PEORIA, Ill. — Marestail control had not been a major challenge for farmers over the years, but that all changed in 2013.

“We became fairly accustomed to getting some pretty good control of marestail and never had to worry about it. But that wasn’t the case this year,” said Aaron Hager, University of Illinois Extension specialist, weed science/integrated pest management.

“Something happened in Champaign County in 2013 that I never thought I’d see. We had chopping crews in Champaign County, and the primary species they were hired to cut out was marestail.”

He said all indications are that in every area that had high populations of marestail this fall there will be a large spread of the weed — setting the stage for a control problem in 2014.

There were many reports of poor marestail control from herbicides applied prior to planting — primarily no-till soybean — especially when burndown applications contained only glyphosate or glyphosate plus 2,4-D.

The increasing frequency of glyphosate-resistant marestail populations, the rush to plant whenever field conditions were conducive and the less-than-ideal environmental conditions when many burndown applications were made contributed to a challenging situation for which a good solution was not always readily available.

“If we understand the biology of marestail just a little bit better, I think ultimately it’s going to make a lot more sense when we talk about the management of the weed going forward,” Hager said at the Corn and Soybean Classic.

Native American Plant

Marestail is native to North America and like many other plant species completes its life cycle in one year. Unlike many other annual species, however, marestail can exist as a winter or summer annual.

Populations of winter annual marestail typically emerge during the fall months, within a few days or weeks after seed is dispersed from the parent plant. Summer annual populations can emerge in early or late spring, perhaps as late as early summer in some instances.

In northern areas of Illinois, most marestail demonstrates a winter annual life cycle, whereas a substantially higher proportion of spring emergence occurs in areas south of around Interstate 70 in Illinois.

Fall-emerging plants form a basal rosette that represents the plant’s overwintering stage. In the spring, plants bolt by rapidly elongating the main stem.

The seeds are produced with an attached “parachute” to aid in wind-borne dispersal.

Research has demonstrated that mature marestail plants can produce in excess of 200,000 seeds, with fall-emerging plants frequently producing more seeds than spring-emerging plants.

Marestail seed can travel long distances with its dispersal mechanism, which becomes especially important when considering the spread of herbicide-resistant biotypes.

Mature seeds do not demonstrate much dormancy, but rather germinate soon after contact with the soil surface. Seeds do not remain viable in the soil seed bank for very long.

Field studies of marestail in soybeans at Ohio State University found yields of 65 bushels per acre where burndown was effective and residual herbicide used. Yields were reduced to 57 bushels per acre when the burndown treatment was effective but no residual herbicide was used.

Fields with marestail where the burndown failed to control emerged plants had yields of 51 bushels per acre.

Glyphosate Resistance

Marestail was the first weed species that evolved into glyphosate resistance, and there have since been confirmations of its resistance to other herbicides in the U.S.

“We have not confirmed acetolactate synthase resistance in Illinois. The only reason we haven’t confirmed it is we haven’t looked for it. We suspect it,” Hager said.

Herbicides and tillage can be effective tools to manage marestail, but resistance to certain herbicides in Illinois marestail populations and adherence to no-tillage production practices can increase the difficulty in controlling marestail before soybeans are planted.

Recommendations were developed by Midwest extension weed scientists in a projected funded by the United Soybean Board.

“Liberty Link soybeans are a solution even right now if you want it. It’s very effective on marestail,” Hager said.

A burndown and residual herbicide is recommended. Liberty should be applied post before marestail plants exceed six inches in height. A second post application of Liberty should be applied as needed.

“The most effective approach to this is going to be a systems approach. The days of throwing one thing in the tank and getting a field cleaned up are done. Actually they were done five or six years ago, but it hasn’t caught up to everybody yet,” Hager said.

“We’re talking about using multiple sites of action with a burndown application and multiple sites of action with residual products also.”

A fall burndown followed by a spring burndown and a residual is an effective way to try to manage marestail.

“You could also do a split burndown in the spring where you burndown early with a low rate of residual say in late March and put down the remaining burndown and residual close to planting,” Hager said.

The residual herbicide also will control waterhemp, a weed that has been problematic to control and herbicide-resistant.

Hager also recommends scouting fall-treated fields before spring planting and taking appropriate measures — that is, supplemental herbicides and tillage, for example — to control any remaining marestail and emerged summer annual species, including spring-emerging marestail.

“Do not simply assume that fields treated with fall-applied herbicides will be free of marestail next spring,” he said.

“This is a species that we’re going to have to keep our eye on and even more so in 2014 because of all the seed that was made in 2013. I expect this one is going to be very, very difficult. You have a plant species with mature seed that really don’t have a lot of dormancy.”

“Seeds are dispersed over long distances. This becomes really important with the spread of marestail and also the spread of herbicide-resistant marestail,” he said.