COLUMBUS, Ohio — Marestail (horseweed), a prevalent weed throughout Ohio crop fields, has reached its stage of development where herbicide treatments become less effective, especially in no-till fields.
Jeff Stachler, an Ohio State University Extension weed specialist, is encouraging growers who have yet to apply a burndown program in their soybeans to do so soon.
"The larger the weed gets, the stronger the burndown program one needs for effective control. So growers shouldn't be waiting," said Stachler. "Preferably, you want to get rid of that vegetation before you plant and it does help the soil dry out faster. A clean start bodes well for the crop because there are no weeds present at the time of planting to compete."
Marestail is now bolting, meaning it's producing a prominent upright stem from which flowers will develop. The larger the weed gets, the harder it becomes to control. Marestail in Ohio can be either glyphosate resistant, ALS-resistant (acetolactate synthase), or a combination of both, and herbicide treatment is dependent upon what type of resistance marestail may exhibit in a grower's field. Not all plants in a field or all fields in Ohio have herbicide-resistant marestsail.
"The minimum rate for glyphosate on any marestail population regardless of resistance is no less than 1.1 pounds acid equivalent per acre. The rate range is anywhere from 30-48 ounces per acre depending on product formulation," said Stachler. "But to get control of all herbicide-resistant biotypes, a grower is going to have to do some mixing."
A strictly glyphosate treatment program will only kill the most sensitive and ALS-resistant marestail plants. Using glyphosate in addition to 2,4-D (at least 0.5 pounds active ingredient per acre), and adding an ALS herbicide such as FirstRate (also in Gangster), or a product containing chlorimuron will improve control. For another possibility, Stachler suggests a treatment with a combination of Gramoxone Max (at least 1.5 pints per acre), 2,4-D (at least 0.5 pounds active ingredient per acre, and Sencor (at 8.0 ounces per acre). Once the stems are greater than 7 inches, the minimum Gramoxone Max rate should be 2.1 pints per acre.
"There are other combinations available depending on which types of resistance you have, but a glyphostate-2,4-D-ALS treatment, and the Gramoxone treatment are considered premiere programs," said Stachler.
Glyphosate-resistant marestail is widespread throughout western Ohio, while ALS-resistant marestail is prevalent throughout the entire state. Only three sites have been confirmed showing multiple resistance.
"Marestail is one of those weeds if you do the wrong thing at the wrong time, it's going to be present at harvest time and potentially at high numbers," said Stachler.