Palmer amaranth, bluegrass, foxtail rank among the toughest weed problems in grassy crops

Debra Murphy, 2016.

A survey that ranks the most common and troublesome weeds in 2020 was recently completed by the Weed Science Society of America (WSSA).

More than 315 weed scientists from across the U.S. and Canada weighed in on seven categories of grass crops, covering weeds found in corn, rice, sorghum, turf, spring cereal grains, winter cereal grains, pastureland, rangeland, and other hay.

There were some surprises, and some results from the survey that weren’t a surprise, like Palmer amaranth being the most troublesome weed to control in corn.

Lee Van Wychen, executive director of science policy for Weed Science Society of America, based out of Washington, D.C., recently joined RealAg Radio host Shaun Haney, to talk about the survey’s results.

Palmer amaranth was one of the not-so-surprising hard-to-handle weeds that is still prevalent across the survey area, especially since there’s growing resistance to herbicides. Van Wychen says that out of the corn, sorghum, rice, and other cereal grain crops surveyed last year, one would think that removing a broadleaf weed for those crops would be easy.

“[It’s] sort of an eye-opener and kind of gives credit to how tough it is and it’s different growth characteristics — the root system and seed production, it’s photosynthetic use efficiency and nutrient use efficiency — it’s just a very awesome weed, to put it no other way,” says Van Wychen, adding that producers won’t be able to spray their way out of having Palmer amaranth or its cousin, waterhemp.

Listen to the full conversation, story continues below:

Bluegrass weed species are on the rise, the reason for which is tied to herbicide resistance. The last time the cereal crops were surveyed, annual bluegrass was on the top 10 list, but it shot up to number two in last year’s results.

“I believe there are ten different mechanisms that bluegrass is resistant to now, around the world,” says Van Wychen.

When it comes to troublesome and most common weeds for small cereal grains, foxtail species are on both lists. Wild oat is typically up there on those lists as well as common lamb’s-quarters, but foxtail makes the top of the lists.

Kochia in Western Canada is popping its head up in more spots, says Van Wychen, and herbicide resistance is spreading rapidly.

“If I had to regionalize some of the worst cropping system weeds, Palmer amaranth is clearly in the southern parts of the United States, and then waterhemp in the Midwest, and then kochia in the western states and provinces,” says Van Wychen. “So in those three the herbicide resistance is continuing to expand in both the number of active ingredients that it’s resistant to and the acreage that you see out there.”

Canada thistle tied for ninth place on the troublesome list and the most common list, which isn’t something to celebrate.

As far as an integrated approach to managing these weeds, Van Wychen says that cropping system is still the biggest weapon — the more you can rotate, diversify, or change up the system, the better.

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