This is one contest few farmers will celebrate. The pigweed species continue to take home the gold as the most troublesome weeds in corn, with Palmer Amaranth edging waterhemp by one vote in the most recent survey by the Weed Science Society of America (WSSA). Morningglory, giant ragweed, common lambsquarters, Johnsongrass and kochia round out the top six most difficult to control weeds. The ranking basically remains unchanged since weed scientists first voted in 2015. The only exception is foxtail, which dropped out of the top six.
In 2020, the WSSA polled members across the U.S. and Canada to determine the most common (weeds seen most frequently) and the most troublesome (most difficult to control) weeds in grass crops, pasture and turf. They change their survey every three years between broadleaf crops (2019 survey), grass crops (2020) and Aquatic/non-crop areas (2021).
Common lambsquarters garnered the top vote in 2020 as the most common weed found in corn, followed by foxtail, waterhemp, pigweed, Palmer amaranth and morningglory. Two weeds have dropped off the top six list from 2015, velvetleaf and common ragweed.
“If you talk to a weed scientist in the north, they’ll tell you waterhemp is just cleaning our clock. We can't control it. And the southern weed guys are always talking about Palmer amaranth. However, Palmer amaranth is actually spreading more and more to the north,” says Lee Van Wychen, Executive Director of Science Policy for the WSSA.
They’re both terrible problems to have, spreading millions of seeds. “If you let one weed go to seed, you're paying for it years down the road,” Van Wychen adds.
Herbicide resistance is the common denominator in both weeds. For example, waterhemp in Illinois has proven resistant to six different herbicide sites of action, along with the recently-discovered metabolic resistance. “We're getting to the point where there's really not much chemistry left to throw at it. The same goes for Palmer amaranth,” he says. “Palmer amaranth and horseweed (marestail) were the first major weeds to spread widely due to glyphosate resistance.”
While the survey examines bigger-picture population trends, it does not gather data on factors causing the worst weed rankings. “However, what we’re seeing is that herbicide-resistant weeds are really climbing the list fast, which will continue until more successful integrated management strategies are implemented,” Van Wychen says.
Integrated management is not just switching to a different herbicide. “It's changing your crop rotation, changing planting dates and cultural practices, even doing a mechanical removal now and then,” he adds.
Van Wychen, like most weed scientists, urges farmers to do everything possible to keep these troublesome weeds from going to seed. “Once they do, you’re behind the curve, causing yourself a lot of problems down the road. The answer is not going to come out of a herbicide jug. It's going to take changing up your techniques, crop rotation [and more] to really get in front of some of these more troublesome species,” he adds.
For instance, Van Wychen cites experiments by some farmers planting earlier. In a corn/soybean rotation, seeding soybeans early can help keep later flushes of warm-season type weeds like waterhemp from germinating and producing seed. “That's a cultural tactic that can change weed control, the same with adding cover crops or a perennial crop like alfalfa in the system. That's a great way to get rid of some of those annual weeds that are always a problem in a pure corn/soybean rotation,” he adds.
Even if a farmer had great weed control in 2021, Van Wychen says farmers should do something different this year. “While it seems counterintuitive since it worked, you really have to change it up if you want to stay ahead of the curve on these troublesome weed species.”
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