Waterhemp seedlings are similar to their pigweed cousins. Courtesy University of Wisconsin.

In the war between waterhemp and crop producers, waterhemp is now winning, according to a University of Missouri weed scientist.

Waterhemp was a significant weed issue in corn and soybean production in the central and western cornbelt in the mid-1990s, but was controlled with certain herbicides.  Populations of waterhemp that are resistant to these herbicides are now widespread in the Midwest.

Kevin Bradley, an associate professor of plant sciences in the MU College of Agriculture, Food and Natural Resources and Extension weed specialist, said the big deal about this weed is that it is now one of the most difficult to control. Coupled with the fact that waterhemp emerges later in the season than most other weed species means that new weed management strategies must be developed.

Kevin Bradley.

“If you’ve got just glyphosate-resistant waterhemp, you should consider yourself lucky,” Bradley said. “We’re seeing waterhemp with multiple resistance to all other herbicides that we would use to control glyphosate-resistant waterhemp, so we have fewer options and it is much more costly to control.”

Different Approach Needed

A different mindset is needed to control this weed, Bradley pointed out. Producers can no longer rely on the simplicity of a glyphosate system.

“It was simple, it was easy, it controlled everything, and now that is just simply not the case,” he said. “As I look into the future, we must become more sophisticated with our weed management. It is not going to get simpler.”

Controlling waterhemp will require more proactive management, spraying much smaller weeds, and rotating to herbicides with different modes of action.

Bradley said understanding the biology of waterhemp and identifying its strengths and weaknesses are critical. And its strengths are formidable: Waterhemp produces on average 300,000 seeds per plant, grows an inch and a half a day during the height of the growing season, and has evolved resistance to just about every herbicide ever sprayed on it.

“We can name strengths all day long. That’s the hard part about waterhemp; it has so many things that enable it to survive,” Bradley said. “But more important are the weaknesses, and waterhemp has two, as I see it.

“Number one, waterhemp seed is relatively short-lived in the soil — four or five years. If you’ve kept waterhemp from producing seed and returning seed back onto that land for four years, you are probably going to virtually eliminate waterhemp from your fields.”

The second weakness Bradley cites is that waterhemp seed does not emerge from low soil depths. He doesn’t recommend it to every grower in every place, but where appropriate, deep tillage can bury that seed and it will not come up.

“In addition to understanding the biology of waterhemp, if we can rotate to multiple modes of action I think we can really get a handle on this problem,” Bradley said.