The problem with pigweeds

Scouting for waterhemp is difficult but critical, Manitoba Agriculture weed specialist Kim Brown told producers during her presentation at St. Jean Farm Days Jan. 10.

“You’re going to hear a lot about this in the next few years because this is just going to become a bigger and bigger problem, but I really believe we can get out ahead of this and we can manage this,” she said.

Why it matters: Waterhemp is a tier one weed under the Noxious Weed Act and farmers are responsible for the weed management that comes with that classification if it is found on their fields.

Waterhemp is a growing concern in Manitoba. First spotted in 2019, it is starting to spread throughout the province. Recent reports identified it in soybean fields in the RM of Emerson-Franklin and the RM of De Salaberry, bringing the number of affected municipalities to 19.

The rapid spread is attributed to flooding and water movement, especially in drainage-prone areas.

Waterhemp is a tier one weed under the Manitoba Noxious Weeds Act. The act mandates the destruction of tier one weeds and places the onus on farmers to prevent their establishment.

“We have three tier one pigweeds on our Noxious Weeds Act,” said Brown. “We’ve got our waterhemp, our Palmer [amaranth], and our smooth pigweed. They’re on our list because, elsewhere in the world, they are resistant to glyphosate.”

Of those three, Brown said she is less concerned about smooth pigweed.

“We have some of this smooth pigweed naturally in Manitoba, but I don’t think that we have any glyphosate resistance here.”

Palmer amaranth

Palmer amaranth has yet to present a significant problem in Manitoba, though there is potential. That weed was first detected in Manitoba in 2021 in the RM of Dufferin. The latest information from Manitoba Agriculture indicates that municipality remains the only one where the weed has been found.

Palmer amaranth has earned its reputation as a yield eater in the U.S. Research out of Iowa State University suggests it can reduce soybean production by 79 per cent.

Palmer amaranth “has displaced waterhemp in a lot of places,” Brown said. “They’re not dealing with waterhemp anymore. They’re dealing with Palmer [amaranth], and it is worse.”

When to scout for waterhemp

Brown said waterhemp, like all pigweeds, is a warm season weed.

“So they don’t come up first thing in the spring when it’s cold and wet,” she said. It prefers a moderate to moist environment as well as high light intensity.

“This is why it is thriving in our row crops. It likes those wide rows of bare, black soil,” said Brown. “It’s hot; you’ve probably got good fertility in there, so this weed just loves that.”

Conversely, it doesn’t compete well beneath the canopy of a competitive crop. Waterhemp is not often observed in canola and wheat, but Brown warns it can still lurk, even if the crop planted that year doesn’t let it express itself.

“What’s saving our butts on this one is that we have other competitive crops in the rotation where it does not take off.”

Producers can expect the biggest waterhemp emergence in June.

“It begins emerging after the herbicide burn down in no-till, or the last tillage, whichever one you’re dealing with. And it continues to emerge throughout the entire growing season,” said Brown.

Even plants that emerge late in the fall can set seed.

“It can set seed within two weeks of emergence. Now, it doesn’t set a lot of seed [if it’s shortly after emergence] but, when you’re talking about a resistant plant, any seed is too much. We cannot let this thing set seed.”

Mature waterhemp is a prolific seed producer. Brown estimated that a big plant with no competition could set up to 4.8 million seeds.

In real field conditions, the total is not quite so high. Research out of Ontario suggests that waterhemp emerging at the same time as soybeans will have about 300,000 seeds. But if emergence can be stalled so it emerges 50 days after the crop, there will only be 3,000 seeds per plant.

“The plants were a lot smaller and they have a lot less seed,” said Brown. “This is good. We need to manipulate this. We cannot deal with thousands of plants per acre, each having 300,000 seeds.”

What to look for

When it comes to pigweeds, it’s hard to tell the mundane from the critical.

“All the pigweeds look alike, especially when they’re smaller,” said Brown. “It’s very hard to identify them visually.”

That’s a problem, since farmers should control waterhemp before it’s four inches tall.

“It looks like lamb’s quarters. It looks like a big ragweed,” she said. “I even find the plants that germinate late in the summer look a lot different than the plants that germinate first thing in the spring.”

Brown points out that Palmer amaranth, waterhemp, and smooth, spiny, red-root or Powell’s pigweeds look almost identical at emergence.

“When we get to the two-leaf stage, it’s no better. And, when we start to add a few more leaves, it pretty much still looks like a pigweed,” she said. “You know it’s a pigweed, but you don’t know which one at this point.”

Despite efforts to categorize young pigweeds by visual characteristics, no methodology has proven reliable.

“Sometimes waterhemp get[s] these longer, skinnier leaves, but not always. Sometimes they’ll talk about a little hair at the end of the leaf, but it depends. Sometimes they have a little ‘v’ mark in here like a chevron, like what’s on clovers, and some people will say, ‘oh, if it’s got that, it’s waterhemp or Palmer [amaranth],” said Brown.

“Nope, because I’ve seen red-root pigweed populations with that chevron. Sometimes it’s purple. Sometimes it’s white. You can’t go by any of those things.”

But one reliable method for eliminating at least one pigweed option is touch.

“Red-root pigweed is hairy and rough feeling,” said Brown, even when it is small. “I think it feels like really fine-grit sandpaper, but without the sharpness. So when you run your fingers up and down that weed and you run your fingers up and down that stem, it feels bumpy.”

All other pigweeds of concern are smooth.

“Waterhemp is very, very smooth. Sometimes you can see some really fine little hairs, but you never feel them. It just feels super, super smooth.”

For a resource on the different species, Brown recommends the Pigweed Species Guide put out by the Ontario Ministry of Agriculture, Food and Rural Affairs.