Watch Out for Late-emerging Waterhemp in Soybeans This Summer
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Waterhemp is known as one of the toughest weeds to control in soybean fields across the Midwest each year. And for good reason: It has developed resistance to six classes of herbicides.1 In addition to its ability to evolve, waterhemp is one of the latest-emerging summer annuals.
“Waterhemp tends to begin emerging later in the spring and can continue emerging well into the growing season,” says Joe Armstrong, a field scientist, Corteva Agriscience. “This means that waterhemp does not come up in a single flush and new seedlings must be controlled over a relatively long period of time compared with many other weed species.”
“Waterhemp tends to begin emerging later in the spring and can continue emerging well into the growing season.”
So, waterhemp can crop up after herbicide applications are complete and rob yield even into the summer months.
Waterhemp Competes With Soybeans
Waterhemp is stiff competition for soybeans due, in part, to its prolific seed creation and size. One female waterhemp creates, on average, about 250,000 seeds. Waterhemp plants can grow very quickly, about 1 inch per day, and reach up to several feet high.1 Those weeds can crowd out smaller, especially younger, soybean plants.
“Waterhemp plants compete directly with soybean plants for light, water, nutrients and space,” Armstrong says.
And that can take a big bite out of yield. If there is season-long competition with more than 20 waterhemp plants in 1 square foot of soybeans, yield can be reduced by up to 44% in 30-inch rows and by up to 37% in 7.5-inch rows. Even waterhemp that’s emerged as late as the V5 soybean growth stage can reduce yield up to 10%.2
Good Conditions for Waterhemp
Armstrong says waterhemp can thrive in many different climates, geographies and farming practices, including in both no-till and traditional operations. There’s just one factor that might have an impact on the weed’s growth: temperature.
“Since waterhemp emerges later than other weeds, it may need slightly warmer temperatures and may not be as prevalent in a cool spring and early summer,” Armstrong says. “However, once the season gets going and temperatures warm up, waterhemp will quickly catch up and will emerge rapidly.”
Late-emerging Waterhemp
Sometimes waterhemp will continue to emerge after your customers’ herbicide applications are complete and it may be too late to go back and spray their beans again. Armstrong says there are still a few measures you can take to help farmers if you find it this year.
“If it is a relatively small patch of waterhemp, removing the plants by hand is a great step to prevent them from adding to the soil weed seedbank and leading to increased populations in future years,” he advises.
“If it is a relatively small patch of waterhemp, removing the plants by hand is a great step to prevent them from adding to the soil weed seedbank and leading to increased populations in future years.”
Armstrong says your customers can also submit a sample of the weed to a diagnostic lab like the University of Illinois Plant Clinic for molecular testing. “This can help provide confirmation of resistance and give growers information about which herbicides are no longer effective on waterhemp in their fields,” Armstrong says.
This information can then help you and the farmer plan a good weed control program in the future to keep soybeans clean.
Controlling Waterhemp in Soybeans
When it comes to creating that program, Armstrong advises using a mixture of cultural practices and herbicide solutions. He says tillage, where possible, can help prevent waterhemp and so can tighter row spacing. “Narrow rows provide earlier canopy closure, which can be an effective cultural control method for waterhemp and other weeds. Earlier canopy closure will help shade out any late-emerging waterhemp plants and help keep fields clean until harvest.”
A weed control program approach that features burndown, preemergence and postemergence herbicides is also crucial to controlling waterhemp in soybeans.
“Growers must use a diverse herbicide program with multiple modes of action to control waterhemp and make timely applications to small weeds to ensure maximum efficacy,” Armstrong says. “Additionally, because of the extended emergence window for waterhemp, it can be advantageous to use an overlapping residual herbicide with the postemergence application.”
Armstrong says a good preemergence to use is Sonic® herbicide, which has two modes of action and extended residual activity. For Enlist E3® soybeans, he advises a postemergence application featuring Enlist One® herbicide and glufosinate. In all, this program provides five effective modes of action against the weed. Another postemergence option for all soybean farmers is EverpreX® herbicide. EverpreX has extended residual activity and is strong against glyphosate-resistant weeds like waterhemp.
Waterhemp can certainly be tough. However, Armstrong says, with the right program approach, you can help your customers control it. Just be on the lookout for waterhemp this summer and, if you find it, take steps to keep it from coming back next year.
1United Soybean Board. 2021. Waterhemp. https://iwilltakeaction.com/weed/common-waterhemp
2Steckel, L. E., and C. L. Sprague. 2004. Late-season common waterhemp (Amaranthus rudis) interference in narrow- and wide-row soybean. Weed Technol. 18(4):947–952.
The transgenic soybean event in Enlist E3® soybeans is jointly developed and owned by Corteva Agriscience LLC and MS Technologies, L.L.C. Enlist Duo® and Enlist One® herbicides are not registered for sale or use in all states or counties. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your area. Enlist Duo and Enlist One herbicides are the only 2,4-D products authorized for use with Enlist™ crops. Consult Enlist herbicide labels for weed species controlled. EverpreX®, Sonic™ and Trivence® are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Consult the label before purchase or use for full details. Always read and follow label directions.