Control of marestail in no-till soybeans
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 remain in the low-growing rosette 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 competes with the soybeans throughout the growing season, and reduces crop yield. Marestail matures in late summer or early fall, late enough to interfere with soybean harvest.
Herbicide activity and resistance in marestail
Herbicide programs must consist of a spring burndown to ensure that the field is free of marestail at the time of soybean planting, and residual (PRE) herbicides to control marestail for another 6 to 8 weeks. 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 average where the burndown treatment failed to control emerged plants
57 bu/A average where the burndown treatment was effective, but there was no residual herbicide
65 bu/A average where the burndown was effective and residual herbicides were used
Marestail is most easily controlled when in the seedling or rosette stage, and spring burndown herbicides should be applied when plants are less than 4 inches tall if possible.
Marestail populations with resistance to glyphosate or ALS inhibitors (e.g. Classic, FirstRate) are widespread throughout Ohio and Indiana, and many populations have multiple resistance to both of these sites of action. Growers should therefore not expect to obtain effective POST control with combinations of glyphosate plus Classic, Synchrony, or FirstRate, except in fields with no history of herbicide resistance or POST control problems.
LibertyLink soybeans are the most effective control strategy
LibertyLink soybeans are the most effective tool for management of herbicide-resistant marestail, especially in fields with high marestail populations.
Use burndown and residual herbicides as outlined on the next page. Apply Liberty POST (29 oz/A) before marestail plants exceed 6 inches in height. Liberty can be applied POST at rates up to 36 oz/A for taller plants or plants that have survived previous herbicide treatments.