Controlling Marestail this year has been a struggle to say the least. It has been most problematic with a number post-emergence herbicide applications in soybean fields Marestail (also known as horseweed) is a winter annual weed that germinates usually in the fall. It can however, also germinate and grow in the spring and summer. This spring, it is prolific in number and given the weather patterns of late, thriving in growth with all of the wet weather. It can grow up to 6 feet in height. Although small in size and many in number, it’s waxy leaves make it difficult for herbicide applications to be absorbed and translocated within in the plant. This is especially true as the plant increases in size.
In Roundup Ready Crops, once it is more than 6” in height, higher rates of a Roundup are required and many times the Roundup needs a tank-mix partner to insure good control. This does not mean it is resistant to Roundup. It means that an appropriate rate increase along with a tank-mix partner provides the best control when the marestail is this big and “out of the range” of labeled control.
Many of the tank-mix partner treatments increase the chance of crop injury. Although this injury is relatively minor, leaf speckling can occur and at this stage of development means little to nothing in regard to yield potential.
An additional option in control is the timing of the herbicide applications that can be made. As mentioned before, marestail is a winter annual by life cycle classification. Fall applications of a number of pre-emergence soybean herbicides such as Authority First, Canopy EX, Sharpen, Sonic and Valor all work well as a Fall burndown of Marestail. They are not in-fallible. But, a fall herbicide application to fields with a historic population of marestail, will give you better control the following year.