ISSUE 13   August 2, 2007


Extension Weed Scientists in the North Central Region have embarked on a plan to develop a series of extension publications on weed management from glyphosate. This includes weeds that are or might become resistant, troublesome, and tolerant in glyphosate systems. The series will also include glyphosate stewardship / management publications.

The following is a list of publications that are available so far.

  • Biology and Management of Horseweed
  • Biology and Management of Wild Buckwheat
  • Biology and Management of C. Lambsquarters
  • Biology and Management of Giant Ragweed
  • Facts about Glyphosate Resistant Weeds
  • Understanding Glyphosate to Increase Performance
  • These publications are housed at a central web site of:

    Dr George Kegode, former NDSU Weed Ecologist and I just completed, "Biology and Management of Biennial Wormwood". It is not a NC Regional publication in that it seems to only inflict ND but will be an NDSU Extension publication and will be in the same format as the other weed publications.

    Hardcopies of these publications, including the Biennial Wormwood publication, can be ordered from the NDSU Extension Distribution Center by emailing Ardis at or calling 701 231-7882. External funding sources paid for the printing but there might a slight charge for shipping and handling.



    Horseweed, also known as Marestail, is a common weed in North Dakota no-till fields. Many know that biotypes have become resistant to glyphosate in several states in the east and mid-west U.S. A few fact about the biology may help us.

  • Single plants can produce over 200,000 seeds. Very small seed (1/16 inch long) with pappus can be carried by wind.
  • Seeds are so light that they fall or settle at a rate of about 1 foot per second.
  • Seeds do not have dormancy and can germinate immediately after maturity.
  • Seeds can germinate under favorable conditions in the fall, spring or mid-summer.
  • Previous research had found that horseweed seeds may travel up to 1500 ft downwind from the mother plants. This distance of travel could easily move horseweed seed within a field, from a fence line into a field, or from one field in to an adjacent field. However, it would not account for horseweed seed moving longer distances to potentially spread glyphosate resistance.

    Researchers in New York and Delaware wanted to determine if horseweed seed was capable of moving long distances in the wind. They described winds in the surface boundary layer, which are 2.5 times the height of the canopy or 15 feet if considering 6 ft tall horseweed. Winds above the surface boundary layer are called the planetary boundary layer (PBL), which generally have greater wind speeds. Winds in the PBL would be responsible for potential long distance seed movement.

    To sample the PBL, the researchers flew specially modified RC planes with samplers downwind from a field with mature horseweed over a 3-day period. The 17 30-minute flights were at heights ranging from 128 to 460 ft above the ground. Remarkably, horseweed seed was collected in 13 of the 17 flights regardless of the height of the flight.

    As a result of collecting horseweed seed at these heights, the researchers concluded that long distance seed movement is possible. If seeds were blown into the PBL in early afternoon, an 11 mph wind could move the seed 45 to 90 miles. Since winds in the PBL are often greater and frequently may exceed 40 mph, such wind could move horseweed seed over 300 miles. With this long distance dispersal of seed possible, it is also possible for more rapid or wide-scale introduction glyphosate resistant (GR) horseweed.

    What might this mean for North Dakota?

  • Horseweed is a common winter annual weed in many no-till fields.
  • Horseweed has some natural tolerance to glyphosate.
  • Glyphosate-resistant (GR) horseweed is certainly possible in ND, but GR horseweed has not been reported to date.
  • Glyphosate-resistant horseweed could develop in a field either from selecting for resistance in that field through repeated use of glyphosate or it could be introduced as GR seed.
  • Once GR horseweed is established, the potential for movement beyond the initial infestation is possible via wind-borne seed.
  • The best defense against "home-grown" resistance or "wind-blown" resistance is a good offensive plan. Use a tank mix in your burndown herbicide program and treat the horseweed before it exceeds 4 to 6 inches in height. It is especially important to control horseweed before planting soybeans because in-crop options are limited. Effective burndown programs before soybeans on seedling or rosette stage horseweed and other options are described in the publication, "Biology and Management of Horseweed".
  • Source: Shields, E.J., J.T. Dauer, M.J. VanGessel, and G. Neumann. 2006. Horseweed (Conyza canadensis) seed collected in the planetary boundary layer. Weed Science 54:1063-1067.

    Richard Zollinger
    NDSU Extension Weed Specialist

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