The first U.S. resistance to glyphosate was detected in 1998 in rigid ryegrass in California. Since then, nine weed species in the U.S. now have confirmed resistance to glyphosate. Among these weeds are strains of common ragweed, common waterhemp, giant ragweed, hairy fleabane, horseweed, Italian ryegrass, johnsongrass, Palmer amaranth and rigid ryegrass. Most of the species that have evolved resistance to glyphosate also demonstrate multiple resistances to other herbicide mechanisms of action.
States with confirmed outbreaks include: Alabama, Arkansas, California, Delaware, Georgia, Kansas, Kentucky, Maryland, Michigan, Minnesota, Mississippi, Missouri, New Jersey, New Mexico, North Carolina, Illinois, Indiana, Iowa, Ohio, Oregon, Pennsylvania and Tennessee. To view listing of weed resistance by biotypes and mode of action, go to: www.weedscience.org
When glyphosate was first introduced for weed control, its unique way of inhibiting protein synthesis and growth in plants led many to believe that resistance would not be an issue. "The herbicide is not the cause of the problem,” says Micheal Owen, Iowa State University extension weed specialist. "The problem is we mismanaged the system. The convenience and simplicity of the use of the product have resulted in an escalating problem.”
Problems in the southern U.S. have led weed scientists to rank weed resistance as the next boll weevil. "Most of the fields still look pretty good across the Midwest. Most of the problems can be seen at field edges,” Owen reports.
"However, currently the majority of the corn and soybean acres grown across the Midwest contain glyphosate-resistant cultivars and glyphosate represents the primary if not sole weed management tactic. Given this incredible selection pressure, it's clear unless proactive tactics are taken that steward this technology, we will see widespread issues,” Owen says.
Have weed escapes? Look for these resistance indicators when monitoring fields.
-- The field has been sprayed repeatedly with the same herbicide or herbicides with a similar mode of action.
-- A patch of weeds occurs in the same area year after year and is spreading.
-- Many weed species are managed, but one particular weed type is no longer controlled.
-- Surviving weeds of the problem species may be in a patch where some are dead and some exhibit variable symptoms, but all are approximately the same age.