Marestail, a prevalent weed in 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, encourages 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," Stachler said. "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 -- commonly known as horseweed -- is now bolting, meaning it is 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, acetolactate synthase (ALS)-resistant or a combination of both. Herbicide treatment depends on 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 lbs. acid equivalent per acre," Stachler said. "The rate range is anywhere from 30-48 oz. per acre depending on product formulation. But to get control of all herbicide-resistant biotypes, a grower is going to have to do some mixing."

A glyphosate-only treatment program will kill just the most sensitive and ALS-resistant marestail plants. Using glyphosate in addition to 2,4-D -- at least 0.5 lbs. active ingredient per acre -- and adding an ALS herbicide such as FirstRate -- also in Gangster -- or a product containing chlorimuron will improve control.

Another treatment option is a combination of Gramoxone Max of at least 1.5 pints/acre, 2,4-D of at least 0.5 lbs. active ingredient per acre and Sencor of at least 8 ounces/acre. Once weed stems are greater than 7 in., the minimum Gramoxone Max rate should be 2.1 pints/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," Stachler said.

Glyphosate-resistant marestail is widespread throughout western Ohio, while ALS-resistant marestail is prevalent in 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 harvesttime and, potentially, at high numbers," Stachler said.