As no-till soybean production gains popularity, horseweed (Conyza canadensis), also known as marestail, is increasing as a weed across Iowa. This plant is a winter annual that germinates in the fall or early spring. It is not a problem in fields where tillage is used to prepare a seedbed since tillage destroys established plants. Few horseweed seeds will germinate when soils reach temperatures suitable for planting corn or soybean.
Horseweed seedlings produce a basal rosette of leaves. The first leaves have smooth leaf margins, and later leaves have a toothed margin. All leaves have a hairy surface. Horseweed plants in Ames currently are 1-2 inches in diameter. Later in the spring, typically in May, the stem elongates, resulting in an erect plant that may reach 6 feet in height. Herbicide tolerance increases when plants begin stem elongation (bolting).
Although no controlled studies have evaluated the competitiveness of horseweed with corn or soybean, its growth habit suggests it would require very high populations to impact crop yields. Horseweed should not interfere with harvest since the plant matures and dries down in mid-summer long before harvest. The primary impact of horseweed in soybean is that it detracts from the appearance of the field. The seedhead of horseweed is produced above the soybean canopy in June/July and remains there the entire season. Consider the lack of competitiveness of this weed when developing management plans.
Horseweed is easily controlled by most burndown herbicides when in the rosette stage. The recommended rate of Roundup for 6-inch horseweed ranges from 16 to 24 ounces, depending on application technique. If horseweed has bolted at the time of burndown application due to delayed planting, increase herbicide rates to maintain control. Before increasing rates, evaluate the potential economic impact of the horseweed by scouting the field to determine the horseweed population.