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Go after resistant waterhemp

Waterhemp is a weed that’s starting to make noise in Indiana. Bill Johnson, a Purdue University weed control specialist, recently issued a press release about waterhemp.

What are others saying about the weed, which up until recently was more of a problem for farmers in other states? “Certainly in west-central and southwest Indiana this year, lack of control of both waterhemp and marestail was cussed and discussed by multiple producers in Knox, Sullivan, Vigo, Clay and Vermillion counties,” says Betsy Bower, a Certified Crop Adviser and an agronomist with Ceres Solutions, Terre Haute.

Key Points

Some resistant waterhemp has been documented in Indiana.

Not all waterhemp that doesn’t go down is resistant.

It’s easier to control this weed in corn than in soybeans.


Both farmers and custom applicators tried a variety of products, she notes. Success varied, but there was no silver bullet. Problems with waterhemp were also noted in eastern Indiana.

“Was all of the waterhemp in question resistant?” she asks. “Probably not. Was some of it resistant? Most certainly. Waterhemp resistance has been documented in Vigo County.”

The weed is easy to spread and does seem more prevalent than a decade ago. Waterhemp is a tough weed to control whether it’s resistant or not, Bower says. But if it’s resistant, glyphosate will be of little help.

Control methods

Bill Johnson and Glenn Nice, also a Purdue weed control specialist, have worked up recommendations for those battling this tough weed. In corn, they note, most pre-plant or preemergence herbicides provide good control, especially when mixed with atrazine. This older herbicide is highly effective against waterhemp, but at higher rates. Many preemergence products do not have a full rate of atrazine.

Increasing the rate of atrazine helps in some situations. Postemergence herbicides with great activity in corn include atrazine, applied on corn up to 12 inches tall, dicamba, Status, Callisto, Laudis, Corvus and Impact.

Glyphosate still works well on Roundup Ready corn in nonresistant populations of waterhemp. If you grow LibertyLink corn, Ignite plus atrazine will control small waterhemp.

Soybean challenge

In soybeans, best recommendations suggest starting with preplant or preemergence products containing Authority, Valor or Dual. These help reduce the waterhemp population that will emerge later and increase flexibility for postemergence application timing.

Effective postemergence choices in soybeans include Ultra Blazer, Cobra, Reflex and Ignite. Because these are contact herbicides, increasing carrier volume is needed for better control. Ignite can only be applied on LibertyLink soybeans. Apply these products when waterhemp is less than 4 inches tall.

If waterhemp is not resistant, Warrant or Outlook can be applied with glyphosate to provide another layer of residual control.

Search for resistance

If you want to know if you have resistant waterhemp plants, or if you just had a hard time controlling them this year instead, there is a way to find out, Bower says. You will need to collect samples from 10 to 20 seed heads from areas at least 50 feet from the edge of the field.

Send your samples to Gurinderbir Chahal in the Botany and Plant Pathology Department. His email is gschahal@purdue.edu. Or contact him at Purdue Weed Sciences, Department of Botany and Plant Pathology, Lilly Hall of Life Sciences, 915 W. State St., West Lafayette, IN 47907.

This article published in the December, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.