June 22, 2006

Early Control Vital

Much of the 2007 winter wheat crop has been subjected to wet weather, which has favored development of diseases such as scab, leaf rust and stripe rust. Other problems included freezes, wheat streak mosaic, root and crown rot, nitrogen deficiency, powdery mildew, bunt, soil borne mosaic, tan spot., loose smut, and fusarium head blight.

Figure 1. The effect of winter wheat crop residue levels on weeds in corn following winter wheat treated with herbicides post harvest. In general, 1 bushel of wheat produces 100 lbs of crop residue or 60 bushels of winter wheat equals 6,000 lbs of crop residue.

Controlling weeds after winter wheat harvest will be a challenge this year, especially with diseases opening up the canopy, resulting in less crop residue (see Figure 1).

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.

Weeds under stress are difficult to control. 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. This year some fields will need to be sprayed before 15-30 days. The key is to prevent weeds from using soil water and producing weed seeds.

Split Treatments

Split treatments, which have a good history of effectiveness, should be especially beneficial this year. In Kansas, there was a 20 bushel increase in corn yields the next year for treatments applied in July vs 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. 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. Liquid ammonium sulfate, with or without a drift retardant, also is available.

It’s difficult to recognize weed stress so it’s wise to always add ammonium sulfate. Improve control by increasing the rate of glyphosate. Allow 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^oF.

With the early harvest and reduced crop residue this year, a second treatment of glyphosate may be needed this summer to maintain weed control.

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 possibly Gramoxone Extra (add surfactant), depending on the amount and size of volunteer winter wheat, downy brome or jointed goatgrass or other weeds present. Several options are available for using nonselective herbicides with difficult to control weeds. With Gramoxone Extra, 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. Other formulations of Gramoxone include Gramoxone Inteon. The active ingredient varies among products 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 only 0.5 quart per acre is often the maximum allowed in one season.

The advantage of split treatments is that they provide excellent control of volunteer winter wheat and other winter annual grasses.

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.

Effective Cultural Practices to Control Weeds

Many options besides increasing herbicide rates 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. Preparing a good firm seedbed, controlling weeds in a timely manner, fertilizing if needed, seeding properly, planting during the optimum time, selecting a competitive winter hardy winter wheat variety, and controlling weeds in the growing wheat offer the best chance of reducing weed population and vigor after harvest. 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.

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. Beyond™ 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