While there are dozens of common weeds that threaten yield each season, many Midwest farmers who grow corn or soybeans are facing off with waterhemp now.
“New cases of resistant waterhemp are daunting to those of us in the ag industry because it threatens the already-delicate ROI ratio,” says Dave Roome, customer technical specialist, Corteva Agriscience, Agriculture Division of DowDuPont. “But waterhemp can grow stronger each year. We have to stay aggressive at reducing the waterhemp seedbank to protect fall yield and get a head start on next season.”
For instance, the recent discovery of six-way resistant waterhemp in Missouri is a major threat to future corn and soybean yield. In the past 25 years, waterhemp has developed documented resistance to six classes of herbicides. Many fields across the Corn Belt also house waterhemp that is resistant to two or more modes of action, which requires farmers to evaluate their control program and incorporate additional, or different, herbicides.
“Waterhemp in my area has a really bad resistance problem with glyphosate, which is one reason we’re using SureStart II,” says Missouri farmer Jimmy Daniels. “For corn, we use a two-pass program with SureStart II as our main first pass with a little atrazine. Then, we’ll come back with a second pass of something other than glyphosate to try to prevent resistance. To have a clean field at the end of the year is a great thing.”
Many farmers are improving yield potential and managing the seedbank by using a mix of regional-specific best practices and a program herbicide approach throughout the growing season.
Roome shares five tips to help farmers manage waterhemp for the remainder of this season and to limit additional resistance development:
- Scout diligently. Identify waterhemp early and continue to check your fields now, during midsummer. Ongoing scouting helps farmers plan timely postemergence herbicide applications and identify potential problem fields for the following year. It can be difficult to identify waterhemp in early growth stages because it looks similar to Palmer amaranth and other pigweed species. The leaves are generally longer and more lance-shaped than other pigweeds.
- Layer residual herbicides. Layering residual herbicides keeps fields clean longer, typically through crop canopy closure, to manage the seedbank. In corn, farmers can apply three unique modes of action with SureStart II herbicide preemergence followed by an additional unique mode of action from Resicore herbicide early postemergence. In soybeans, Sonic herbicide provides two modes of action preemergence followed by DuPont EverpreX herbicide for an additional mode of action postemergence. Farmers may also add atrazine to their corn programs and glyphosate to corn and soybean programs, in areas where waterhemp isn’t resistant, to increase the number of different modes of action.
- Plant narrower rows. Narrow row spacing can help suppress waterhemp growth by allowing crops to reach canopy closure quicker. Also, tillage is an enemy of waterhemp, Roome says. Tillage can help lower waterhemp populations because the seeds must germinate in the top part of the soil; however, tillage may not be a viable method of control on land prone to erosion.
- Prevent weeds from going to seed. Just a few weeds left in a field can present significant problems for the next season. Waterhemp that goes to seed in corn or soybean fields can potentially cross-pollenate with a population in another field and build additional resistance.
- Maximize application technology. Pay close attention to herbicide labels to maximize the efficacy of the product. Not every herbicide can be applied in the same manner with the same nozzles, water volumes, pressures, and adjuvants. Waterhemp requires herbicide control and effective cultural practices, such as rotating crops, which should be planned for more than a single year at a time, Roome says. When rotating crops, think beyond just rotating corn and soybeans to what you can do in that system. Rotating crops also allows farmers to rotate modes of action and adjust tilling plans for corn and soybean fields.