyon
Extension Dryland Cropping Systems Specialist
Panhandle Research and Extension Center, Scottsbluff