July 25, 2008

Russian thistle had grown higher than the combine cutter bar and was cut off at harvest, complicating weed control efforts.

Controlling weeds after winter wheat harvest can be a challenge, especially this year. Heavy rainfall in many wheat producing areas will encourage weed growth and may delay entry into fields for timely herbicide application. Also, a delayed harvest can allow weeds to grow above the height of the combine cutter bar. These weeds can be difficult to control after having been cut back by the combine.

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 include: weed size, cutting off weed tops with the combine, crop rotation, temperature when spraying, rain the day of spraying, weed seed distribution, and streaks caused by sprayers, terraces, dust, straw, and chaff. The amount of residue from this winter wheat crop affects how the next crop will compete with weeds.

Timely Control Measures Essential

In wet areas, weed growth will be rapid and lush. It's a general rule that you can wait up to 30 days after harvest to spray wheat grown as part of a three-year rotation. If the wheat was planted without an 11- to 14-month fallow period, it should be sprayed within 15 days of harvest. Each field should be examined separately. Rapid weed growth may necessitate spraying before 15-30 days. The key is to prevent weeds from using soil water and producing weed seeds.

Split Treatments

In many wheat fields weed control should quickly follow harvest to prevent weeds from getting a stronghold and using valuable soil moisture and producing seed.

Summer Application. Split treatments have a good history of effectiveness. In Kansas, there was a 20 bushel increase in corn yields the next year for treatments applied in July compared to those in mid-August. When using a split treatment, apply the glyphosate products alone (adding surfactant, if needed, plus ammonium sulfate) as the first application in July or early August.

Additives. Some glyphosate products include sufficient surfactant while many products require it to be added. Be sure to check the product label.

For all glyphosate brands, add ammonium sulfate (spray grade) at 17 lb per 100 gallons of spray solution. (The ammonium sulfate is the first item put into the spray tank after the water.) Ammonium sulfate is especially helpful when stress conditions are present. Since it's difficult to recognize weed stress, it's wise to always add ammonium sulfate. Liquid ammonium sulfate, with or without a drift retardant, also is available.

Improve control by increasing the rate of glyphosate and allowing at least six hours — and longer with some weeds — for the glyphosate product to become rainfast. Barnyardgrass may require as much as 24 hours without rain for maximum control. With glyphosates, use a spray volume of 5 to 10 gallons per acre and don't apply when temperatures reach or exceed 95°F.

In areas which received abundant rainfall this year, a second glyphosate treatment may be needed to control repeated flushes of weeds.

Fall application. The second part of the split treatment should be applied in September. It should contain at least 0.5 pound per acre of atrazine and a nonselective contact herbicide, depending on the amount and size of volunteer winter wheat, downy brome or jointed goatgrass or other weeds present. (Also add surfactant.)

Several nonselective herbicides are available for difficult to control weeds. With Gramoxone Inteon (add surfactant), use a minimum of 2 pints of X 77, or equivalent surfactant, per 100 gallons of solution. Use 2 quarts of X 77 per 100 gallons of spray solution if using less than 20 gallons of carrier. Another product containing the active ingredient Gramoxone Inteon is Firestorm. If a glyphosate and atrazine tank mix is used as the second part of the split treatment, increase the glyphosate rate up to the maximum recommended rate. Some atrazine products have been seen to antagonize the performance of glyphosate. Formulations vary among products with similar active ingredients so check labels and adjust rates accordingly.

The atrazine rate varies with soil and rainfall patterns. In southwest Nebraska, use at least 2 quarts of atrazine per acre unless the following crop or soil limits the rate to a lower amount. In the Panhandle often the maximum amount allowed in one season is only 0.5 quart per acre.

The advantage of split treatments is that, along with killing weeds right after harvest that would have used valuable soil moisture, they provide excellent control of volunteer winter wheat and other winter annual grasses.

Impact of Timely Weed Control

Control of volunteer wheat is especially helpful in reducing the spread of wheat streak mosaic disease. Using one quart or less of atrazine before September 10 allows winter wheat to be planted 12 months later in most areas and soils. If sufficient soil water is available the following spring, corn could be planted or if moisture is limited, the field could be fallowed and winter wheat could be planted in the fall.

Cultural Practices To Aid Weed Control

For more information, check these Extension resources:   Cultural Practices to Improve Weed Control in Winter Wheat 2008 Guide to Weed Management in Nebraska UNL Weed Science Web site

In addition to herbicide application, many options are available for weed control after wheat harvest. Combining several options can help achieve maximum weed control. Stands of vigorous winter wheat will compete better with weeds, allowing you to concentrate on weed control in the fallow. You can achieve this and reduce weed population and vigor after harvest:

 

• Prepare a good, firm seedbed
• Control weeds in a timely manner
• Fertilize if needed
• Seed properly
• Plant at the optimum time
• Select a competitive winter hardy winter wheat variety
• Control weeds in the growing crop

In addition, it's essential that you closely watch for weed developments and spray at the proper time to achieve maximum control. Most labels state that weeds must be treated before they are 6 inches tall. If weeds are under severe drought stress, wait for rain and spray about a week later. If downy brome is a problem and a winter wheat fallow rotation is being used, tillage is usually recommended immediately after harvest to plant the seeds and ensure maximum weed germination during the fallow period. Do not till if only a limited amount of crop residue is present after harvest since tillage will make the soil susceptible to wind and water erosion.

See these NebGuides:   Controlling Jointed Goatgrass (PDF 901 KB) Downy Brome Control (PDF713KB) Rye Control in Winter Wheat (PDF1.2MB)

Troublesome Weeds. Herbicides are available to control downy brome in the growing winter wheat and are best applied early. If jointed goatgrass and/or feral rye is a problem, use a rotation where wheat is not planted for at least three years under good moisture conditions and even longer under dry conditions. Herbicide-tolerant winter wheat varieties are available for fields with jointed goatgrass or feral rye problems. BeyondJ herbicide is then applied in the growing wheat. (Grower training is required before this herbicide can be purchased.) Check the label for additional information.

Robert Klein
Extension Cropping Systems Specialist
West Central REC, North Platte