CHAMPAIGN, Ill. — Resistance to multiple herbicides is the new norm for populations of waterhemp, a common agricultural weed. With their herbicide options dwindling and nothing new on the horizon, Illinois growers must be strategic in how they manage waterhemp-infested fields, says a University of Illinois expert on crop weed management.
“So many decisions that farmers have to make now are focused on waterhemp because of how common and widespread it has become, and how challenging it is to control,” said Aaron Hager, a U. of I. crop sciences professor who has tracked the spread of herbicide-resistant weeds in Illinois for more than two decades.
Waterhemp in Illinois has evolved resistance to more herbicide classes than any other weed species in Illinois, Hager said.
So far, researchers have confirmed waterhemp’s resistance to herbicides from six classes in Illinois, Hager said. These include:
- ALS inhibitors, which interfere with the weeds’ ability to synthesize specific amino acids;
- T1R1 Auxin receptors (for example, the herbicide 2-4D), which alter protein synthesis and cell division;
- Photosystem II inhibitors, which disrupt photosynthesis;
- EPSP synthase inhibitors (for example, glyphosate herbicides such as Roundup), which interfere with amino acid synthesis;
- PPO inhibitors, which disrupt chlorophyll and heme synthesis; and
- HPPD inhibitors, which interfere with plants’ ability to make and use protective compounds known as carotenoids.
In reality, waterhemp is likely resistant to more than these six groups, and the number is predicted to keep growing, Hager said.
Perhaps even more troubling is how common multiple-resistance waterhemp has become, Hager said. Resistance to two, three, four and five different herbicide classes have been documented within individual Illinois waterhemp populations.
Illinois growers need to be prepared to deal with herbicide-resistant waterhemp regardless of whether it’s been reported in their county or not, he said. Herbicide resistance isn’t limited by location – it can occur in any plant or field across the state.
Herbicide resistance can spread quickly through waterhemp populations because of how the plant reproduces. Waterhemp has separate male and female plants, and inputs from both are required for seed production. Inheriting genes from two different parents increases the chance that the offspring will end up with herbicide-resistance genes.
Unfortunately, herbicide resistance isn’t the only challenge associated with waterhemp control, Hager said.
“Even without resistance, this is a species that's just flat-out well-adapted to modern farming practices. It'll thrive,” he said.
Waterhemp can germinate throughout much of the growing season – from March through August. This means that multiple herbicide applications and mechanical treatments are needed to keep the plant in check.
To further complicate things, female waterhemp plants are prolific seed producers. Plants average 250,000 seeds each and can produce more than a million seeds under optimal conditions. The seeds themselves are tiny and mobile. These traits allow waterhemp to invade and infest new areas extremely quickly.
“We have to be more proactive, because what you invest now to preclude any further seed production is going to pay returns for years and years,” Hager said.
Growers need to think of waterhemp management as “yield protection” rather than “weed control,” he said.
If left unchecked, waterhemp can decrease soybean yields by up to 42 percent and corn yields by 48 percent. Money spent on the highest-yielding varieties of corn and soybeans is essentially wasted if waterhemp infestation cannot be controlled, Hager said.
Some farmers don’t control for waterhemp, thinking they can deal with the yield losses, Hager said. However, these growers may not be prepared for the repercussions of a closely related species, Palmer amaranth. Palmer amaranth can bring a yield to zero, and it’s incredibly difficult to tell apart from waterhemp. Apathy to waterhemp can lead to the devastation of a farm by Palmer amaranth, he said.
“The key to winning the battle is not looking for another herbicide to spray on it. We’ve lost that battle. We will continue to lose that battle. We need to focus all of our attention on targeting the weak stage of the life cycle of the plants. That’s the seed,” Hager said.
Unlike other common agricultural weeds, whose seeds can survive more than 50 years in the soil, waterhemp seeds are only viable for seven to 10 years, Hager said. That’s their weakness.
One inexpensive way to reduce the waterhemp seedbank is to let as many seeds as possible germinate prior to planting. Before planting, apply a chemical or mechanical treatment so that the field “starts clean,” Hager said.
If growers want to continue to use herbicides, they can, but they will have to be very selective in which they use. Every time growers make an application, they are selecting for resistance, he said.
“Waterhemp is not going to change to suit us – we’re going to have to change to defeat it,” Hager said.