Horseweed. (Image courtesy Billy E. Warrick, Texas Cooperative Extension.)

New Tests Screen Weed for Resistance to Major Herbicide

By Jim Core
June 20, 2005

Two rapid, nondestructive tests have been developed by Agricultural Research Service (ARS) scientists to screen a troublesome weed for resistance to the world's most-used herbicide.

ARS scientists Clifford H. Koger III and Dale L. Shaner developed the test to determine if horseweed plants were resistant or susceptible to the herbicide glyphosate.

Koger is an agronomist with the Crop Genetic and Production Research Unit at Stoneville, Miss., and Shaner is a plant physiologist with the Water Management Research Unit at Fort Collins, Colo. Koger was based in the ARS Southern Weed Science Research Unit at Stoneville when the research was done.

In 2000, horseweed (Conyza canadensis) became the first weed species to develop resistance to glyphosate in cropland where glyphosate-resistant soybeans were grown. Glyphosate-resistant biotypes of horseweed have now been confirmed in 13 states east of the Mississippi River.

Glyphosate is effective at killing all plant types including grasses, broadleaves and sedges, as well as perennial and woody plants. After emergence, glyphosate-resistant crops are capable of tolerating multiple applications of the herbicide, while weeds are killed. However, repeated use over many years has left several weed species resistant to glyphosate.

The two tests can be used together. One method, which involves dipping a whole leaf into a glyphosate-based mixture and looking for signs of injury, is quick and easy to perform. To achieve double confirmation of the weed's status, a second assay can be used. This method takes advantage of glyphosate's mode of action, which involves inhibiting amino acid metabolism in what is known as the shikimic acid pathway. Leaf tissue samples are removed, and amino acid levels are measured with specialized laboratory equipment.

If glyphosate resistance is confirmed, the tests should help reduce the spread of resistant horseweed populations because growers will use different herbicides to manage the resistant weeds.

Koger and Shaner are testing both assays to see if they're useful for screening other weed species for resistance to glyphosate.

ARS is the U.S. Department of Agriculture's chief scientific research agency.