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Recognize the threat of these invasive weeds

How big a problem can waterhemp and Palmer amaranth really be? Let’s not find out

Although it has yet to be confirmed in Ontario, Palmer amaranth is capable of rapid spread and should be treated as glyphosate resistant.

Has everyone been crying wolf about waterhemp? After all, haven’t we been hearing warnings in southern Ontario about the weed for the past 15 years, and who has lost a bushel?

Well, it was actually 17 years ago — 2002 — when the species was first confirmed in Lambton County, and the good news is that since then the weed has been kept largely under control, limited to just two or three hot spots in the region.

The bad news is that it is starting to spread. And that it may keep spreading if more producers don’t think it worthwhile learning how to distinguish it from its close cousin, redroot pigweed.

If it seemed as though there was more activity on social media concerning common waterhemp in 2018, you were right. Agronomists, crop advisors and growers were issuing frequent updates about waterhemp (Amaranthus rudis), mostly getting the word out about its presence as well as about some of its key differences compared to redroot pigweed (Amaranthus retroflexus) or green pigweed (Amaranthus powelli).

That, says Richard Anderson, is the greatest concern as the 2019 growing season draws nearer. In 2014, Anderson brought the first glyphosate-tolerant waterhemp to the attention of Dr. Peter Sikkema from the University of Guelph-Ridgetown Campus. He can’t say enough about the identification issue being the biggest impediment to limiting waterhemp’s spread.

“Waterhemp is a very problematic weed in terms of how it can impact yield and how growers have approached their weed management practices,” says Anderson, business representative for BASF.

“This is a more difficult weed to control than the species that we’re used to. Growers should be concerned about it, and the key is identification. Scouting for it, knowing where it is, what it looks like, how it moves and what kind of time frame during the year that growers need to be looking for it are the most important components.”

For the record, the visual differences in waterhemp compared to redroot pigweed come down to the presence of hair: redroot pigweed has dense short hairs on the stem but waterhemp has none. Waterhemp leaves have a waxy appearance, and a pointed apex versus duller leaves and a notched apex on redroot pigweed. Its seed heads are also long and narrow and interspaced on the stem, not short and compact as with redroot pigweed.

A hand lens is needed to see the fine hairs on the stem of redroot pigweed (l). By contrast, the stem on common waterhemp is hairless (r). photo: Courtesy of Richard Anderson, BASF Canada

It’s also a greater concern in soybeans than corn. Anderson points out that corn’s competitive nature and its options in terms of available modes of action make it less of an issue. On the other hand, he says, if waterhemp gets an early start and isn’t quickly controlled in corn, it can be just as big an issue as in soybeans.

“In soybeans, some kind of canopy closure is harder to achieve and the chemistry groups are limited in terms of what we can use,” adds Anderson. “For instance, we don’t have a Group 27 for soybeans, and that’s a prominent class in corn. We do have Group 15s and the Group 4s that can work in both.”

Time on our side?

One positive is that waterhemp doesn’t spread as quickly as a species like Canada fleabane with its airborne “parachute-like” seeds. But growers need to be actively aware of how weed seed can and will move about on a grower’s operation: it’s easily vectored by trucks, planters, tractors and combines.

“Learning the slight differences between waterhemp and pigweed — which is probably already in their fields — and understanding what it looks like is most important,” says Anderson, adding that he’s been updating his Twitter feed regarding waterhemp. “The biggest thing I run into when talking to growers is after it’s been identified, they discover that they may have had it for several years.”

Compounding that difficult lesson is how the weed’s march across the U.S. Midwest has unfolded in the past 20 years, where reports of “stacked” resistance in waterhemp first emerged in 1998, namely with triazines and ALS inhibitors.

Colour is not a determining factor in distinguishing common waterhemp from redroot pigweed. These samples are from different areas and soil types. photo: Courtesy of Richard Anderson, BASF Canada

By 2012, several counties in Illinois confirmed resistance to five different modes of action, including triazines (Group 24), ALS inhibitors (Group 2), glyphosate (Group 9), PPO inhibitors (Group 14) and HPPD inhibitors (Group 27). In 2018, Randolph County in Missouri confirmed resistance to six different classes in 16 per cent of a waterhemp stand, adding 2,4-D (Group 4) to the list. Only dicamba (also a Group 4) and glufosinate (Group 10) provided effective control.

“We have the benefit of seeing what’s happened to the south and being able to work with existing chemistries or existing cultural practices, whether it be tillage or cover crops or anything we can to try to slow it down,” says Anderson.

“It’s an opportunity to look at what they’ve learned and to make sure that we are using multiple modes of effective action on these weeds. And trying to use different practices in terms of ground cover with soybeans, getting them up as quickly as possible and getting ground cover to maximize the amount of shade, and try to slow the later-germinating weed like waterhemp — and Palmer amaranth — as well.”

And now… Palmer amaranth

Palmer amaranth is another of the amaranthus species that could affect growers in Ontario in the next few years (photo at top of page). Like waterhemp, Palmer amaranth has created considerable angst in the Deep and Mid-South regions in the U.S., where glyphosate-resistant populations have robbed growers of soybean yields and added considerably to the cost of weed management over the past 10 years.

By 2012, Palmer amaranth had moved into the lower tier of Illinois and researchers, including Dr. Aaron Hager of the University of Illinois, were gathering evidence to determine whether it could thrive in central regions of the state.

Although it was confirmed in one of the southernmost counties of Michigan in 2010, Palmer amaranth hasn’t made any significant advance towards Ontario since then; but again, it may only be a matter of time. Its arrival in Michigan allegedly came in some cotton residue commonly used in cattle feed. Palmer’s prolific seed heads carry as many as 500,000 seeds, although those are often vectored by mechanical means. Since its initial foray into Michigan, it’s also been confirmed in North Dakota and Minnesota in 2017 and South Dakota in 2018.

Steve Johns, agronomic sales representative with Syngenta Canada, concedes he hasn’t had any experience with Palmer amaranth in his region of Ontario, but has spoken with a colleague from Tennessee. They said that in spite of the challenges that waterhemp can create, it’s Palmer amaranth that is the greater concern: once established, it’s very difficult to deal with.

“The one thing we have going for us is that growers here tend to have better rotations — corn, soybeans and wheat — and some are even more complex,” says Johns. “It’s better than growers in the U.S. where it’s continuous corn or those parts of southern Ontario where some grow soybeans year after year.”

Johns echoes Anderson’s statement about waterhemp’s mistaken identity being a problem. Since it’s also an amaranthus species, there’s the common default to assume it’s something familiar — namely, redroot pigweed.

“One of the main problems with waterhemp, and the same is true with Palmer amaranth, is people might be inclined to say, ‘That looks like pigweed,’ so it’s treated like pigweed,” says Johns. “But most growers really don’t want to start spending what they consider extra money to prevent resistance on their land. Most wait until they have a problem and then they’ll deal with it — and that’s human nature.”

Time for a new approach

What is changing — perhaps out of necessity — is the realization that the days of effective weed management from single-pass, single modes of action herbicides are gone. It’s happened with glyphosate in multiple weeds in the U.S. and the more prominent example here is with Canada fleabane. It’s highly unlikely that the chemical industry can develop another inexpensive, effective, one-pass product like glyphosate.

Yet Johns is content with the idea that most farmers know they need some other herbicide in their program. They can pencil out what it’s costing them in terms of weed escapes and impacts on yield.

“I think new herbicides will come but as an industry, we’ve gone through a lengthy period where it’s been tough to invest millions of dollars looking for a new mode of action when glyphosate was so cheap and effective,” he says. “That may be affecting the research and development efforts, but it’s almost easier to put some genetic tolerance into our corn and soybean crops to support modes of action that are going to be really useful.”

Anderson agrees: the change in weed species has driven growers to try more strategic means of addressing new — and existing — weed species. Some are taking a more multi-pronged approach to their fields, adding different modes of action and different timings of applications or different use patterns, along with cover crops or other cultural practices to address new weeds.

“But it’s a good thing to help with the spread of another species, where we’ve actually been able to spread control measures for weeds we currently have,” he adds. “That will help us when we’re trying to employ a system against something like waterhemp.”

Like Johns, he knows there are new products on the way, but he also follows the same line of thinking that working with new tools and combining them with existing means will require a higher premium on management skills. Growers will also have to guard against overusing those new products and allowing the same thing that happened with glyphosate.

“We have to approach it with adding the new tools into our current use patterns to improve what we’re doing,” he says. “We do have new tools, new timings for old chemistries as we get into different genetics. We can use the tools that we’re already comfortable with, and we can add those to the new tools that are coming or that are already here to manage these new problems.”

This article was originally published in the February 2019 issue of the Soybean Guide.

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Ralph Pearce

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