NEBRASKA – Winter and summer annual broadleaf weeds have an important economic impact on Nebraska winter wheat. They compete for water, light, space and nutrients, reducing Nebraska winter wheat yields by an estimated 10 percent each year. The dollar loss, with wheat at $6 per bushel, is approximately $35 million per year in Nebraska. Weeds also slow harvest and increase combine repair costs. Producers may be docked at the elevator for excessive grain moisture and/or weed seeds in wheat.
In 1982, only 11 percent of the winter wheat acres in Nebraska were sprayed with herbicides to control weeds. In 1987, 29 percent were sprayed. By 1998, however, approximately 60 percent were sprayed. The 2009 survey still showed that about 60 percent of the acres were sprayed. Effective weed control in winter wheat can eliminate losses due to weeds and increase net returns.
Success with reduced and no-till programs is improved when winter wheat stubble remains weed free after harvest. Allowing weeds to go to seed will cause problems in future crops. These potential problems underscore the importance of broadleaf weed control in winter wheat. An effective weed control program considers the entire cropping system. This approach involves the use of preventive, cultural, and chemical weed control methods.
Preventive weed control
Prevention or stopping the advancement of weed infestations, is an important part of a total weed management program. It requires diligence from the producer and offers low cost, and effective control. Some rules of preventive weed control are:
1. Use crop seed that is weed free.
2. Clean tractors, implements, trucks, and combines before moving them from infested to clean fields.
3. Keep uncropped areas (fencelines and field borders) weed free.
4. Do not allow livestock to move directly from infested to clean areas.
5. Prevent weed seed production in all areas.
6. Screen irrigation water when pumping from a canal system.
Cultural weed control
Cultural weed control involves manipulating the crop/weed environment so that conditions are favorable for crop plants, but unfavorable for weeds. Crop competitiveness and crop rotation are two important cultural control practices in winter wheat production.
Crop competition involves establishing a vigorous crop that can compete more effectively than weeds for water, light, space, and nutrients. Several factors contribute to competitive crops, including proper seedbed preparation, adequate fertilizer, high quality crop seed, careful variety selection, and proper rate, date, and depth of seeding. In a properly established, healthy winter wheat crop, it is difficult for summer annual weeds to survive, and the result is high winter wheat yields.
Crop rotations that include late spring seeded crops break the life cycle of problem winter annual broadleaf weeds such as tansy mustard and field pennycress, and allow the use of tillage or herbicides that may not be feasible in a winter wheat monoculture. Adapted rotational grain crops include corn, grain sorghum, proso millet, soybean, and sunflower, depending on location.
Chemical weed control
Several herbicides provide excellent broadleaf weed control with minimal wheat injury. However, some varieties are more sensitive to herbicides than others. Injury varies with herbicide, variety, and growth stage. Research has not been conducted on the herbicide sensitivity of many of the varieties currently planted. The following are fundamentals that should be considered before selecting a herbicide treatment:
1. Identify the problem weed(s).
2. Spray when weeds are small and actively growing. Spray at the proper winter wheat growth stage for the herbicide used.
3. Use proper spray equipment that is in good condition and not contaminated with previously used herbicides.
4. Calibrate the sprayer to ensure application accuracy.
5. Read and follow directions on the herbicide label.
6. Know your rotational plans to avoid herbicide carryover problems to sensitive crops.
7. Be aware that crop disasters such as winter injury, hail or disease occur and previously applied residual herbicides may limit the choices for recropping.
8. Identify neighboring fields or locations with sensitive crops.
Herbicides recommended for broadleaf weed control in winter wheat are Affinity BroadSpec (thifensulfuron + tribenuron), Agility SG (dicamba + thifensulfuron + tribenuron + metsulfuron), Aim (carfentrozone-ethyl), Ally XP (metsulfuron), Ally Extra SG (metsulfuron + thifensulfuron + tribenuron), Amber (triasulfuron), 2,4-D, Banvel (dicamba), Clarity (dicamba), Curtail (clopyralid + 2,4-D), Finesse (metsulfuron + chlorsulfuron), Harmony Extra SG (thifensulfuron + tribenuron), Huskie (pyrasolfotole + bromoxynil), Peak (prosulfuron), Rave (triasulfuron + dicamba), Starane NXT (fluroxypyr + bromoxynil), rane(fluroxypyr), Sterling (dicamba), and WideMatch (fluroxypyr + clopyralid). Some of these products should be combined to control a wider spectrum of broadleaf weeds in winter wheat. Herbicide combinations are also recommended for management of potential herbicide resistance development by weeds. Ally XP, Amber, Affinity BroadSpec, Agility SG, Express XP, Finesse, Harmony Extra XP and Peak are, or contain, sulfonylurea herbicides, which are ALS-AHAS inhibitors.
Many broadleaf weeds commonly found in Nebraska winter wheat fields can be controlled at a modest price with amine or low volatile 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.
Winter wheat must be between four tillers and the joint stage when 2,4-D is applied. In Nebraska, winter wheat generally is in the proper growth stage for 2,4-D application in March to early May, depending on planting date, the season, and location. Winter wheat planted Sept. 10 should be ready to spray by March 1, but wheat planted Oct. 1 may not be adequately developed until April 1 or later.
Winter wheat is considered fully tillered when it has six 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. Winter wheat yields were reduced more than 20 percent when 2,4-D was applied in the fall to winter wheat with two to four leaves (Table III). Winter wheat yields also were reduced with spring applications. To reduce injury with 2,4-D use low rates and apply in early spring to fully tillered wheat.
Adapted from Robinson and Fenster, 1973. Agro. J. 65:749-751.
Dicamba 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 and before jointing to avoid crop injury.
The sulfonylurea herbicides and Curtail have soil persistence and will control germinating broadleaf weeds for about four weeks after application. A surfactant (at 0.25 percent v/v) should be added to the spray solution whenever the sulfonylurea herbicides are used, unless liquid fertilizer is being combined with the herbicide. Ally XP, Amber, Finesse or Peak alone (without 2,4-D) can be applied in the fall to control winter annual broadleaf weeds.
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 Ally XP, Amber, Finesse or Harmony Extra SG and a surfactant improves weed control and helps prevent the development of resistant weeds. Higher rates of 2,4-D and surfactant may injure the wheat.
The sulfonylurea herbicides have rotational restrictions from one to 36 months which limit their use in areas where susceptible crops are grown in rotation with wheat. This is especially important to consider 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 sulfonylurea herbicides should not be applied to soils with a pH greater than 7.9 to avoid the risk of rotational crop injury. Producers should follow label directions carefully and determine rotational plans before using these products.
Wild buckwheat has become a significant 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
Producers have combined liquid nitrogen fertilizers (UAN - 28 and 32 percent) 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.
Liquid nitrogen fertilizers
Ally + 2,4-D amine (4L), 2,4-D ester (4L) and glyphosate are registered for spraying winter wheat prior to harvest (check labels for rates and timing). Not all brands of 2,4-D are labeled for this use.
Winter wheat must be treated after the dough stage and treated at least seven days before harvest. Wheat sprayed with 2,4-D while the wheat nodes are green may result in stem breakage. A 10-day preharvest interval is needed for wheat sprayed with Ally + 2,4-D amine (4L).
It is important for growers to examine fields early and treat where weed densities justify. One year, thousands of acres were sprayed with 2,4-D prior to harvest. Many treated fields had unsatisfactory desiccation of the weeds, plus many complaints were received on drift to susceptible crops, including corn. Most of these fields should have been sprayed during the tillering stage.
Weeds cause yield and quality losses in Nebraska’s winter wheat. Producers can implement weed management systems that include preventive, cultural, and chemical control methods to limit these costly losses. Herbicide treatments are available to control broadleaf weeds in winter wheat at reasonable costs. Producers should read and follow directions on the herbicide label to ensure the safe and effective use of herbicides. For the most current herbicide recommendations see University of Nebraska–Lincoln Extension Circular EC130, Guide for Weed Management.