ST. PAUL, Minn. (2/20/2012) —Waterhemp, a summer annual weed species in the pigweed family, has been a significant issue for corn and soybean growers in the central and western Corn Belt states for more than a decade. The latest challenge in this fight is managing for waterhemp's growing resistance to various herbicides, including glyphosate.
Glyphosate-resistant waterhemp was first confirmed in southern Minnesota in 2007 and continues to increase, likely due to the continued planting of Roundup Ready® crops and the exclusive use of glyphosate.
Waterhemp begins emerging near mid-May and continues through the beginning of August. Three factors make it especially difficult to keep under control: an ability to produce nearly 1 million seeds per plant, continual germination throughout the growing season, and an increased frequency of biotypes, or new weeds that are genetically adaptable to a diverse array of herbicide chemistries.
The longevity of waterhemp seeds in the seed bank is short compared to most species, with only 1 to 12 percent survival after four years. For that reason, complete control (zero seed production) of all waterhemp plants over a three- to four-year period should allow producers to take control of this difficult weed problem.
To reduce the selection pressure for glyphosate-resistant waterhemp, Extension crop specialists recommend using pre-emergence residual herbicides, increasing crop rotation diversity in the cropping system, and focusing on the use of Roundup Ready® crops in the rotation where the fewest alternative herbicides to glyphosate exist.
For more detailed information regarding crop rotation and herbicide effectiveness, see the Extension publication "Pre and Post Herbicide Diversification Options," (PDF) at www.extension.umn.edu/go/1096. For more information on how to proactively manage for waterhemp in sugarbeet, soybean, corn and wheat, visit www.extension.umn.edu/go/1097.
How to identify waterhemp
The stems of waterhemp plants have little to no hair compared to redroot pigweed, and the leaves are usually longer and narrower. Waterhemp seedlings have oval-shaped seed leaves and are hairless, appearing waxy or glossy-looking. Waterhemp can range from 4 inches to 12 feet high, but generally grows to about 4 or 5 feet in most field crop situations. In the mature stage, one way to differentiate waterhemp from the other pigweeds is to compare the seed heads. Redroot and smooth pigweed have denser, more compact seed heads than waterhemp.
For more Extension crops resources, visit www.extension.umn.edu/crops.
Any use of this article must include the byline or following credit line: Dave Nicolai is a crops educator with University of Minnesota Extension.
Media Contact: Catherine Dehdashti, U of M Extension, (612) 625-0237, firstname.lastname@example.org