Horseweed (or marestail) is the most widely distributed glyphosate-resistant weed in the U.S. After the initial confirmation in Delaware in 2001, glyphosate-resistant (GR) horseweed has been reported 15 states, including Illinois to our south and Nebraska to our southwest. The two sources for GR horseweed are either home-grown (i.e. resistance developed on a specific farm) or GR seed (or pollen) introduced from another field. The potential for horseweed seed to move from one farm to another or even from one county to another is an interesting question. 

 

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 GR horseweed.

 

1. Horseweed is a common winter annual weed in many no-till fields in Wisconsin.

2. Glyphosate-resistant horseweed is certainly possible in Wisconsin, but GR horseweed has not been reported to date.

3. 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.

4. Once GR horseweed is established in Wisconsin, the potential for movement beyond the initial infestation is possible via wind-borne seed.

5. 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 include glyphosate + 2,4-D or Gramoxone + Sencor. On elongating horseweed less than 6 inches tall, the glyphosate rate should be increased to 1.1 lb ae/a and still tank mixed with 2,4-D or 2,4-D should be added to the Gramoxone + Sencor mix. Chlorimuron products (an ingredient in Synchrony and Canopy) or FirstRate can also improve glyphosate’s control 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.