Editor’s note: The following was written by Amit Jhala, University of Nebraska Extension weed management specialist, and Roger Elmore, Extension cropping systems agronomist, for the university’s Crop Watch website Oct. 25.
Abundant rain has promoted the relatively early emergence of winter annual weeds, particularly marestail.
Marestail, also known as horseweed in the eastern Corn Belt, is a winter or summer annual weed in Nebraska.
Historically, marestail was found in waste areas and field edges and along roadsides and railway tracks; however, no-till crop production systems over the last 20 years have favored establishment in crops.
A single marestail plant can produce up to 200,000 seeds, about 80% of which can germinate immediately after falling from a mature plant.
Studies show that most marestail plants emerge during fall and survive the winter as basal rosettes. However, significant spring and early summer emergence also have been observed in Nebraska. Therefore, scouting should be done in the fall as well as in early spring to make an effective management plan.
Marestail is the first glyphosate-resistant weed reported in the United States. Marestail resistant to glyphosate and ALS inhibitors such as Classic, Permit, Pursuit and Raptor is becoming more common.
But several reports have noted marestail is sensitive to most herbicides labeled for its control early in its growth, i.e. the rosette stage.
Preliminary data for eastern Nebraska suggests a fall burndown applied with a residual herbicide may eliminate the need for an early spring burndown for marestail control. However, this would not replace an at-planting residual application for management of additional troublesome weed species such as waterhemp and Palmer amaranth.
For successful marestail management in the fall, apply herbicides following harvest while weather conditions remain favorable. Fall herbicide application is a more consistent strategy because weather conditions frequently interfere with early spring applications.
Glyphosate-resistant mares-tail is widespread across eastern Nebraska, thus 1 lb. a.e. 2,4‑D per acre is recommended as the base treatment for marestail burndown.
Glyphosate or other products such as Sharpen or Gramoxone may be tank-mixed with 2,4-D to provide broader spectrum control of winter annuals and certain perennial weeds.
We generally do not recommend including residual herbicides in fall applications since they provide little benefit in managing weeds that emerge the following spring. However, if infestation of marestail is high in the field and the field has a history of marestail seed bank, it would be advantageous to add a residual herbicide such as Authority, Valor, Autumn Super or other metribuzin products.
Fall herbicide application is unlikely to eliminate the need for a burndown application at planting. Weeds adapted to cool temperatures, such as mares-tail, are likely to emerge before planting, making it necessary to control them.
Tillage is an integral part of the integrated management of marestail. Tillage in the fall or early spring after most marestail plants have emerged can provide effective control.
A good stand of cover crops such as cereal rye planted before or after crop harvest in the fall can suppress marestail and other winter annual weeds.
Early planting of cover crops is the key to effective suppression of marestail. Cover crops should have emerged by the time marestail emerges.
Inter-seeding cover crops while corn is at the reproductive stage is a good practice for the early establishment of cover crops.
A recent study at the university’s Agronomy Research Farm reported excellent suppression of marestail in soybean when cereal rye had been interseeded in the previous corn crop at the R6 growth stage.