Preharvest herbicide application in winter wheat
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The primary benefit of pre-harvest herbicide application in small grains is in aiding harvest. This treatment will not increase yield. However, there are several negative aspects of a pre-harvest application including: the damage to the small grain caused by driving over the field with the application equipment and potential off-target injury. Therefore, this practice is generally not recommended.
2,4-D is registered for pre-harvest application in wheat, barley, rye and oats. Many amine forms and low volatile ester forms can be used in wheat, barley and rye, and the labeled rates are 0.5-1 lb ai/acre (1-2 pints/acre of a 4 lb/gal formulation). Only certain amine forms are labeled for use in oats. Check the herbicide label for details. Application can be made after the small grain has reached the hard dough state. Earlier application carries a serious risk of crop injury.
When applying 2,4-D as a pre-harvest treatment, the two pints/acre rate will be most effective. Remember, that the weeds being treated will be very large and that one pint/acre of 2,4-D may not be adequate. The best choice between the amine and ester form of 2,4-D will depend upon the individual situation. The ester will be more effective on the weeds; however, the risk of off-target injury from volatilization must be considered. To minimize this problem, the ester form of 2,4-D should not be used if the temperature is expected to exceed 80°F within one day of treatment. The amine form of 2,4-D will be less effective, but does not have the risk of volatilization. Both herbicides can cause off-target injury from spray particle drift. Therefore, applicators must be very cautious about wind speed.
Both forms of 2,4-D have activity on broadleaf weeds, but not on grasses or yellow nutsedge. Expect that a minimum of ten days will be required for a pre-harvest application of 2,4-D to desiccate weeds sufficiently to aid in harvesting. Following harvest, the straw must not be fed to livestock.
Several glyphosate products are labeled for pre-harvest application in wheat and feed barley. This treatment should not be applied to wheat grown for seed. Application can be made after the hard dough stage (30 percent or less grain moisture) and at least seven days prior to harvest. Refer to the specific product label for maximum rate, tank mixtures with other herbicides and spray volume. Ammonium sulfate should be added to all glyphosate products in order to minimize the negative effect of hard water on glyphosate activity. The addition of ammonium sulfate also can improve glyphosate’s efficacy on some species, especially velvetleaf, regardless of water quality. Dry ammonium sulfate should be labeled “spray grade” quality and applied at 17 lbs/100 gal. Liquid ammonium sulfate products are equally effective if applied at a rate equivalent to 17 lbs/100 gal.
Glyphosate applied at 0.75 lb ae/A will provide some control of perennial broadleaves, such as milkweed and Canada thistle. If temperatures are high at the time of application, quackgrass will not be actively growing and, therefore, will be much more difficult to control.
In general, the most effective system for controlling perennial weeds in small grains is to harvest the wheat, allow the weeds to re-grow, and apply an herbicide in the fall. If the patches of perennial weeds are too dense to harvest, they can be mowed following the wheat harvest.
Several factors must be considered when deciding whether or not to treat. These include the severity of the weed problem (both density and percentage of the field affected), the weed species and the risk of off-target injury.