Preharvest herbicide options for soybeans: Weeds may be an issue
Consider using a preharvest herbicide application in weedy soybean fields this year.
The lack of good row closure and challenges with weed control this season has left several soybean fields weedy as harvest approaches. Without a killing frost, the “green” stems and leaf tissue of these weeds can cause issues with harvest. Currently in Michigan there are four different herbicide options labeled as “harvest aids” to help desiccate or dry down the green stem and leaf tissue of weeds that can hinder soybean harvest. These herbicides are glyphosate (Roundup and several other formulations), Gramoxone 2SL (paraquat and other formulations), Clarity 4L (dicamba) and Aim 1.9EW (carfentrazone). Differences in these products include the speed of activity, preharvest intervals, recropping restrictions and effectiveness. Below outlines the guidelines and additional use precautions to consider prior to using each of these herbicides.
Glyphosate (Roundup and several other formulations) is one of the more common herbicides used as a harvest aid in soybeans. Glyphosate can be used in both Roundup Ready and non-Roundup Ready soybeans. However, depending on the soybean trait the use instructions, including the amount of product that can be used and the preharvest intervals are different. Glyphosate should be applied after soybean pods have lost all green color. In Roundup Ready soybeans, the maximum use rate of glyphosate as a preharvest treatment is 0.75 lb a.e./A (equivalent to 22 fl oz/A of Roundup PowerMax). There is a 14-day preharvest interval for glyphosate in Roundup Ready soybeans.
In non-Roundup Ready soybeans, glyphosate can be applied at rates that range from 0.75 to 3.6 lb a.e./A (equivalent to 22 fl oz/A up to 3.3 qt/A of Roundup PowerMax) and there is a seven- day preharvest interval. It generally will take 10 to 14 days for maximum herbicide activity.
Preharvest applications of glyphosate are not recommended for soybeans grown for seed regardless of trait, and these applications will not control glyphosate-resistant weeds. As with all glyphosate applications, I recommend including ammonium sulfate (AMS) at 17 lb/100 gal of spray solution.
Gramoxone 2SL (paraquat) is also a popular harvest aid herbicide treatment. Of the harvest aid options, Gramoxone provides the quickest speed of activity, often times showing greater control of weeds three days after treatment compared with other herbicides. However, generally by 10 days after treatment, weed control with Gramoxone and glyphosate is often similar.
Gramoxone is a contact herbicide, so desiccation is dependent on good spray coverage (20 gallons of water per acre). The use rate of Gramoxone ranges from 8 to 16 fl oz/A. I would generally recommend the 16 fl oz/A use rate. A non-ionic surfactant (NIS) at 0.25 percent v/v must be applied with Gramoxone. The application timing for Gramoxone is when at least 65 percent of the soybean pods are a mature brown color, or when seed moisture is less than 30 percent.
There is a 15-day preharvest interval between application of Gramoxone and soybean harvest. Gramoxone is also a restricted-use pesticide, so a private or commercial pesticide applicator’s license is required for use of this product.
Clarity 4L (dicamba) is another preharvest herbicide option. However, this treatment is less common. Clarity can be used to help control or suppress annual, biennial and perennial broadleaf weeds at harvest.
The use rate for Clarity ranges from 8 to 32 fl oz/A. Clarity can be applied once soybean pods have reached a mature brown color and at least 75 percent leaf drop has occurred. There is a seven-day preharvest interval between application of Clarity and soybean harvest. Do not use Clarity as a preharvest treatment for soybeans grown for seed. It is not recommended that Clarity be used as a preharvest treatment if you are planning on planting winter wheat.
Aim 1.9EW (carfentrazone) is probably the least effective preharvest herbicide for weed desiccation. Velvetleaf would be one of its strengths. However, if this is the herbicide that you choose to use, there are a few guidelines that need to be followed. The application timing is when the crop is mature and grain has begun to dry down. The Aim use rate is 1.5 fl oz/A and Aim should be applied with 1 percent v/v MSO. The preharvest interval for Aim is three days.