Weed Management in Winter Wheat
Weeds compete with winter wheat for soil water, light, space, and nutrients. Weed competition reduces wheat yields and profitability, slows harvest, and increases combine repair costs. At harvest growers may be docked at the elevator for having excessive weed seeds in their grain. Weeds also may serve as hosts for insects or diseases that can injure winter wheat plants and reduce yields.
An effective weed control program considers all aspects of the cropping system, including tillage program, crop rotation, herbicide rotation, soil fertility, disease and insect management, and the complex of weeds targeted.
An integrated weed management plan uses a combination of practices to manage weeds. These include:
- Preventive weed control
- Crop rotation
- Fallow weed management
- Seed bed preparation
- Wheat variety selection
- Quality seed
- Seeding time and rate
- Seeding depth
- Row spacing
- Row direction
- Pest management
- Timely weed scouting
- Chemical weed control
There are two major classes of weeds in wheat: winter annuals and summer annuals. Winter annuals emerge in fall, winter, or early spring. Summer annuals emerge in spring when soils reach a certain temperature. Winter annuals have the greatest effect on wheat yields. It is estimated that winter annual weeds reduce wheat yields by an estimated 10% each year; of these, fall-germinating weeds having the greatest impact.
There are both winter annual grass weeds and winter annual broadleaf weeds. The three winter annual grass weeds that are the biggest problems in Nebraska are downy brome, feral rye, and jointed goatgrass. These weeds are best controlled when herbicides are applied in the fall shortly after emergence, when plants are growing rapidly but before they become well tillered. Winter wheat fields that look like a lawn probably have winter annual grassy weeds filling in between the rows of wheat.
Maverick®, Olympus®, and OlympusFlex® herbicides provide selective control of downy brome and other Bromus species in winter wheat. Maverick and Olympus provide similar control of downy brome (70% to 95% control in UNL trials) when applied in the fall. Spring applications have been less consistent, ranging from 35% to 85% control. Plant growth rate, stage of development at the time of application, and weather conditions after application influence the level of control.
All three products have important rotation restrictions. Olympus Flex has a little less soil residual than Olympus, which allows a few rotational crops, such as soybean, to be planted a little sooner than is the case with Olympus. However, the differences are small and may be of little practical significance in non-soybean production regions.
Clearfield Wheat for Feral Rye and Jointed Goatgrass
Growers who have seeded a Clearfield wheat variety can use Beyond or ClearMax herbicide to selectively control downy brome, jointed goatgrass, feral rye, cheat, wild oat, and minor populations of Italian ryegrass. Of these weeds, feral rye control has proven to be the most difficult and least consistent. The best control of feral rye has been achieved by applying 5 oz/ac of Beyond in the early fall before rye plants have formed a tiller. To improve control, add UAN and a surfactant to the spray mixture. Fall control of feral rye with Beyond has ranged from 70% to 90%, while spring applications of Beyond have been inconsistent and are not advised in most situations.
Unlike with feral rye, the control of jointed goatgrass with Beyond has been very effective and consistent. Fall and spring applications of Beyond at 4 oz/ac have ranged from 85% to 100% control. Surfactant and UAN should be added to the spray mixture. Herbicide resistance is a concern with jointed goatgrass, so growers should be careful not to overuse this technology or it may soon lose its usefulness. We do not recommend that growers use Beyond herbicide more than twice in six years. Although downy brome control with Beyond is usually good, downy brome can be controlled more economically with the previously discussed herbicides.
Winter Annual 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 crop damage has already occurred. The sulfonylurea herbicides Ally®XP, Amber™, Finesse™ Cereal and Fallow or Peak™ can be applied alone, without 2,4-D, in the fall to control winter annual broadleaf weeds. Herbicide applications made in late winter or early spring must be applied before weeds begin to bolt, or stems elongate, for effective control.
Blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts early. If timed correctly, 2,4-D (8 oz/ ac of LV4 ester or 16 oz/ac of 4 lb/gal amine) provides low cost and effective control of winter annual broadleaf 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 SG 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 weed seed production, it may reduce the amount of weed seed produced, but such late control may not prevent yield loss.
Warm Season Broadleaf Weeds
Many broadleaf weeds in winter wheat can be controlled at a modest price with amine or ester formulations of 2,4-D. Generally, ester formulations of 2,4-D provide better broadleaf weed control than amine formulations because they are oil soluble and readily penetrate plant foliage. Amine formulations are water soluble and do not penetrate foliage as easily, resulting in reduced control of weeds such as kochia and Russian thistle. However, amine formulations provide greater crop safety than ester formulations.
To reduce injury with 2,4-D, use low rates and apply in early spring to fully tillered wheat, prior to stem elongation (jointing). Winter wheat is considered fully tillered when it has five to nine tillers; however, the number of tillers depends on the seeding rate and date. Wheat injury and yield loss can be significant if 2,4-D or other herbicides are misapplied.
Dicamba (Banvel™, Clarity™, etc.) and 2,4-D are combined to control a wider spectrum of broadleaf weeds, including wild buckwheat, which is not controlled by 2,4-D alone. Dicamba plus 2,4-D must be applied to well tillered wheat, but before jointing, to avoid crop injury.
Sulfonylurea herbicides have soil persistence and will control germinating broadleaf weeds for about four weeks after application. A surfactant (at 0.25 % v/v) should be added to the spray solution whenever the sulfonylurea herbicides are used, unless liquid fertilizer is being combined with the herbicide.
Among the weeds that may (or have) become resistant to the sulfonylurea herbicides are kochia, Russian thistle, and prickly lettuce. The use of 2,4-D (4 lb/gal) at 1/2 pint per acre applied with one of the sulfonylurea herbicides and a surfactant improves weed control and helps prevent resistant weed development. Higher rates of 2,4-D and surfactant may injure the wheat.
The sulfonylurea herbicides have rotational restrictions of one to 36 months that limit their use in areas where susceptible crops are grown in rotation with wheat. This is especially important when the crop is lost to hail or other crop failures.
The degradation of sulfonylurea herbicides in soil is slowed by high soil pH. Some of the sulfonylurea herbicides should not be applied to soils with a pH greater than 7.2 to avoid the risk of rotational crop injury. Follow label directions carefully and determine rotational plans before using these products.
Wild buckwheat has become an increasing problem in winter wheat fields. Wild buckwheat is best controlled when herbicides are applied before it produces vines. Herbicides with short residuals applied before wild buckwheat germinates will not provide adequate control. Herbicides with the greatest efficacy on wild buckwheat include Curtail, dicamba + 2,4-D, Rave, and Starane NXT.
Liquid Nitrogen Fertilizers and Herbicides
Producers have combined liquid nitrogen fertilizers (UAN at 28% or 32%) and herbicides to control weeds and fertilize the crop with one application. In some situations, the winter wheat showed evidence of crop injury when sprayed with these mixtures. When wheat is under stress, spraying herbicides with UAN may cause yield reduction regardless of the crop stage. Do not add surfactant to the liquid fertilizer-herbicide mixture. Adding sulfur increases crop injury.
An alternative to this program may be to strip band (20 inches) nitrogen fertilizer, if needed, as soon as field conditions permit in the spring and apply the herbicide later. The advantages of strip-banding over broadcast for nitrogen fertilizer application are probably great enough to pay for the second application.
Weed control in winter wheat requires an integrated system that relies on numerous management decisions related to maximizing crop growth and minimizing weed growth. The use of multiple cultural practices for weed control frequently provides synergistic benefits greater than the added effects of using just one or two cultural practices. Timely field scouting is essential in good weed management.
Extension Western Nebraska Crops Specialist