Giant ragweed, a prolific and efficient weed, robs soybeans each year of precious sunlight, nutrients and water, slashing yield. The trick to beating this thief could be easier than one might think—destroy the weed’s seed bank.
Don’t discount the simplicity of this solution. 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 mechanisms of action and to catch these weeds when they are smaller than 4” tall. If a single giant ragweed reaches seed production, it will shed around 5,000 seeds that will compete with next season’s crops.
“Since the ragweed seedbank is short-lived, our research shows it is possible to manage fields infested with giant ragweed by simply eliminating weeds that emerge before they go to seed,” says Jared Goplen, researcher with the University of Minnesota.
Employ a zero-weed threshold this season. According to University of Minnesota research, this approach reduces the weed’s seed bank by 96% in two years. Researchers also found that the weed’s presence can be lowered by moving beyond a corn and soybean rotation. They recognized 38% fewer giant ragweed plants when wheat or alfalfa is placed in the rotation.
Unchecked, one giant ragweed plant per square meter can reduce yield up to 52% in soybeans, and two per square meter can reduce corn yields by up to 37%. Scout fields regularly and destroy giant ragweed before seed development.
Look for these characteristics:
· Cotyledons leaves are round, thick and large with purple hypocotyl.
· Hairy stems.
· True leaves are opposite, three- or five-lobed, hairy, with toothed edges and can be 4” to 8” wide by 6” long.
· Small, green flowers, but crop experts urge growers not to let giant ragweed get to the flowering stage.