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Waterhemp weed showing greater resistance to glyphosate

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Written Thursday, September 29, 2011  

A Purdue Extension weed specialist urges farmers to take necessary steps now to better control yield-reducing waterhemp next year as it spreads through Indiana with increasing resistance to glyphosate.

Farmers need to learn more about waterhemp because it has shown greater resistance to the herbicide the past 2-3 years, Bill Johnson said. The weed has been present in some Indiana field surveys for about 10 years.

"More recently, we've had an increasing number of complaints about glyphosate's failure to control waterhemp in soybean production," Johnson said. "The number of calls we have been getting on that has essentially doubled each year over the last couple of years."

Waterhemp now is found throughout Indiana. It is most prevalent in the southern, southwestern, northwestern and east-central portions of the state.

Johnson has produced the educational document "Waterhemp an Emerging Weed Problem in Indiana" to raise awareness among farmers and help them develop a plan to better control the weed next year. The document, which includes pictures to help farmers identify waterhemp, is available online at http://www.ag.purdue.edu/btny/weedscience/Documents/Waterhemp-11.pdf

Waterhemp is among a dozen species in the pigweed, or amaranth, family. Other pigweed found in Indiana includes redroot pigweed, smooth pigweed and Palmer amaranth.

"We've had pigweed for a long time, but this one - waterhemp - has really emerged as a major threat to soybean production," Johnson said.

One waterhemp plant can produce as many as 1 million seeds. Just a few untreated weeds in a field can lead to a major infestation within a couple of years, Johnson said.

Waterhemp is more of a problem for soybeans than it is for corn. A heavy infestation in a soybean field can reduce yields by 30-50 percent, Johnson said.

Waterhemp has proven difficult to control. Although Johnson said pre-emergence herbicides greatly reduce waterhemp density, it emerges throughout the growing season and usually also requires a postemergence herbicide treatment, especially for soybeans.

"It is very common to have to spray a field multiple times to get the waterhemp under control," Johnson said. "One-pass herbicide programs are not going to be effective on it because it has a very long emergence pattern."

Because waterhemp is dioecious male and female flowers on separate plants two plants always mix genes when they reproduce, increasing genetic diversity within a population and the potential for spreading herbicide resistance and other traits that enable the plant to survive.

Johnson recommended that farmers now going through their fields at harvest time take note of whether waterhemp is present and, if so, the level of density and whether there appears to be more of it this year than last. He said that will help them determine a plan for better controlling it.

Waterhemp first was controlled in the mid-1980s by herbicides known as ALS- and PPO-inhibitors before Roundup Ready soybeans, resistant to glyphosate, were introduced. But glyphosate-resistant waterhemp also has been found in recent years in Illinois and other states in addition to Indiana.

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