Marestail (aka horseweed) has two primary periods of emergence - from late summer into fall, and from late March through June. Spring-emerging marestail has been the most problematic to manage in the southern half of Ohio and Indiana, especially the plants that emerge in May and June.

Marestail plants overwinter in the rosette stage, and remain in this low-growing stage through late April, followed by stem elongation (bolting) and growth to an eventual height of 3 to 6 feet. Plants that emerge the previous fall will start stem elongation earlier than spring-emerging plants.

Marestail is most easily controlled when in the seedling or rosette stage, and burndown herbicides should be applied prior to stem elongation.

Marestail competes with the soybeans throughout the growing season, and reduces crop yield. Marestail matures in late summer or early fall, and large mature plants can interfere with soybean harvest.

Marestail plants can produce up to 200,000 seed that are transported by wind, providing for effective spread of herbicide-resistant populations.

Soybean yield loss due to marestail

Herbicide programs must consist of: 1) fall and spring burndown treatments (or two spring treatments - early spring and at plant) to ensure that the field is free of marestail at the time of soybean planting, and 2) spring-applied residual (PRE) herbicides to control marestail for another 6 to 8 weeks after planting.

Failure to follow these guidelines can result in poor control and reduced soybean yield. We observed the following soybean yields in a 2010 OSU marestail study:

51 bu/A - the burndown treatment failed to control emerged plants

57 bu/A - the burndown treatment was effective, but there was no residual herbicide

65 bu/A - the burndown was effective and effective residual herbicides were used

Herbicide resistance in marestail

Most populations of marestail in Ohio and Indiana are resistant to glyphosate, and will not be controlled by burndown or postemergence applications of glyphosate alone.

Many marestail populations are now multiple-resistant, to both glyphosate (group 9) and ALS-inhibiting herbicides (group 2 - e.g. Classic, FirstRate). Growers should therefore not expect to obtain effective POST control in soybeans with combinations of glyphosate plus Classic, Synchrony, or FirstRate. Postemergence PPO-inhibiting herbicides (group 14), such as Flexstar, Cobra, and Cadet, also do not control marestail.

Other Impacts of glyphosate + ALS multiple resistance

Fall-applied Canopy or other chlorimuron- or cloransulam-containing herbicides will not provide residual control of ALS-resistant marestail into spring, and residual herbicides should largely be reserved for spring application (at least with regard to marestail - residual control of other weeds can occur).