Identify Palmer amaranth presence to better manage for herbicide resistance
The excitement of spring can come to a halt when Palmer amaranth finds its way into seed lots. Two new tests created by the University of Illinois could help slow the spread of the prolific weed through early identification and management.
“We have a test that can detect one Palmer seed out of 100 seeds and we can ID leaf tissue, too,” says Suzanne Bissonette, assistant dean at the University of Illinois (U of I) College of Agriculture and Natural Resources Extension and plant clinic director.
This past year, farmers in more than 40 Midwestern counties learned no good deed goes unpunished when Palmer amaranth seed snuck into their Conservation Reserve Program (CRP) or pollinator plot seed mixes. This problem could potentially have been avoided with this test, which can be ordered by pollinator or CRP seed companies or farmers.
The test also shows if the weed is resistant to glyphosate or PPO inhibitors, which means farmers can cater their weed management strategy to actually kill the weed.
If Palmer is present in fields, the U of I tissue test can help identify the species (Palmer can look similar to its less aggressive cousin tall waterhemp) and determine the best herbicide.
Identifying herbicide resistance in-season helps farmers make management decisions in real time. However, the U of I herbicide tolerance test only checks for specific resistance pathways, which means if a new mode of resistance develops or if it isn’t in their library for testing, farmers could still have resistance even when tests don’t indicate it.
“Palmer is an enormous weed that wreaks havoc on yield, seed quality and harvestability,” Bissonette says. Prevent seeds from entering fields and stop growth as early as possible to avoid more weed seed taking root.
Best Strategy: Kill the Seed, Stop the Weed
While Palmer amaranth dominates headlines for its aggressive nature, it’s not the only troublesome resistant weed. Giant ragweed robs crops of precious sunlight, nutrients and water. The trick to beating this thief is to destroy the weed’s seed bank.
Don’t let the simplicity of the solution fool you. Giant ragweed is resistant to EPSP synthase inhibitors (group 9, glyphosate) and ALS inhibitors (group 2) and can grow up to 16' tall. It’s vital to use effective herbicide sites of action and to catch these weeds when they are smaller than 4" tall. If you don’t kill the weed before it reaches seed production, it can drop 5,000 seeds for you to compete with next season.
“Since the ragweed seedbank is short-lived, our research shows it’s possible to manage fields with giant ragweed by simply eliminating weeds that emerge before they go to seed,” says Jared Goplen, researcher with the University of Minnesota.
According to University of Minnesota research, a zero-weed threshold reduces the weed’s seed bank by 96% in two years. In addition, a rotation with wheat or alfalfa results in 38% fewer giant ragweed plants; a corn/soybean rotation doesn’t cut it.