Waterhemp has quickly established itself as one of the nation’s most devastating weeds. Resistant to six herbicide groups, the weed can steal between 40% and 70% of yields, according to University of Illinois research.
Groups to which waterhemp is resistant include: ALS (2), T1R1 Auxin receptors (4), Photosystem II inhibitors (5), PPO Inhibitors (14), EPSP synthase inhibitors (9, glyphosate) and HPPD inhibitors (27). Control is trickier because waterhemp has developed resistance to multiple herbicides in a single plant.
“We essentially confirmed that we can’t control this population with three classes of herbicides: the HPPD-inhibitors, the ALS-inhibitors, or the PSII-inhibitors. The weight of everything together points to the fact that this population is using resistance mechanisms that we haven’t seen before,” says University of Illinois Weed Scientist Aaron Hager in a recent university publication.
Hager and others at U of I found this increased level of resistance in McLean County, Ill., but resistance such as this has become increasingly common across the Midwest. Hager says it’s rare to find waterhemp resistant to only one herbicide group.
When to Attack Waterhemp
“Ultimately, we know how to win the battle,” Hager says. “If we attack waterhemp at the most vulnerable stage in its life cycle—the seed—we could beat this thing in five to seven years.”
Since waterhemp seeds have a relatively short shelf life of four to five years, tillage could be an excellent defense against the weed--its seeds don’t grow from low soil depths. Hager encourages farmers to let the weed seed germinate and then mechanically work the soil while it’s young to stop it from going to seed and make that seed ineffective in future years. Keep an eye out for late-season stragglers and plan ahead to gain control over the weed before it puts on seeds.
Certain herbicide groups are still effective, but need to be used with caution so waterhemp doesn’t develop more resistance. Talk to local agronomists and chemical experts to determine what herbicides work in your area. To perform your own research, visit the International Survey of Herbicide Resistant Weeds and search by state and county. Be sure to use multiple, effective herbicide groups to delay resistance.
One of the keys to gaining control over waterhemp is catching it early. It's easier to kill chemically and mechanically before it reaches 4” tall.
While scouting for waterhemp, look for these characteristics:
- Stems can be bright red or green.
- Seedling leaves are oar-shaped.
- First true leaves are oval with a notch at the tip.
- True leaves are alternate, oval, hairless and waxy.
- Flowers are green to dark pink with spikes.
- Summer annual found in eastern and central U.S.
- Can reach up to 9” tall.
Even if you have no or limited resistance to waterhemp in your state, the weed can still steal yield. The weed grows up to 2.5” per day in certain conditions and has a growth cycle opposite of typical spraying windows. Since it can often dodge the herbicide bullet, a weed seedbank could be building in your fields.
How is waterhemp pressure in your area? What herbicide or mechanical control methods do you use?