FARGO, N.D. -- Horseweed is becoming a bigger, localized weed threat in some areas of North Dakota, particularly where no-till farming techniques have become popular.

The weed was one of the main topics at this year’s North Dakota State University Wild World of Weeds, an annual conference, Jan. 21, at the Fargodome.

NDSU scientists brought in Mark VanGessel, an Extension weed specialist for University of Delaware at its Georgetown, Del., Research and Education Center, was a keynote speaker.

In 2000, VanGessel became the first in the country to confirm glyphosate resistance in horseweed, about the same time colleagues in Ohio, Indiana and Tennessee were finding it.

‘It was the first broadleaf weed resistant to glyphosate,” VanGessel acknowledged. Ryegrass had been found to be resistant. Later, there would be glyphosate-resistant Palmer amaranth, waterhemp and common ragweed. “It was kind of a harbinger of what was to come, unfortunately,” he said.

Weed-hunting

Kirk Howatt, an NDSU weed scientist who specializes in annual weed control in small grains and annual seed crops, said horseweed has been present in the southeast and south-central parts of North Dakota, as well as north-central regions.

It is a “winter-annual,” Howatt said of horseweed. The weed seeds usually germinate in the fall, overwinter, and are ready to grow quickly in the spring. “It tends to like pasture and rangeland -- non-crop type areas. It does really well where we do not do any cultivation,” he said.

If farmers can hit it in the fall with products like Paraquat, Sharpen, Valor, or Spartan, they can eliminate much of the weeds in the fall so the field is cleaner in the spring.

No-till is great for soil conservation, but has relied heavily on glyphosate for managing weeds, including horseweed. Glyphosate has been the “primary, sometimes the only chemical option for controlling that weed,” Howatt said. Mild, shallow tillage easily disrupts the weed’s fine root system, even when the weed is a couple of inches in diameter.

Countermoves

Glyphosate was effective on it until the 2000s, but much of the weed’s population now is resistant to glyphosate herbicides, as well as ALS-inhibiting herbicides. Scientists in recent years have focused more research on understanding the genetics, biology, and alternative control strategies.

“If you don’t control it, it’s going to rob yields -- particularly no-till yields,” VanGessel said, adding it “an be very, very competitive and have a huge impact on yield.” The seed travels like dandelion seed in that it produces a tuft of hair and blows in the wind, spreading quickly.

Like Palmer amaranth, a new weed with a long germination period, horseweed’s germination period is longer. It can germinate in the fall, overwinter, as well as germinate in the spring.

Howatt said that before a new crop is in place, farmers can use herbicides like gramoxones or or Liberty-type products.

“Those products are contact-only,” Howatt said. “They are very effective only on the very small plants.”

Once the plants get larger and start to elongate and start to develop height, then it becomes difficult to control with those products as well, he added.

Howatt has heard of fields that are so infested that farmers will simply combine around an affected part of the area -- corners or a low slough area. “Late in the season, the stems get fairly stiff and rigid and can start to clog up the combine machinery,” he said.