Two rapid 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 tests to determine if horseweed was resistant or susceptible to the herbicide glyphosate.

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.

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.