What is in this article?:
- Avoidance is the best, least costly, approach to herbicide resistant weed management.
- At least 76 weed species have known resistance to some herbicide.
- Specialists offer several recommendations, depending on location, crop choices and management options.
Palmer amaranth is one of the most worrisome herbicide resistant weeds.
A numbers game
The numbers prove his point. At least 76 weed species have known resistance to some herbicide, and weeds have shown resistance to at least 18 different modes of action. “It’s a big problem.”Glyphosate is the first herbicide that comes to mind in a resistance discussion but that may not be the worst problem. Weeds and grasses are also resistant to ALS and ACCase inhibitors as well as to glyphosate.
But the glyphosate issue is huge with 99 percent of soybeans planted to Roundup Ready varieties. That’s 90 percent for cotton and 80 percent for corn.
York said observers have identified four weed species with glyphosate resistance in North Carolina. A small stand of glyphosate-resistant common ragweed has been identified. “It was in a small geography but it is spreading. We also have a long history of resistant Italian ryegrass problems in wheat. We’re now seeing glyphosate resistant ryegrass.”
Marestail/horseweed is “a big problem. But Palmer amaranth is the big one. It has been a game changer for us. It’s amazing that one weed species can change everything we do.
“We have to go through a denial phase before we confirm resistance,” York said. A 2005 survey showed less than 20 percent of Palmer amaranth resistant to glyphosate and less than 15 percent resistant to ALS herbicides. “That resistance was relatively isolated and Palmer amaranth was a relatively new weed for North Carolina.”
A 2010 survey showed resistant Palmer amaranth “all over the state and it became our number one concern.”