Weed Woes

Missouri field trials confirm waterhemp is resistant to glyphosate. Jerilyn Johnson

Published on: Jul 12, 2006

University of Missouri researchers have confirmed that tall waterhemp is the sixth glyphosate-resistant weed in the U.S. and the ninth such weed in the world.

Kevin Bradley, a University of Missouri Extension weed scientist, and graduate student Travis Legleiter found that tall waterhemp from a field near the Missouri River in Platte County could survive despite being treated with up to eight times the labeled rate of glyphosate.

Glyphosate is the active ingredient in Roundup herbicide. The fields where resistant waterhemp was found had been in continuous Roundup Ready soybean production since 1996.

Kevin Bradley, University of Missouri weed scientist, explains new waterhemp research trials at the MU Weed Resistance Management Field Day last month near Columbia.

While all resistant weeds are worrisome, Bradley says resistant tall waterhemp is especially troubling. "Waterhemp is one of Missouri's toughest weed problems," he says. "It has developed resistance to a number of other soybean herbicides." That resistance has been known to spread quickly. Waterhemp plants are either male or female, which means females rely on pollen shed from surrounding male plants.

"If the resistant trait is carried in the pollen, which we are fairly confident it is, then you have pollen traveling to fields all around the resistant plants," Bradley adds. Each female waterhemp plant produces hundreds of thousands of seeds, ensuring a ready supply of plants for the following season.

In light of the confirmed case of glyphosate-resistant waterhemp, Monsanto is recommending farmers use a pre-emergence soil residual herbicide in their Roundup Ready soybean system if they have dense stands of common broadleaf weeds such as waterhemp. A residual herbicide product such as INTRRO or Valor applied prior to soybean emergence helps farmers eliminate early season weed competition and adds another herbicide mode of action into the cropping system.

"Farmers who are growing no-till Roundup Ready soybeans year after year or any farmer who has a dense stand of tough-to-control weeds such as waterhemp should use a residual herbicide prior to planting," says Monsanto Roundup Technical Manager Jennifer Ralston. "A continuous, no-till Roundup Ready soybean production system where only glyphosate is used increases the pressure on glyphosate as the sole tool for weed control."

This waterhemp population is also resistant to ALS herbicides, further reducing herbicide choices for tank mix recommendations. For the few growers that have glyphosate-resistant waterhemp, Monsanto provides the following recommendations:

  • In Roundup Ready soybeans, use a pre-emergence residual product at planting such as INTRRO or Valor. Tank mix Roundup WeatherMAX with Cobra or Flexstar if post-emergence waterhemp control is needed.
  • In Roundup Ready Corn 2, use a pre-emergence residual such as Harness Xtra or Degree Xtra at planting. Tank mix Roundup WeatherMAX with 2,4-D or dicamba (Clarity, Banvel or Distinct) if post-emergence waterhemp control is needed.

Positive news

Bradley and Legleiter have found good news in their field plots. The glyphosate-resistant waterhemp is killed by a number of popular pre-emergence soybean and corn herbicides. Bradley reported on this strategy at the Weed Resistance Management Field Day June 20 near Columbia, hosted by MU and Monsanto.

The pair plan at least two seasons of examining whether the resistant plants can be brought under control economically in continuous soybeans - using pre-emergence and post-emergence herbicides - or whether it is better for farmers facing resistant weeds to alternate plantings of corn and soybeans. The rotation opens up a wider array of herbicides labeled for use in corn.

The eight other confirmed glyphosate-resistant weeds throughout the world include common ragweed, buckhorn plantain, goosegrass, hairy fleabane, horseweed (a.k.a. marestail), Italian ryegrass, palmer amaranth and rigid ryegrass.