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.
Steps for effective management of marestail
1. Use fall herbicide treatments in fields with a history of problems or where marestail seedlings are observed in the fall. The primary goal of a fall treatment is control of emerged plants, and it is not a substitute for a spring preplant herbicide treatment. An application of burndown and residual herbicides is still required in the spring in fields that were treated in the fall. Products containing chlorimuron can provide residual control into the spring if the marestail population is not ALS-resistant, but do not expect a fall herbicide treatment to adequately control marestail that emerges in May or June. Authority, Valor, and metribuzin products and mixtures should be applied in the spring, not in the fall. Where a fall application is necessary, we suggest minimizing costs by applying one of the following:
glyphosate + 2,4-D; or dicamba + 2,4-D; or 2,4-D + a low rate of Canopy/Cloak EX or DF
2. Apply effective burndown herbicides in spring. Do not plant into existing stands of marestail. Start weedfree at the time of planting by using one of the following preplant herbicide treatments, applied when marestail plants are less than 4 inches tall. Note - tillage also effectively removes marestail.
- 2,4-D ester plus glyphosate (1.5 lb ae/A)
- 2,4-D ester plus Gramoxone (3 to 4 pts/A) plus a metribuzin-containing herbicide
- Liberty (29 to 36 oz/A) or Liberty plus a metribuzin-containing herbicide
- Saflufenacil product (Sharpen, Optill, or Verdict) plus MSO (1% v/v) plus either glyphosate or Ignite
The mixture of glyphosate and 2,4-D ester has become more variable for control of marestail over time in some fields. Plants should be in the rosette stage at the time of application for best results. In fields where this mixture has previously failed to provide effective control, use one of the other burndown treatments listed above (2,4-D ester can be added to any of these).
Use the highest rate of a 2,4-D ester product that is allowed, based on the interval between application and soybean planting. For all 2,4-D ester products, rates up to 0.5 lb active ingredient/A must be applied at least 7 days before planting. Rates between 0.5 and 1.0 lb/A should be applied at least 30 days before planting, with the the exception of several products (E-99, Salvo, and Weedone 650) that allow 1 lb/A to be applied 15 days before planting.
In ALS-sensitive populations, the activity of any of the above can be improved with the addition of a herbicide that contains chlorimuron (Canopy/Cloak/Fallout, Valor XLT, Envive, Authority XL) or cloransulam (Gangster, Sonic, Authority First).
3. Include residual herbicides with the burndown treatment. Add one of the following herbicides or herbicide combinations to the burndown herbicides, for residual control of marestail until the soybean leaf canopy develops. Where a saflufenacil product is used for burndown, do not use a residual herbicide containing flumioxazin (Valor, Valor XLT, Envive, Enlite, Gangster) or sulfentrazone (Authority products).
- Valor, Valor XLT, Envive, Enlite, or Gangster
- Authority First, Sonic, Authority XL, Authority Broadleaf, or Spartan
- Canopy/Cloak DF + metribuzin (total metribuzin rate must be at least 0.38 lb ai/A)
- metribuzin (Metri DF, Tricor, etc) or Boundary (metribuzin rate must be at least 0.38 lb ai/A)
4. Herbicides for POST control. Liberty is the only consistently effective herbicide for POST control (for use only in LibertyLink soybeans). If marestail plants escape the measures outlined above and require POST control in Roundup Ready soybeans, try a combination of glyphosate plus Classic or FirstRate at the highest allowable rates. These combinations are likely to be variable in their effectiveness but may work in some fields.