Marestail, also known as horseweed, is top of mind for many corn and soybean growers preparing for the 2018 growing season. Because of its ongoing threat, Syngenta stresses the importance of a start-clean, stay-clean residual herbicide strategy to help maximize crop yields and manage future resistant marestail.
According to Purdue University, marestail was one of the first glyphosate-resistant weeds identified in U.S. row crops. It’s been reported in more than 10 states since its initial occurrence in 2000. Marestail seeds are highly mobile and can easily spread to new areas. This is especially concerning as each plant can produce up to 200,000 seeds and grow up to 6 feet high.
“Once you have a 4-inch horseweed in your soybeans, it becomes a major limiting factor,” stresses Bryan Young, a weed scientist at Purdue University. “If it’s ALS-resistant, we don’t have those herbicides to use on it. If you’re in a glyphosate system, you don’t have an option unless you go to dicamba- or glufosinate-tolerant soybeans.”
According to the United Soybean Board, marestail populations with evolved resistance to glyphosate and ALS-inhibiting herbicides are widespread, and it is easier to control the weed in the seedling, or rosette, stage. In addition, it has two primary periods of emergence, late March through June and then late summer through late fall.
"If you don’t get marestail early in the spring with spray, you’ll need a tillage tool to uproot them or cut them out of the ground. Otherwise, they are just there for the rest of the season," says Joe Humes, a soybean, corn and wheat grower in Wyaconda, Missouri. "The most important thing to stress to growers is to not cut rates and watch weed height because the bigger it gets, the harder it is to kill. Some guys want to wait and do it all in one pass, and the weeds just get too big. At a certain point, they can’t be controlled."
Brad Hemeyer, a farmer in Gilliam, Mo., says, “A two-pass program is pretty much standard. In my programs, I plan ahead for that. I try to get out to my fields as early as I can, as far as burndown options. And I’m looking more into fall application, too, especially in our no-till fields with marestail problems.”
In addition to residual herbicides with multiple non-ALS, non-glyphosate modes of action, growers can adapt to manage the spread of resistant marestail through a variety of other methods.
"There are so many practices that help growers maintain the viability of their herbicide tools," says Dane Bowers, Syngenta herbicide technical product lead. "Crop rotation, cover crops, cultivation and harvest weed seed control are some of the ways to develop a truly diversified program, one that does not depend solely on herbicides. There may be an additional investment up front, but the return on investment is strong over time if we can prevent resistant weeds, like marestail, from developing."