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Are weeds growing resistant or shifting?

The notion of weed shifts versus resistance development is part of the push behind the concept of weed management versus weed control.

Editor’s note: Since the time this article was submitted, extension personnel with the Ontario Ministry of Agriculture, Food and Rural Affairs and researchers with the University of Guelph confirmed the existence of a glyphosate-resistant giant ragweed plant on a farm in Ontario. Top Crop Manager will be examining this issue in greater depth in coming editions.  RP

The appearance of glyphosate-resistant weeds in the United States has put more focus on the need to manage the use of the popular herbicide. With the wide adoption of technology like Roundup Ready soybeans and corn, it was only a matter of time before some weed species developed resistance. In particular, there are populations of Canada fleabane and common ragweed in the US that are resistant to glyphosate. Meanwhile, there are other species that are either suspected of being resistant or have increased their tolerance to common rates. So far Ontario has yet to confirm a resistant species. While that may be good news, farmers are still being urged to keep an eye on glyphosate management.

For Pat Lynch, agronomist for Cargill AgHorizons, the larger issue is “weed shifts” as opposed to resistance. To illustrate his point, he relates the story of atrazine, a herbicide that was used widely in corn production in the 1960s. “When I was growing up on the farm, I can remember how excited we were to go from 2, 4-D, when you had a bunch of weeds left, to atrazine. The corn fields were clean with atrazine and corn oil. A gallon of corn oil and a pound of atrazine, and it was just like walking through the living room – not a weed left.”At first, one pound per acre of atrazine was effective. This rate controlled many broadleaf weeds. But Lynch says continued use of this practice led to a shift in the weed populations from broadleaf weeds to foxtail. This weed was then controlled by higher rates of atrazine. “As growers continued to use atrazine on many acres something else occurred. There were populations of lamb’s quarters that were mainly susceptible to atrazine but there were also small numbers that were resistant to atrazine. With continued use of atrazine that controlled the other weeds, the populations of triazine-resistant lamb’s quarters grew,” says Lynch. “Eventually these weeds grew to the point that they significantly reduced yield. This shift to a resistant population of lamb’s quarters on many fields meant that growers had to switch to other herbicides to control lamb’s quarters.”

Other weeds, other shifts

According to Mike Cowbrough, tank-mixing is more a matter of lengthening the control window and less about preventing resistance.

Lynch adds that another weed shift is underway as a result of continuous glyphosate usage and the decrease of other herbicides. Weeds like bindweed and perennial sow thistle are becoming a bigger problem. He explains that these species have not yet emerged or have not advanced enough to be controlled when glyphosate is applied in a Roundup Ready crop. However, they grow and produce seeds after the application. “If people believe that we can use glyphosate in corn and have clean fields, and use nothing else, they are absolutely deluding themselves,” warns Lynch. “It’s not that weeds are vengeful, but weeds are opportunistic. If there is no herbicide there, they will multiply.”

In addition to the weed shifts, Lynch says other weeds like lamb’s quarters and its “cousin”, spreading atriplex, are becoming more tolerant to glyphosate, and therefore, are becoming harder to kill. “To me, in the next one to five years, weed shifts that escape glyphosate represent a much bigger issue than resistant weeds,” notes Lynch. He is not as concerned about the glyphosate-resistant species in the US such as Canada fleabane and common ragweed because other herbicides can be used to control them. “Weed shifts like bindweed and perennial sow thistle are taking yield away right now,” adds Lynch.

As glyphosate has become the weed control method of choice for the lion’s share of the soybean crop and is gaining in popularity for corn, growers have been advised to manage their use of the herbicide to minimize the risk of resistant weeds. Some of the options have included rotating herbicides with different modes of action and tank-mixing glyphosate with residual herbicides where appropriate. These methods are not only being monitored in North America, they are also being scrutinized in other major growing regions. Dr. François Tardif, associate professor, Department of Plant Agriculture at the University of Guelph, says Australian researchers have developed computer models to gauge the effectiveness of these practices.

“The main conclusions were that mixtures are much better than rotation. If you go the rotation route, you’re just delaying resistance. You’re not keeping resistance in check, as opposed to going with mixtures if they’re done properly.”

In other words, Tardif says rotating different modes of action is putting off the inevitable. Instead of experiencing weed resistance in five years, Tardif says it may occur in 10 years. That said, Tardif says tank-mixing is not an easy solution either. “Let’s say you’re worried about pigweed. You put two or three herbicides on the weed, but you also want to control the other areas of the field. It can get a bit messy because you risk compromising the efficacy of the herbicide in the tank,” says Tardif. He concedes that the situation is rare, but adds that it can happen.
Tank-mixing a tricky option
Mike Cowbrough, weed management specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, also questions the soundness of the tank-mixing route. His reasoning is that the targeted weeds are the ones that have already emerged. “The second mode of action may or may not have the same spectrum of weeds as glyphosate. And most, inevitably, won’t,” explains Cowbrough. “Tank-mixing with glyphosate for resistance management purposes would be effective only if the tank-mix partner had the exact same spectrum of weeds that glyphosate has, and it had residual activity. And part of the challenge is that if you look at a lot of the tank mix partners, they’re not going to control as many species as glyphosate.” Cowbrough notes that tank-mixing is better than just applying glyphosate. But he adds that people are not likely to use tank-mixes to prevent resistance. “You do it because you want to lengthen the window of control or deal with difficult-to-control species.”

Another concept that is receiving more attention is to adopt a pre-emerge program with a soil-applied herbicide. The next step is to come back, post-emerge, with glyphosate. Cowbrough says this method has two positive aspects. “It eliminates the urgency of coming in with that first application of glyphosate because you have something down. But then secondly, after using a broad spectrum herbicide, you’re then putting one or two different modes of action down. You’re controlling weeds at a different time.”

He says the concept is one of a few factors that have helped Ontario manage the risk of developing glyphosate-resistant weeds.

The “pre/post-emergence” strategy has received a higher level of acceptance in corn production. This, in turn, has opened the door for IP soybeans, explains Cowbrough. “So in some cases you see growers using glyphosate-tolerant corn and then going back to IP soybeans. I think any time you eliminate in-crop application of glyphosate every year, that’s good.” Furthermore, he says that putting wheat into the rotation has also gone a long way toward breaking the cycle of using glyphosate every year.

Lynch agrees with Cowbrough’s assessment of using a pre-emerge, soil residual herbicide. But he also notes that it is not being widely used for growing glyphosate-tolerant soybeans. “A lot of corn growers are doing that in corn, but they’re not using a residual product in Roundup Ready soybeans. Growers are using a reduced rate of something like Primextra, Frontier or Prowl in their Roundup Ready corn, and then coming back post-emerge with glyphosate before the crop closes in.”

Lynch suspects the reason the soil residual herbicide program has not taken off in soybeans is that weed shifts appear to be less of an issue when compared with corn. He also points out that scouting and records should also be part of the solution. “Scout the fields, not with the combine, but in-season, and record where the perennial sow thistle is, where the bindweed is and then address those two perennial weeds in your crop rotation."