The march of glyphosate-resistant Canada fleabane through Ontario has been nothing short of striking, going from Essex County in 2010 to the Ottawa Valley by 2015. And now other resistant weeds are also spreading.
In the U.S., waterhemp and Palmer amaranth are the new “dynamic duo” with their own resistance movement.
It’s to the point now where researchers and specialists in the U.S. are sounding the alarm on herbicide resistance, referring to it as a “tipping point” and forecasting the possibility that for some weeds, we may not have any adequate chemical options at all.
In a bulletin circulated last October, University of Illinois ecologist Dr. Adam Davis noted that Palmer amaranth and waterhemp have gone beyond mere “hard-to-control” weeds. Palmer amaranth has seen a rapid migration from the Deep South and mid-south states into the U.S. Midwest similar to Canada fleabane’s through Ontario. And waterhemp in east central Illinois is now resistant to five of the six modes of action common to corn-soybean rotations.
It begs the question: Are farmers in Canada looking at a similar kind of “tipping point?”
Dr. François Tardif doesn’t believe we are, and he maintains that what we’re seeing in the U.S. is a small action and a strong reaction. He concedes there are concerns about resistance and the lack of new modes of action, but the bulk of weed control is still based on herbicides.
“Even if you have four out of five resistant modes on your weeds, you can still spray the remaining mode of action,” says Tardif, an associate professor in the department of plant agriculture at the University of Guelph. “If your weeds show resistance to all of the available modes of action, then that would be the tipping point. We’ll reach that level when a significant proportion of fields will be uncontrollable and farmers will rely more on mechanical control, cover crops and weed-seed destruction as the main methods of management, not just as supplementation to herbicides.”
Asked whether Canadian farmers are protected by the tendency towards longer rotations, Rob Miller believes growers are more aware of weed challenges in their crops. They also lean towards greater diversity with their rotations.
“While we’ve been dealing with weed resistance for a number of years, the increase in glyphosate-, Group 2- and triazine-resistant weeds over the last few years has brought more attention to this challenging issue,” says Miller, technical development manager for BASF in Eastern Canada. “Growers are starting to look at ways to take an integrated pest management (IPM) approach to tackling some of these weed challenges.”
It’s not that we’re ahead of the learning curve relative to the U.S., nor are we “protected,” notes Miller. We’re just at a different point using different strategies. He believes more growers here are incorporating biological and cultural methods with chemical means, creating greater diversity in the soil. They might lengthen their crop rotations, and by doing so change the weed spectrum and perhaps create a more competitive environment.
“As we introduced more conservation tillage and more no till, we actually started to see some weed shifts to more perennials, and we started to see more weeds like dandelion start to show up,” says Miller. “That’s when growers had to change their mindset… that pest always seems to adapt.”
It’s by no means a new challenge, but with tighter margins and increasing costs, one of the larger hurdles across the agri-food sector is convincing growers to spend more money on their weed management strategies. Agronomic sales representative with Syngenta Canada Steve Johns says, human nature being what it is, growers are more likely to commit to dealing with the problem only after it arrives on their farm, and he points to the situation with Canada fleabane as a prime example.
About 20 years ago, without new modes of action coming to the market, chemical companies determined the best course of action was to develop tolerance or resistance to existing chemistries and put them into crops. Roundup Ready and LibertyLink technologies were the first arrivals, and Xtend and Enlist are the latest to enter the marketplace. Next, growers might see a stacking of various chemical resistance factors, like glyphosate with glufosinate and mesotrione.
“All of those things will be stop-gaps for a while, and overall, we can usually find a way to manage things,” says Johns. “It might take a totally new technology — like RNA interference (RNAi) and adding that to glyphosate, so waterhemp that was affecting its enzyme system can no longer do that, allowing the glyphosate to work.”
The arrival of those stacked chemistries should have been the first step in the evolution of tolerances and resistances, according to Tardif. Models show that three-way mixes are better at preventing resistance than rotation or alternation of chemistries.
“We should have had three-way stacked crops from day one — for example, glyphosate, glufosinate and auxinics together, or other combinations of traits,” says Tardif. “But historically and commercially, that was impossible. Once you spend millions of dollars on developing a new technology, you want to recoup your investment as quickly as possible.”
In the University of Illinois document, it suggested one way to reduce the reliance on glyphosate would be to ramp up its price, saying “herbicide susceptibility in weeds should have been viewed as a finite resource all along, like the global oil supply. As resources start to dwindle, prices should theoretically go up.”
In the case of herbicides, prices in the past 30 years have decreased. Yet Tardif quickly dismisses the analogy citing the many subtle differences among herbicides. There are different formulations and ionic variations, but oil will always be oil.
“With herbicides, glyphosate is not dicamba, is not sethoxydim and is not mesotrione,” Tardif says. “They differ in their efficacy, spectrum, residual and cost of manufacturing. A herbicide may be very good on the market — good weed control, good crop safety — but then may be too expensive to manufacture, and that may take it out of the market.”
Another way of lengthening the life of a technology is to shift the mindset on the use of residuals. Both Johns and Miller believe this is happening.
In a way, says Johns, the industry was fortunate that when Roundup Ready corn came to market, many growers continued to use residual products in addition to glyphosate on their crop to maximize yield and profits.
“We really tried to get that message out to not let your crop see weeds, to use residual chemistry, and overall, a lot of farmers continued to use residual chemistry in Roundup Ready corn,” says Johns.
Johns maintains that most growers understand the connection between the continued need for residual chemistries and any long-term use of stacked herbicides, such as glyphosate with dicamba or with 2,4-D.
Changes in cultural practices are also on the list of action items from Miller’s perspective. Longer rotations provide different application timings for different crops. As well, renewed interest in cover crops may help in the resistance fight as well. The addition of cover crops — after wheat or soybeans and inter-seeded into corn — helps create competition in a beneficial way. Best of all, they’re an effective part of IPM strategies.
“The use of chemistry is a vital tool in the tool box,” says Miller. “But it’s best used as part of an IPM strategy.”
This article was originally published in the January 2018 issue of the Corn Guide.