While there are dozens of common weeds that threaten soybean yields, a recent survey found a clear winner when it comes to robbing retailers of more sleep — and farmers of more yield potential.
Corteva Agriscience asked a group of 100 retailers from across 12 north-central states this question: “What is the most challenging herbicide-resistant weed in your area?” The definitive winner was waterhemp — which was named by 58 percent of retailers — more than all other weed species combined.
Marestail was a distant second, at 23 percent, followed by giant ragweed at 7 percent, and Palmer amaranth and kochia, both at 6 percent.
Jeff Moon wasn’t surprised by the results.
“It’s consistent with recent conversations we’ve had with customers, whether out in the field or at industry events,” Moon says. “We work diligently to keep tabs on the currently challenging weed issues, so we can be ready to help with tailored solutions.”
Why it’s challenging, and tips for more effective management
Waterhemp is seen as the most challenging for a couple reasons. First, it’s become resistant to multiple herbicides — not just glyphosate. In fact, researchers have identified a waterhemp population in Missouri that is resistant to a record-breaking six herbicide mechanisms of action.1 Second, it shows no signs of slowing its spread to infest soybean fields in new states.
Of particular concern is that this season many farmers could be looking at a one-two punch from marestail and waterhemp. That’s because it’s expected that marestail will emerge in high numbers this spring after the cool, wet conditions many areas experienced last fall. These conditions, along with a delayed harvest, provided an ideal situation for winter annuals like marestail to germinate and become established. If farmers pass on a burndown application, marestail can be expected to quickly cause problems in soybean fields.
“A good burndown herbicide application — either in fall or early spring — is very effective against actively growing winter annuals like marestail,” Moon says. “Elevore herbicide provides excellent control of marestail up to 8 inches tall, and works in challenging conditions.”
But burndown herbicides won’t work on later-emerging summer annuals like waterhemp. For that, Moon recommends farmers scout often, implement a diverse action plan and use a program herbicide approach with multiple modes of action. Below he provides several tips for better waterhemp management:
- Scout early and often. It’s critical to identify waterhemp early, then continue to check fields through midsummer. Ongoing scouting helps farmers plan timely postemergence herbicide applications. While scouting, make note of potential problem spots for the following year. Waterhemp is often misidentified with its cousins in the pigweed family, such as Palmer amaranth. When identifying waterhemp, check the leaves. Waterhemp leaves are generally longer and more lance-shaped than other pigweeds.
- Reduce row spacing at planting. Planting narrower rows can help suppress waterhemp growth by reducing the time it takes for crops to reach canopy closure.
- Layer residual herbicides. Layering residual herbicides keeps fields clean longer, typically through crop canopy closure, to manage the waterhemp seedbank. In soybeans, Moon recommends Sonic® herbicide for two modes of action preemergence, followed by an application of EverpreX™ herbicide over the top of soybeans for an additional mode of action. Farmers may also add glyphosate, in areas where waterhemp isn’t resistant, to increase the modes of action.
- Keep weeds from going to seed. Just a few waterhemp weeds left in a field can mean significant problems next season. Waterhemp that goes to seed in soybean fields can potentially cross-pollinate with a population in another field and build additional resistance. Tillage also can help lower waterhemp populations because in order to germinate and emerge, its seeds must be in the top inch of soil and they are relatively short lived.
- Maximize application technology. Pay close attention to herbicide labels to maximize the efficacy of the product. Not every herbicide can be applied in the same manner with the same nozzle, water volume, pressure and adjuvant.
- Rotate crops. Waterhemp requires herbicide control and effective cultural practices, like rotating crops. This should be planned for more than a single year at a time, Moon says. Rotating crops also allows farmers to alternate modes of action and adjust tilling plans for corn and soybean fields.
“It’s critical to start with a strong treatment plan for winter annuals like marestail, then be ready with a diverse plan of action for waterhemp,” Moon says. “Otherwise, you risk a significant drop in yield potential, and the weeds will only get tougher to control down the line.”