Figure 1.  Letting weeds get the upper hand after wheat harvest is likely to mean reduced yield for the next crop. Weeds use valuable soil water and serve as a bridge between one crop and the next for virus-transmitting insects. (Photos by Robert Klein)

August 16, 2013

Annual broadleaf weeds have an important economic impact on Nebraska winter wheat. They compete for water, light, space, and nutrients, reducing winter wheat yields by an estimated 10% each year. 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.

Figure. 2. A wheat field near Grant where annual broadleaf weeds have begun encroaching on the crop.

The effectiveness of post-harvest weed control is influenced by production practices associated with the previous wheat crop, such as winter wheat variety selection, fertilizer practices, row spacing, planting date, and seeding rate. Other factors influencing weed control include

• weed size;
• cutting off weed tops with the combine;
• crop rotation;
• temperature when spraying;
• rain the day of spraying;
• streaks caused by sprayers;
• terraces;
• dust, straw, and chaff; and
• weed seed distribution.

Less residue from a winter wheat crop also will make the next crop less competitive with weeds. Weeds under stress are very difficult to control. Each field should be examined separately. The key is to prevent weeds from using soil water and producing seeds.

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.

The following practices are paramount to any successful weed management program.

1. Correctly identify weed(s).
2. Apply herbicides when weeds are small and actively growing and the crop is at the proper growth stage,
3. Use spray equipment that is clean and in good condition to avoid chemical contamination.
4. Calibrate spray equipment to ensure accurate application.
5. Read and follow directions on the herbicide label.
6. Know your rotational plans to avoid herbicide carryover problems and the development of herbicide-resistant weeds.
7. Be aware that crop disasters occur and previously applied residual herbicides may limit recropping options.

Cultural Control

Cultural weed control involves manipulating the crop/weed environment so conditions are favorable for crop plants, but unfavorable for weeds. Cultural practices that improve weed control include crop rotation, timely weed control during summer fallow and crop growth, a firm seedbed at wheat seeding, proper variety selection, use of quality seed, optimal seeding dates, seeding rates, row spacings, row direction, seeding techniques and soil fertility.

Chemical Control

Several herbicides provide excellent broadleaf weed control with minimal wheat injury; however, some varieties are more sensitive to herbicides than others. 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.

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. It is important for growers to examine fields early and treat where weed densities justify.

Read and follow directions on the herbicide label to ensure the safe and effective use of herbicides. For the most current herbicide recommendations see the UNL Extension Guide for Weed Management in Nebraska (EC 130).

Fallow Weed Control

A good stand of vigorously growing winter wheat will have fewer weeds than a thin stand. Poor wheat stands and lack of plant vigor often result from poor soil moisture management during the fallow period before winter wheat seeding. Precipitation and temperature greatly influence crop and weed growth in the semiarid areas of the central Great Plains. In western Nebraska, where fallow is practiced, annual average precipitation varies from 14 to 24 inches.

The purpose of fallow is to store water in the soil to stabilize winter wheat yields and manage weeds. With good fallow practices, sufficient soil water is usually available to establish winter wheat so it can compete with weeds.

Steve Young
Extension Weed Ecologist, West Central REC