Herbicide resistant pigweed dominates farmers’ concerns at research field day

Aug. 10, 2009
Contact Information:

Dr. Ken Smith, Extension Weed Scientist
870-460-1091, smithken@uamont.edu

Dr. Robert Stark, Professor of Agricultural Economics
870-460-1091, stark@uamont.edu

Dr. Fred Bourland, Director, Northeast Research and Extension Center
870-526-2199, fbourland@uaex.edu

By Fred Miller, Division of Agriculture, University of Arkansas System
479-575-5647 / fmiller@uark.edu

Download story
jason norsworthy at test plot

Scientists at the University of Arkansas System’s Division of Agriculture are conducting research on the growing problem of glyphosate-resistant weeds in Arkansas. In this test plot, weed scientist Jason Norsworthy is tracking the rate at which a one-square-meter patch of glyphosate-resistant Palmer amaranth, or pigweed, spreads through a cotton field.

See related video at http://aaes.uark.edu/nerec_video.html.

KEISER, Ark. — Farmers, agricultural consultants and county agents who turned out for a field day at the University of Arkansas System Division of Agriculture’s Northeast Research and Extension Center all had at least one question in common: what to do about herbicide resistant pigweed?

Ken Smith, extension weed scientist at the Division of Agriculture’s Southeast Research and Extension Center in Monticello, said that eight years ago, morning glory was the problem weed on every grower’s mind. Today, “herbicide-resistant pigweed has choked out the morning glory,” he said.

Smith discusses the problem and management of herbicide resistant pigweed in a Division of Agriculture Web video: http://aaes.uark.edu/nerec_video.html

The growing problem is glyphosate resistant Palmer amaranth, known best to farmers as Roundup resistant pigweed. First confirmed in Mississippi County in 2005, Smith said, the problematic weed has spread to most of the counties in eastern Arkansas.

Research technician Ryan Doherty said glyphosate resistant pigweed has been confirmed in 21 counties throughout the state. A research location has been established to study control programs in a field situation. Research on those plots is revealing the nature of the resistance and helping to develop management strategies.

Doherty said the most resistant pigweed population identified by division scientists was found in Lincoln County.

“The farmer had already put two 22-ounce applications of Roundup on that field before he called us,” Doherty said. “We put on another 44-ounce application of Roundup and it didn’t hurt it at all.”

Even another application of 128 ounces of Roundup did not kill the pigweed biotype found in that field. Doherty said all those plants probably came from a single female plant.

Smith said division scientists confirmed this month that there are two distinct patterns of distribution in Arkansas from pigweed plants with two different mechanisms of tolerance. In one pattern, called segregated, the herbicide resistant plants are scattered throughout the field randomly among plants that are not resistant. Spraying these fields with glyphosate kills about 80 percent of the pigweed. The remaining plants are scattered randomly throughout the field. Smith cautioned that resistance is creeping up in these pigweed populations; 80 percent may be killed this year, but next year it may be only 70 percent.

Smith calls the second pattern non-segregated. In these fields, the resistant plants are clustered tightly together and glyphosate herbicide does not kill any of them. “All the offspring of these plants have high levels of resistance,” he said.

“When you see these,” Smith advised, “Do whatever you have to do to take them out.”

Smith said Division of Agriculture scientists had devised a number of strategies to control glyphosate resistant pigweed, most involving a combination of different herbicides beginning with a preplant application. Roundup is still a valuable weed control product, he said, because it controls more than 100 other weeds. But it will have to be part of a new program for weed control.

 “There is no prescription that works in every cotton or soybean field,” Smith said. “But in any program, soil residual herbicides are going to be essential for controlling these pests.”

Smith said farmers should overlap soil residual applications to keep them on the field all the time. He recommended scouting for pigweed at the same time growers would be scouting for insects. Catching and killing pigweed before it matures and goes to seed is important in controlling the spread of the weed.

Agricultural economist Bob Stark said glyphosate resistant Palmer amaranth is the main economic concern among all herbicide resistant weeds.

Stark said there are a lot of different chemical choices, but Division of Agriculture economists were putting together an economic analysis of four management strategies. The key point, he said, was to begin planning now for next season, to consider what herbicides are available and to consult county extension agents about what plans will work best.

“I know we don’t have the 2009 crop in yet,” Stark said, “but this is not too early to begin looking at 2010.”

An overnight thunderstorm forced the field day program indoors because the fields and roads on the Northeast Research and Extension Center were too muddy to drive or walk on. Other topics covered:

Division entomologists Tina Teague and Glenn Studebaker discussed management of insect pests, particularly tarnished plant bugs. They gave an update on insecticide efficacy based on division research programs.

In a series of presentations by division agricultural economists, Archie Flanders described comparative costs and returns for field crops, Bob Stark discussed the economic impact of herbicide resistant weeds and Scott Stiles gave an update and talked about the inherent risks for commodity prices.

Extension crop specialists Jason Kelley, Tom Barber, Jeremy Ross and Chuck Wilson discussed crop conditions and challenges for corn, sorghum, wheat, cotton, soybeans and rice.

Erika Chudy, district director for Arkansas Congressman Marion Berry gave a legislative update and said the congressman will be in Arkansas in coming weeks during a Congressional recess.