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Dave Bilyea of the University of Guelph’s Ridgetown Campus says the most striking thing about Palmer amaranth is that it has the very long petioles. The petiole is longer than the leaf itself as it grows. Photo courtesy of Christy Shropshire.

There’s some debate about when to sound the alarm on Palmer amaranth. Those struggling to help farmers combat the weed in the United States insist the sooner, the better. Folks who know Eastern Canada assure farmers it isn’t time to panic. Yet.

Palmer amaranth, known more commonly among American farmers as Palmer pigweed, has been traveling north through the midwestern states, Ohio, Pennsylvania and now Michigan. Scattered plants have also been reported in Ontario, but Dave Bilyea, a weed science technician at the University of Guelph’s Ridgetown campus, confirms none of these isolated cases are of any concern so far. However, he does believe farmers in Ontario do need to be vigilant moving forward.

“They are very difficult weeds to control in the U.S., so we sure don’t want to see them in any large populations in Ontario,” Bilyea says. “People could deal with small pockets of these weeds fairly easily but if they were to be ignored for any length of time, they could be problematic on a farm by farm basis.”

For a root-bound organism, Palmer amaranth is demonstrating an excellent ability to travel. As a weed seed, it is exceptional at hitching rides on agricultural equipment, hidden within grain or wrapped up in bales of all kinds. Bilyea says that the plants that have been found in Ontario would seem to have travelled here by rail. He notes that a recent study by the University of Missouri has proven that the weed isn’t depending on human traffic alone however. Both geese and ducks feed on Palmer amaranth and water hemp, including in areas where the weed has manifested a resistance to glyphosate, and the seed is still viable after passing through the digestive system.

“That might be one way that resistant species could show up here but, that being said, we haven’t seen that yet. It could happen because we’re on a major flyway here,” Bilyea says. He adds that weeds could start to appear on the shorelines of waterways where these geese and ducks are more likely to land, but this is all just theoretical.

But Jason Norsworthy, a leading weed specialist at the University of Arkansas, says rail travel is certainly not theoretical and he has a report, conducted by one of his post-doctorate students a few years ago, which shows rail to be one of the major means of transport for resistant weeds and Palmer amaranth in particular. The challenge – which has been well documented in the States – he says, is that even when farmers know the weed is in their area, they historically do not notice it until it’s too late.

“Generally 20 per cent of a field or population has to succumb to a level of resistance to be noticeable,” he says. “With Palmer amaranth, at 20 per cent, the cat’s out of the bag.”

Norsworthy says farmers would have to get down on their hands and knees to identify the differences between very young redroot pigweed and Palmer amaranth. Misidentifying the two can result in a few hundred thousand more seeds per plant, however.

Norsworthy says this weed’s ability to produce seed dominates others so entirely that, if Palmer amaranth is found in a field, every weed management decision made will be centered on Palmer amaranth.

Soybeans are hit the hardest because atrazine remains effective and is still used widely for corn production, while the winter wheat crop season naturally evades the summer annual. Tillage buries the seed and reduces the likelihood of survival, so no-till systems are at a higher risk of infestation and he says cover crops offer a good physical barrier management option. Still, Norsworthy insists growers think hard about spraying glyphosate on any Palmer amaranth alone.

“I understand commodity prices are tight and times are hard, but if you want to still be using glyphosate five years from now, you’d better start using some residual herbicides,” he says.

In research studies where several collaborators have worked together to find herbicide management strategies that can control the troublesome weed, Norsworthy says they had to move to the new trait technologies in order to achieve 94 per cent control. He said they found that Monsanto’s coming Xtend system and Dow Agrosciences’ Enlist technologies were comparable to one another, while HPPD technologies, if applied in a timely manner, were superior.

If application timing was delayed in the HPPD system from three or four weeks to six or seven weeks, however, these fields were far more difficult to salvage. He says his recommendation for keeping control is to burndown with glyphosate and dicamba four or six weeks before planting, then apply a paraquat plus a residual herbicide at planting, then spray a glyphosate tank mix 21 days later. Waiting six weeks is too long for good post-emergence control, he says.

“Yes, we were able to kill a lot of weeds, but we were never able to get back to the level of weed control that we had when we made those timely applications of a residual followed by a subsequent post application at three to four weeks,” Norsworthy says. “When we got into a salvage situation, it turned into just that, even with the new technologies.”

Both Norsworthy and Bilyea have been actively speaking to farmers about Palmer amaranth throughout the winter, trying to raise awareness so farmers are well prepared to identify the weed in the coming growing season.

Norsworthy says it’s something he believes farmers in Ontario should be at least a little bit afraid of, or else he’s afraid they won’t be motivated enough to protect themselves against it.

No matter what the perceived risk, Bilyea would simply rather see farmers participate in survey initiatives such as the one he has initiated this year. He hopes to use the results in a seminar he’ll be delivering this July as part of the Ontario Ministry of Agriculture, Food, and Rural Affairs’ annual Southwest Crop Diagnostic Days.  


April 5, 2016
By Amy Petherick

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