May 8, 2009

Figure 1. In Nebraska soybean field, marestail that survived regular application rate of glyphosate.

Weed resistance to herbicides is not a new thing. It began when man started using chemicals for weed control. Well documented, worldwide information suggests that more than 150 weed species are resistant to one or more of 15 herbicide families. In most of these cases, resistance developed because of repeated use of the same herbicide, a factor that growers can control. (For more information on these weed species, see the Weed Science Society of America.)

Widespread use of glyphosate-tolerant crops and repeated use of glyphosate-based herbicides is causing single selection pressure on weed populations, resulting in glyphosate resistance in some. Prior to the introduction of glyphosate-tolerant crops, only a few weed species worldwide (ryegrass and goosegrass) had become glyphosate-resistant. However, the number of glyphosate-resistant weeds increased sharply in just over eight years of repeated glyphosate use over a large area worldwide (more than 200 million acres) due to introduction of Roundup Ready® crops.

Glyphosate Resistance Nationally and in Nebraska

Fifteen weed species have developed resistance to glyphosate worldwide, including 10 in the U.S. These are waterhemp, rigid and Italian ryegrass, lambsquarter, giant ragweed, common ragweed, palmer amaranth, hoary fleabane, johnsongrass, and marestail (horseweed).

Marestail (Figure 1) is the only species in Nebraska with confirmed glyphosate-resistance. A UNL study of a marestail population showed a glyphosate resistance requiring three to 6 times the normal level of herbicide. For example, 90% control of a susceptible population was achieved with 32 oz/ac of glyphosate (3 lb/gal acid equivalent, as 1X rate), while the resistant populations needed about 100 oz/ac (3 times the label rate) and 200 oz (6 times the label rate) in order to achieve the same level of control.

The Weed Science Society of America Web site lists weeds resistant to various herbicides by state and includes atrazine, ALS, AHAS, glyphosate, and 2,4-D. Some of Nebraska's neighboring states have considerably more resistant types, including Kansas, 17, Missouri, 10, Iowa, 13, and Colorado, 4.

Management

We believe that glyphosate- and herbicide-tolerant crops, including those based on glyphosate herbicide, can remain useful components of a crop production system only with proper management. It is easy to overuse glyphosate, versus combinations of preemergence herbicides or tank mix partners, when one Roundup Ready crop is grown after another. Proper use of herbicide-tolerant technology, as a component of an integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many problems of misuse.

More Information

For more information about glyphosate resistance contact your local UNL extension office or me at at 402-584-3808 or sknezevic2@unl.edu.
Stevan Knezevic, Extension Weeds Specialist
Haskell Agricultural Laboratory, Concord