Marestail continues to be one of the biggest problem weeds on many soybean fields in Kansas. It is a little easier to control in corn since it is still susceptible to atrazine, dicamba and other herbicides that can be used in corn.

Marestail has historically been considered a winter annual weed, but can also germinate in the spring or summer and act as a summer annual. In fact, marestail appears to be shifting to more spring and summer germination in Kansas. Individual plants can produce an abundance of tiny seed that can be easily dispersed by wind. Seed can germinate soon after it is produced, but also can remain viable in the soil for several years, making it a hard weed to control with crop rotation. In addition to those problems, many populations of marestail in Kansas now appear to have some level of glyphosate resistance, while some populations may also be resistant to ALS-inhibiting herbicides.

Herbicide effectiveness on marestail depends largely on stage of growth and size of plants. Marestail generally is most susceptible to herbicides when it is small and still in the rosette stage of growth. Once marestail starts to bolt and exceed 4 to 6 inches tall, it becomes very difficult to kill with most herbicides. Since marestail can germinate throughout much of the year, a single herbicide application probably will not provide season-long control, particularly in no-till.

In soybeans, marestail control should begin with a burndown application of 2,4-D while the marestail is in the rosette stage – which may be in the fall or any time in the spring and early summer. A tankmix of 2,4-D and glyphosate can burn down a broad spectrum of annual broadleaf and grass weeds.

Fall applications can be effective even into December as long as applications are made to actively growing weeds during a stretch of mild temperatures. In fact, for fall applications, it may be better to wait until November to allow most of the fall-germinating winter annuals to emerge. A residual herbicide such as the Valor or Classic (unless ALS resistant) containing products can be added to help control marestail through winter and early spring, but don’t expect a residual herbicide applied in the fall to provide residual marestail control through the spring and summer of the next year. If a fall treatment isn’t applied, early spring treatments in March to early April should be applied to help control the fall-germinating marestail. 

For marestail that germinates in the spring or summer, 2,4-D is generally very beneficial for early-season control, but its use is limited as planting time approaches. A waiting period of 7 days is required after application of up to 1 pt/A 2,4-D LV4; 15 days for up to 1 pt/A 2,4-D amine; and 30 days between application and planting of soybeans for rates greater than 1 pt/A for either ester or amine 2,4-D products.

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.