Dozens of weeds threaten soybean yields, but a recent survey by Corteva Agriscience, agriculture division of DowDupont, found that one weed robs retailers of more sleep and farmers of more yield potential than any other.
A group of 100 retailers from across 12 north-central states, were asked: “What is the most challenging herbicide-resistant weed in your area?” Their answer: waterhemp — in a landslide. Waterhemp received more votes than all other cited weed species combined. Of the retailers surveyed, 58% listed it as the most troublesome weed. Marestail was second, at 23%, followed distantly by giant ragweed at 7%, and Palmer amaranth and kochia, both at 6%.
The results weren’t all that shocking to Jeff Moon, market development specialist, Corteva Agriscience. “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.”
Root of the problem
Why did so many retailers name waterhemp as their biggest problem weed? First, it’s developed resistance to multiple herbicides — not just glyphosate. Researchers have identified a waterhemp population in Missouri that is resistant to a record-breaking six herbicide mechanisms of action. Second, its continued expansion affects soybean fields in new states.
Making matters worse, in 2019 many farmers could be looking at a one-two punch from marestail and waterhemp. It’s expected that winter annuals, especially marestail, will emerge in high numbers this spring, after last fall’s cool, wet conditions in many areas. The delayed harvest provided ideal conditions for weeds to germinate and become established. If farmers don’t apply a burndown application, marestail can be expected to quickly cause problems this spring.
“A good burndown herbicide application — either in fall or early spring — is very effective against actively growing winter annuals like marestail,” Moon says. But burndown herbicides don’t work on later-emerging summer annuals like waterhemp. For that, he recommends farmers scout often, implement a diverse action plan and use a program herbicide approach with multiple modes of action.
6 management tips
Moon provides these six tips for better waterhemp management:
1. Scout diligently. Identify waterhemp early and 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.
2. 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.
3. Plant narrower rows. Narrow row spacing can help suppress waterhemp growth by allowing crops to reach canopy closure quicker.
4. Stop 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 crosspollinate with a population in another field and build additional resistance. Tillage can also help lower waterhemp populations, because to germinate and emerge, its seeds must be in the top inch of soil.
5. 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 nozzles, water volumes, pressures and adjuvants.
6. 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. 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.”