Remain Offensive Against Glyphosate-Resistant Horseweed

The most widely distributed glyphosate-resistant (GR) weed in the United States is horseweed (or marestail), currently confirmed resistant in 14 states. In the U.S., GR horseweed was initially reported in Delaware in 2000. There are two possible sources for GR horseweed -- home-grown resistance, which occurs when resistance develops on a specific farm, and resistant seed or pollen introduced from another field.  Horseweed seeds are very small and light and easily carried by wind; can germinate immediately after maturity; and have the ability to germinate in the fall, spring and mid-summer.

Horseweed flourishes in areas of reduced tillage and little diversity. In places such as western Tennessee, where no-till use is heavy, horseweed has emerged as one of the most problematic weeds for growers.  Last summer, researchers at Southern Illinois University were also investigating populations of horseweed uncontrolled by glyphosate that showed similarities to the rapidly spreading populations in Tennessee (http://www.greenbook.net/viewStory.asp?StoryID=186).

Although GR horseweed is a serious threat, there are management practices that can help delay the onset of resistance.  Chris Boerboom, extension weed scientist at the University of Wisconsin, recommends an offensive plan to guard against resistant horseweed.  This plan should include a tankmix in burndown herbicide programs and making sure to treat horseweed before it exceeds 4 to 6 inches in height.  It is especially important to control horseweed before planting soybeans since in-crop options are limited.  For the full report, click here. What has been your best defense against glyphosate-resistant horseweed?

Visit the Resistance Fighter library or click here for specific GR horseweed management tips for cotton, corn and soybeans, provided by Syngenta.