Marestail control essential to protect soybean yields

No-till practices save soil and offer many other benefits, but soybean producers know there’s at least one big disadvantage: Not tilling gives weeds, particularly problematic marestail, a chance to thrive.

“The biggest challenge we have in no-till soybeans across Ohio and surrounding states is control of glyphosate-resistant marestail in the spring,” said Ohio State University Extension weed specialist Mark Loux.

Marestail emerges in fields from late March through June, and again in late summer through fall. Spring-emerging marestail competes with soybeans throughout the growing season, eventually bolting to a height of 3 to 6 feet, enough to interfere with harvest. It’s more of a problem in the southern two-thirds of the state, though it’s moving north, Loux said.

Loux said a one-two punch is necessary for marestail control in no-till fields: An effective burndown herbicide treatment to ensure planting is done in weed-free fields, and a residual treatment controlling the growth of any new weeds until early to mid-June, when the leaves of the soybean plants are large enough to form a canopy that provides plenty of control.

In a 2010 marestail study, Loux found soybean yields were severely affected when marestail wasn’t controlled:

• In fields where a burndown treatment failed to control marestail, yields averaged 51 bushels per acre.

• In fields where the burndown treatment was effective, but no residual herbicide was applied, yields averaged 57 bushels per acre.

• In fields where both burndown treatment was effective and residual herbicides were used, yields averaged 65 bushels per acre.

This spring, it should be easier for farmers to make an effective burndown application, Loux said. Because last fall was so dry, most of the marestail that emerged at that point
(which is normally difficult to eradicate from fields in the spring) didn’t thrive. But a burndown treatment is still needed for spring-emerging weeds, he said.

Timing of the burndown application is an important factor, Loux said. Cold temperatures can reduce herbicides’ effectiveness. Don’t apply treatments when nighttime temperatures could get down to 35 or 40 degrees. Wait until conditions warm up and nighttime temperatures are steadily 40 or 50 degrees, he said.

Weed problems are also an issue in no-till corn fields, Loux said, but farmers generally have a wider array of effective herbicides to handle those problems.

“For soybeans, the choices are fewer as to what will actually control the vegetation that’s out there, and you have to use the right rates and the right combinations to really get control,” he said.

Specific recommendations for both burndown herbicide treatments and residual control are listed in an updated fact sheet, “Control of Marestail in No-Till Soybeans,” by Loux and Purdue University weed specialists Bill Johnson and Glenn Nice. It is available on OSU Extension’s Agronomic Crops Team website,

Additional information is available in the 192-page “Weed Control Guide for Ohio and Indiana, 2011,” available at county Extension offices and on OSU Extension’s e-Store, at

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