Gage County
1115 West Scott St.
Beatrice NE 68310
Phone: (402) 223-1384
FAX: (402) 223-1370

News Column

Paul C Hay, Extension Educator

Glyphosate Resistance is Here

It was bound to happen. Roundup (glyphosate) is widely used now in Nebraska on corn and soybeans acres. We talked about this at private applicator training meetings for the past five years. We knew we could have resistance weeds unless we changed mode of actions or mix mode of actions with herbicides. Now, University of Nebraska - Lincoln researchers have confirmed the first glyphosate resistant weed species in Nebraska -- marestail, also know as horseweed. While Nebraska researchers first suspected resistance had developed almost a year ago, it took tests and growing plants from last year's seeds to confirm it.

The location of resistant marestail includes weeds in Saunders County. Seeds of three suspect marestail populations were collected in the Fall of 2005 near Mead. Greenhouse bioassays were conducted this summer and dose response curves for Glyphosate were defined for each marestail population. Curve comparisons clearly showed Glyphosate resistance requiring three to six times the rate needed in a susceptible population. (Resistance level varied with the marestail population). For example, 90% control of a susceptible population was achieved with 32 ounces of a Glyphosate herbicide (3 pounds/gallon acid equivalent) (1X rate), while the resistant populations needed about 100 ounces/acre (3X rate) and 200 oz (6X rate) in order to achieve the same level of control.

Weed resistance results from repeated use of the same herbicide. It is really not a lot different than when Penicillium first came out. Penicillium saved a lot of lives but eventually there became Penicillium resistant strains of bacteria because of overuse. Too much of a good thing. Widespread use of glyphosate tolerant crops and repeated use of glyphosate herbicide has resulted in selection pressure on weed populations in recent years. Now we have 11 species of weeds worldwide and 7 species of weeds in the U.S. that are resistant to glyphosate. These include giant ragweed, lambsquarters, common ragweed, waterhemp, and palmer amaranth. We have these weeds in Nebraska so time will tell is the resistant types spread or develop here.

In the Midwest it is now suspected that 5 million acres of soybeans have resistant populations of marestail weeds due to using glyphosate. The number of glyphosate resistant weeds tripled in just over eight years of repeated glyphosate use over a large land area (over 50 million acres) due to introduction of Roundup-Ready crops.

to Contact our Staff

Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska-Lincoln cooperating with the Counties and the United States Department of Agriculture.
UNIVERSITY OF NEBRASKA-LINCOLN EXTENSION educational programs abide with the nondiscrimination policies of the University of Nebraska and United States Department of Agriculture. We assure reasonable accommodation under the Americans with Disabilities Act.