Control strategy for marestail in soybeans
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.