Control strategy for marestail in soybeans
Clarity herbicide has proven to be more effective than 2,4-D for control of marestail, but has more restrictive preplant limitations. However, it may be a good alternative as a fall or very early spring treatment in some areas. Clarity use as a preplant herbicide treatment ahead of soybeans is prohibited in areas that average less than 25 inches of rain per year. In areas with greater than 25 inches of rain, a waiting interval of at least 14 days is required following accumulation of at least 1 inch of rain or irrigation after application of Clarity at rates up to 8 oz per acre.
Sharpen is a new herbicide that has provided very good control of maresetail. It can be applied anytime prior to soybean emergence and provides a short period of residual control. Sharpen works best if applied with methylated seed oil and in combination with 2,4-D or glyphosate. Because Sharpen is a contact herbicide, using higher spray volumes (15-20 gal/a) will help increase herbicide coverage on plants. Sharpen works very fast and quickly desiccates marestail foliage, but larger bolted marestail can sometimes regrow from axillary buds one to two weeks after treatment. Sharpen can not be applied postemergence to soybeans. Sharpen is also available as a premix with Pursuit in the product OpTill or with Outlook in the product Verdict.
In addition to a burndown application made in fall or early spring, most fields will benefit from use of residual herbicides that include a Valor, Classic, or FirstRate component in the spring, along with another dose of a burndown herbicide if needed. The use of a residual preplant or preemergence herbicide at planting time, tankmixed with a burndown herbicide, will help provide additional control of marestail, as well provide early-season weed control and help manage or prevent the development of other glyphosate resistant weeds such as waterhemp, ragweed, Palmer amaranth, or kochia.
If marestail are not controlled in fall or early spring and have started to bolt before they are treated, Ignite herbicide has proven to be one of the best treatments for control of larger bolted marestail. Ignite can be used as a burndown treatment prior to emergence of any type of soybeans, or as a postemergence treatment in Liberty Link soybeans. However, Ignite efficacy is often reduced under lower humidity.
Postemergence control of large marestail in soybeans can be very difficult, especially if the marestail is glyphosate resistant. FirstRate, Classic, and Synchrony herbicides are probably the best postemergence options, unless marestail is also ALS-resistant. The combination of these herbicides with glyphosate on Roundup Ready soybeans seems to work best, even on glyphosate-resistant marestail.
Glyphosate-resistant marestail and other glyphosate resistant weeds have developed due to over-reliance on glyphosate for weed control. Integration of other herbicides into the weed control program and proper timing of herbicide applications is a key factor to help manage and prevent the development of glyphosate-resistant weeds.