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The latest on waterhemp

Incredibly, the weed is already resistant to four modes of action in Ontario, with more in danger

In U.S. farm media circles, it seems that every week there’s another report, another bulletin, another research paper about waterhemp. Often it’s a confirmation of another find in a midwest county or state. More recently, reports indicate the weed is resistant to seven different modes of action with the latest — Group 15 herbicides — officially announced early in 2019.

Waterhemp has been an unwelcome find in farmers’ fields in Ontario, first discovered in 2002 and largely confined to two areas of Lambton and Essex counties. But it’s managed to spread to other parts of the province, thanks to two factors that make it harder to manage than most other species. For one, waterhemp is capable of germinating throughout the growing season, unlike an early-season flush of lamb’s quarters. It’s also harder to distinguish from its amaranth cousins, redroot and green pigweed, at least without the use of a hand lens. (The stem of a waterhemp plant is hairless, lacking the dense short hairs found on the stem of redroot pigweed.) As a result, farmers often dismiss waterhemp as one of the other pigweed species.

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In 2014, glyphosate-resistant waterhemp was first confirmed in Ontario and since then Dr. Peter Sikkema, along with other weed scientists, has been conducting research on the weed and its mechanism of resistance. In that time, it has spread from Lambton and Essex to Kent, Middlesex, Niagara, Wentworth and Huron. Resistant biotypes have been found in Quebec as well.

“The latest thing we’ve found is that some populations in Ontario are also resistant to the Group 14 herbicides,” says Sikkema, professor of field crop weed management at the University of Guelph’s Ridgetown Campus. “Work done by Dr. Pat Tranel at the University of Illinois indicates there is resistance to the Group 14 herbicides in some fields in Ontario.”

That means growers now face waterhemp biotypes that can be resistant to four different modes of action: Group 2 (ALS inhibitors), Group 5 (photosystem II inhibitors), Group 9 (glyphosate) and Group 14 (PPO inhibitors). Sikkema says almost all of the waterhemp in the province is resistant to Group 2 herbicides, making its management more challenging. That means Pursuit, Pinnacle, Classic, First-Rate and Broadstrike will not work in soybeans, and Broadstrike will not work in corn, nor will Permit or Peak. That takes out a lot of weed management tools.

It’s easy to see how waterhemp can be overlooked in a soybean field.
photo: Courtesy Richard Anderson, BASF

To be clear, when dealing with waterhemp in some fields, Sikkema stresses the word “some.” It’s not that every field with waterhemp is resistant to Group 14 products, but in some fields, four-way resistance has been confirmed, making waterhemp control very complex. In others, it might be resistant to Group 2 and 9 herbicides, and others to Group 2, 5 and 9 herbicides. Interestingly, within Group 5 herbicides in some fields, waterhemp is resistant to both atrazine and metribuzin, while in others, it is controlled by Sencor and resistant to atrazine.

Identification starts with location

An unpleasant fact with waterhemp, beyond the increasing complexity of its on-farm management, is that researchers can only report fields where they’ve collected seed. Weed identification and testing resistance in various biotypes is part of what they do, but if they’re not contacted about a weed in a field of soybeans that just won’t die, there’s very little chance they’ll find out about it.

“We’re playing catch-up all the time,” says Sikkema, conceding the difficulty in identification and the everyday workload that challenges farmers, retailers and advisors. “If a farmer, ag retailer or crop consultant doesn’t contact us, we’re never going to find it.”

Identification definitely tops the list of management practices in dealing with waterhemp, and Richard Anderson says the industry is doing its part in helping growers as well as retailers and advisors boost their awareness of waterhemp. Understanding some of the weed’s biology and timing also helps with management plans.

In spite of its ability to germinate throughout the growing season, waterhemp doesn’t overwinter like Canada fleabane. It also emerges later in the spring, usually after fleabane and giant ragweed, which is why advance planning, including the use of residuals is so important.

Waterhemp has hairless stems but it’s hard to distinguish without a hand lens.
photo: Courtesy Richard Anderson, BASF

“We miss it with our burndown because it’s at the time of burndown that fleabane, giant ragweed and other weeds are there,” says Anderson, business representative for BASF. “Waterhemp comes after and that’s why good residuals are so important.”

If there was one benefit of the late spring in 2019, it was that many burndown applications actually controlled some of the waterhemp, since those emerged a little later in the growing season. It’s usually from August 1 through the middle of the month that Anderson gets most of his calls, where waterhemp begins to appear above the soybean canopy.

As vital as proper identification is to finding waterhemp in a field, crop planning is the next urgent step, and the longer the outlook, the better. Planning two to three years out and co-ordinating crops, modes of action and chemistries is best, says Anderson. A multiple-layered, multiple-mode-of-action approach in each crop provides the best chance to manage waterhemp.

Still management options available

In spite of the loss of many products in the fight against waterhemp, Anderson echoes Sikkema’s reminders about dealing with other resistant weed species: there are still options for the control of resistant bio-types in Ontario.

“That’s the biggest factor with any resistant weed, whether it’s waterhemp or Canada fleabane or giant ragweed,” says Anderson. “There are different modes of action that can be applied at different time periods during the corn or soybean growing cycle that give us an option to control it. We just want to make sure that when you do have resistant weeds, you’re using every option to control those weeds.”

It’s also important to understand that waterhemp is not a fast-moving species, like Canada fleabane. Anderson says it won’t spread throughout a farm in one year, which means a grower has the chance to manage and plan, including minimizing its mechanical spread. Again, its ability to germinate throughout the growing season means a combine can be just as effective as a planter in moving seed from field to field.

“Once you know you have it, you want to create a plan, and a strong plan with multiple-layered residuals and also crop rotations,” Anderson says. “You want to make sure you have different modes of action in different crops to attack that weed. Don’t overuse whatever we do have that works.”

By the numbers

In research terms, five years since the confirmation of glyphosate resistance is a relatively short period of time, yet Sikkema has investigated several aspects of waterhemp, besides its resistance to different modes of action. For instance, average soybean yield loss due to waterhemp interference is 43 per cent and in corn, it’s 17 per cent.

“That’s less than giant ragweed or Canada fleabane,” notes Sikkema, adding that it’s a function of the relative time of weed and crop emergence. “If you’re clean on June 15 with early-germinating weeds, in a lot of cases you’re clean until harvest. But that is not the case with waterhemp where you get your first flush shortly after your last tillage operation in the spring or your burndown application in no till. This weed just never stops coming up. If you have a rain event on September 10, you’ll have a flush of waterhemp on September 17.”

“The other thing is that after conducting more than 200 field experiments, 40 per year for the last five years, I think we have solutions for Ontario farmers,” says Sikkema. “I believe multiple-resistant waterhemp can be managed in corn and soybean, but it’s going to cost farmers more money to manage this resistant weed biotype, and the presence of these herbicide-resistant weeds does reduce farm profitability.”

There’s no doubt, Sikkema adds, that as growers move from single mode-of-action resistance to biotypes with two, three or now four modes of action, their weed management is growing increasingly complex.

The answer, says Sikkema, comes down to diversity, be it in crop rotations, herbicide modes of action, the use of cover crops — even diversity in tillage practices.

Says Sikkema: “If farmers could somehow plan a crop management program that goes three to five years into the future and plan for diverse crop management systems, it’d help in terms of reducing the evolution of multiple-resistant weed biotypes.”

This article originally appeared in the October 2019 issue of the Soybean Guide.

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