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Less than perfect wheat stands this past spring and ample spring and early summer precipitation allowed weeds to get established in many winter wheat fields. These weeds have flourished after wheat harvest. Additionally, hail storms were common in western Nebraska near harvest and volunteer wheat is abundant in many stubble fields.

Volunteer wheat serves as a "green bridge" to carry wheat curl mites, which carry wheat streak mosaic, High Plains disease, and Triticum mosaic virus, to the next wheat crop. Controlling weeds and volunteer wheat is critical to having successful crops in 2012.

Controlling Grasses and Broadleaves

Good precipitation earlier this summer allowed weeds to get a strong start in wheat. To conserve water for the next crop and prevent weed seed production, these weeds need to be controlled within 15 to 30 days of harvest. It's essential that volunteer wheat be controlled at least two weeks prior to emergence of the next crop to break the "green bridge" to disease development.

Weeds growing in wheat stubble after wheat harvest use water that is best saved for the 2012 crop, be it corn, sorghum, sunflower, proso millet, or wheat. If allowed to go to seed, these weeds also pose problems for subsequent crops. The general rule of thumb is to wait 15 to 30 days after harvest to spray wheat stubble as part of a three-year rotation that includes a summer crop following winter wheat.

The key is to allow the weeds to recover some from being cut off at harvest, while not allowing so much growth that weeds become difficult to control with herbicides. As with all weed control, it's essential that you closely watch for weed development and spray at the proper time to achieve maximum control. Most herbicide labels state that weeds must be treated before they are six inches tall. If weeds are under severe drought stress, wait for rain and spray a week later.

Glyphosate is the most commonly used herbicide for weed control in wheat stubble. Some glyphosate products include sufficient surfactant while many products require more. 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. Glyphosate usually provides excellent control of grass weeds, if they are actively growing, but the control of broadleaf weeds can vary widely depending on species, size, and rate of growth. Adding 2,4-D with the glyphosate will help control broadleaf weeds. If temperatures are above 80^oF, use the amine formulation of 2,4-D to reduce the risk of vapor drift to nearby crops. For additional information, treatments, and rates go to the Ecofarming Section (page 25) of the 2011 Guide for Weed Management.

Controlling Volunteer Wheat

By far, the greatest risk for developing serious wheat streak is from volunteer wheat that results from hail during the three weeks prior to harvest. This volunteer allows for a continuous "green bridge" to carry mites and virus to the next wheat crop. Uncontrolled pre-harvest volunteer wheat virtually assures a serious infection of wheat streak in surrounding fields the following year!

The risk of developing wheat streak from volunteer wheat emerging after harvest is much lower than the risk from pre-harvest volunteer, but this risk will depend on the length of time between its emergence and when wheat emerges in the fall.

Breaking the Green Bridge

The most likely source of mites carrying wheat streak mosaic virus are the fields right next to a wheat field. Therefore, controlling volunteer wheat will reduce the risk of developing problems in adjacent fields. However, when large hailstorms result in widespread volunteer and a number of uncontrolled volunteer fields are present throughout a community, the risk of developing serious problems is increased for the entire community. A community-wide effort is necessary to control volunteer in those areas where widespread pre-harvest hail resulted in extensive volunteer.

The most effective way to manage this disease is to break the over-summering "green bridge," and thus, avoid the buildup of mites and virus before winter wheat is planted in the fall. To be effective at breaking the green bridge, all volunteer plants must be completely dead. The goal must be to completely eliminate pre-harvest volunteer for two weeks between harvest and emergence of fall-seeded wheat.

Control Methods

Volunteer can be effectively controlled by tillage or chemical means. Weather conditions will influence the effectiveness of the method that is used. If conditions following harvest are warm and dry, shallow tillage can provide rapid and highly successful control of volunteer wheat. Tillage is less effective at providing adequate volunteer control when soils are wet or cool conditions exist.

When plants are growing well, glyphosate will provide excellent control of volunteer. If plants are stressed, glyphosate efficacy is reduced and growers should consider shallow tillage or a herbicide containing paraquat, for example Gramoxone Inteon.

Drew Lyon, Extension Dryland Crops Specialist, Panhandle REC, Scottsbluff
Gary Hein, Entomologist and Director of the Doctor of Plant Health Program, Lincoln
Greg Kruger, Extension Cropping Systems Specialist, West Central REC, North Platte