Late-season rains and later-than-normal canopy closure have led to some weedy soybean fields in parts of the Midwest. Christy Sprague, crops specialist at Michigan State University, says these weeds can lead to difficulties with harvest if stems or leaves are still green.
"Frost usually helps desiccate these weeds," Sprague says. "However, if soybeans are ready to be harvested before weeds have dried down, there are a few herbicide options that growers can use as harvest aids to help desiccate or dry down 'green' stem and leaf tissue."
Currently, there are three different herbicide options labeled in Michigan for preharvest applications in soybeans, including glyphosate, Gramoxone Inteon 2SL (paraquat and other formulations), and Aim 1.9EW (carfentrazone). Differences in these products include the speed of activity, preharvest intervals and effectiveness.
Sprague says you should check product labels for uses in your state.
She says glyphosate is the most effective herbicide for broad-spectrum weed desiccation followed closely by Gramoxone Inteon. Glyphosate generally takes 10 to 14 days after application for maximum activity.
Even though most soybean fields are Roundup Ready, Sprague says glyphosate should not be applied until soybeans are mature and have lost all of their color. Earlier applications may result in glyphosate residues being found in the harvested beans.
There is a seven-day preharvest restriction between glyphosate application and soybean harvest. Sprague says you should not use glyphosate as a harvest aid for soybeans that are grown for seed.
There are several glyphosate formulations that are labeled for preharvest applications in soybeans, and the maximum use rate is often dependent on the product, she says. It's important to examine the individual product labels to find the maximum labeled rate that can be used.
In most cases, the use rate of 0.75 lb ae/A — which is equivalent to 22 fl oz/A of Roundup WeatherMax and Roundup PowerMax; or 24 fl oz/A of Durango DMA and Touchdown Total; or 32 fl oz/A of a 3 lb ae/gal glyphosate formulation — will generally desiccate most annual weeds. Ammonium sulfate (AMS) at 17 lb/100 gal should always be added, regardless of glyphosate formulation.
Gramoxone Inteon 2SL (paraquat) can also be used as a harvest aid to help desiccate uncontrolled weeds that may interfere with harvest. Sprague says Gramoxone Inteon has generally been shown as the herbicide with the quickest speed of activity. Since it is a contact herbicide, desiccation is dependent on good spray coverage.
The use rate of Gramoxone Inteon is 12 to 16 fl oz/A. A non-ionic surfactant (NIS) at 0.25 percent v/v or crop oil concentrate (COC) at 1% v/v must be applied with Gramoxone Inteon. The application timing for Gramoxone Inteon is when at least 65% of the soybean (indeterminate varieties) pods are mature brown (seed moisture less than 30%).
Sprague warns not to apply Gramoxone Inteon within 15 days of harvesting soybeans. Gramoxone Inteon is also a restricted-use pesticide, so a private or commercial pesticide applicator’s license is required for use of this product.
Aim 1.9EW (carfentrazone) is another option that may be used as a harvest aid. However, the spectrum of weed desiccation is not as broad as the other herbicides labeled for this purpose, Sprague says.
Aim is also a contact herbicide, so desiccation is dependent on good spray coverage. Aim should be applied to mature soybeans once pods have lost all their color. The Aim use rate is 1 to 1.5 fl oz/A and Aim should be applied with 1% v/v COC or a methylated seed oil. The pre-harvest interval for Aim is three days.