Herbicide-resistant weeds can have devastating effects on crop production. Experts provide tips on how to reduce selection pressures for resistance.
by Kate Ayers
Herbicide resistance has increased exponentially across the globe over the last four decades.
In fact, “resistance has been found in 263 species worldwide. That covers 23 of the 26 known herbicide sites of action. So, resistance is pretty wide-spread,” explains Dr. Charles Geddes.
Geddes is a weed ecology and cropping systems research scientist at Agriculture and Agri-Food Canada in Lethbridge, Alta.
“Within Canada, about 37 weed species are resistant, evenly split across Eastern and Western Canada. We have found resistance to 12 different sites of action.”
But how does resistance affect Canadian growers?
Herbicide resistance costs producers between $1.1 and $1.5 billion each year because of increased herbicide use and decreased crop yield and quality, a Manage Resistance Now article says.
Fortunately, producers have an arsenal of tools and strategies they can use to reduce the likelihood of resistance on their farms.
This month, Better Farming speaks with weed management specialists, agronomists and a crop research scientist to learn how Ontario cash crop farmers can prolong effective modes of action to control or suppress weed species.
Problematic weeds
The first case of herbicide resistance in Ontario emerged in 1957 when stakeholders found wild carrot resistant to 2,4-D. Field crops in the province were first affected in 1973 when triazine-resistant lamb’s-quarters were discovered in Ripley, a Manage Resistance Now article says.
Glyphosate-resistant weeds later emerged in 2008. Giant ragweed was the first species to join this group in Ontario. Other species now resistant to glyphosate include Canada fleabane, common ragweed and waterhemp.
The most challenging weeds to control last year included lamb’s-quarters, Canada fleabane, giant ragweed and foxtails, a December report by Mike Cowbrough says. He’s OMAFRA’s weed specialist for field crops and is based in Guelph.
Cowbrough surveyed 27 agronomists from across the province to gather information on weed control challenges and successes during last year’s growing season.
The increasing number of multiple herbicide-resistant weeds makes control more challenging for growers.
“There are numerous herbicide-resistant weeds in Ontario. There is Group 1-resistant crabgrass, 10 species that are resistant to Group 2 herbicides, one species resistant to Group 4 herbicides, 10 species resistant to Group 5, and two species resistant to Group 6,” says Dr. Peter Sikkema, a professor of field crop weed management at the University of Guelph’s Ridgetown Campus.
In addition to the four glyphosate-resistant species, farmers also face biotypes of waterhemp that are resistant to four herbicide modes of action.
“There are waterhemp biotypes that are resistant to Group 2, 5, 9 and 14 herbicides. That really decreases the number of effective herbicides to manage those weed biotypes, especially in soybeans,” Sikkema adds.
A surprising finding from last year’s growing season was the difficulty some farmers had controlling common ragweed, Adam Bent says. He’s the eastern territory manager for Nufarm Agriculture and is based in Reaboro.
“This weed, especially in IP soybeans, is widespread and difficult to control. That was quite an eye-opener,” he adds.
The long-term culprits including dandelions, sow thistle and other perennial weeds can be difficult to control as well, Rob Miller says. He’s BASF Canada Inc.’s technical development manager for Eastern Canada.
“Lamb’s-quarters, which can be Group 2- and triazene-resistant, are challenging to control in IP soybeans or dry bean fields, for example. We are somewhat limited by the number of classes of chemistry we can use in those crops. It is important to have a plan in place so that when you have crops such as corn, you can control those weeds by using a residual herbicide at planting as well as in-crop.”
Authority brand herbicides from FMC Corporation offer growers extended control of tough weeds and the chance to combat resistance by introducing Group 14 and Group 15 modes of action into their soybeans.
As the list of herbicide-resistant weeds grows, the list of effective modes of action shortens. This shift can have adverse effects on the productivity and profitability of crop enterprises.
Effects on production
“The biggest issue with herbicide resistance is that there is a greater abundance of weeds that escape herbicide applications. Those weeds end up competing with the crop, resulting in yield losses,” Geddes says. While it is difficult to estimate the percentage of yield loss producers will experience due to resistance in individual fields, farmers can easily calculate the effect it has on their bottom lines.
For example, “glyphosate-resistant weeds increase farmers’ costs of production. Historically, their pre-plant burndown would require one litre of glyphosate of the old formulation or 0.67 L of the new one. This application would cost about $6 per acre,” Sikkema says.
“Glyphosate-resistant Canada fleabane is the most widespread glyphosate-resistant weed in Ontario. The cost of pre-plant burndown has increased from $6 to about $30 per acre in soybeans. So, let’s say the average yield of soybeans is 50 bushels per acre. Forty bushels would cover variable costs such as rent and mortgage payments, seed, herbicide, planting and combining, and 10 bushels are profit. If soybeans sell for $12 per bushel, your profitability has been reduced by at least 20 per cent because now over two bushels must cover the additional cost of the burndown herbicide.
“Farmers in the province can manage herbicide-resistant weeds with current management tools, but it reduces their profitability,” he says.
However, up to this point, herbicide resistance has not had significant impacts on crop production.
“Largely, the course of action over the last 40 years, is that a resistant weed pops up and then a solution comes to solve that problem. It’s mostly been in the form of a jug. A new herbicide or crop-tolerant technology have come around that allows farmers to use a new product. Stakeholders then ride out that solution until a new resistance presents itself,” Cowbrough says.
“People are rewarded for not proactively managing resistance, but eventually this approach will no longer work.”
While few new active ingredients have been launched on the market, new crop technologies offer new management options.
“In the last couple years, we’ve seen new soybean seed technologies such as Corteva’s triple-stack herbicide-tolerant Enlist E3 soybeans and Bayer’s Roundup Ready 2 Xtend and XtendFlex soybeans,” Miller says.
“With those new trait platforms, we can use additional modes of action on soybeans and products including BASF’s Liberty 200 SN herbicide.”
Nufarm recently released Fierce EZ, an upgraded product available to farmers this year.
“This herbicide replaces Fierce WG, which is a dry product. We did some extensive field trials last season in various locations across the province,” Bent says.
Management strategies
Soil health and fertility are incredibly important in the fight against challenging weeds, our experts agree.
Weed management “starts with soil fertility. Make sure fields have baseline levels of nutrients – 20 parts per million (ppm) phosphorus and 120 ppm potassium – so that crops can be vigorous and canopy as quickly as possible,” Cowbrough says.
“Make sure crops come up in advance of the weeds.”
Bent agrees.
“The best thing you can do is grow a good crop to take care of most of your weed control,” he says.
“We have some dynamite herbicides that will give you five to eight weeks of residual control, but that period does not cover the whole growing season like crops do.”
In addition, farmers who keep the ground covered with a growing crop for more months of the year can reduce weed pressures in their fields, Bent notes.
For example, an “effective management tool for control of glyphosate-resistant Canada fleabane is to seed a cover crop after winter wheat harvest,” Sikkema says.
“Wheat comes off in July and if farmers immediately plant a cover crop, it is really interesting how effective a vigorous cover crop can suppress the growth of glyphosate-resistant fleabane. It reduces the density of fleabane in the following corn crop in rotation.”
Miller agrees.
“Mother Nature does not like bare soil. Cover crops can outcompete some of those weeds, decrease weed pressure and they offer soil health benefits,” he says.
The most ideal “cover crop depends on a farmer’s rotation, soil type and overall production practices.” Certified crop advisors can provide recommendations.
For farmers who are starting out with cover crops, “take baby steps and keep expectations low,” Cowbrough says.
“Corn-soybean rotations are more common in Ontario than corn-soybean-wheat. If you grow corn and soybeans, you could seed 60 pounds of oats after soybean harvest. It doesn’t need to be much.
“It just needs to cover the ground and create an inhospitable environment for weed seeds to germinate. Oat is a low-cost cover crop that doesn’t require a lot of complex management and serves its purpose,” he says.
Farmers who have a solid understanding of their herbicide products can reduce the risk of resistance in their fields.
“It’s one thing to say that you are using multiple modes of action, but it’s another to say you are using effective modes of action. It’s not enough to use more than one mode of action in a tank mix, but rather make sure those modes of action are effective on the target weed species,” Geddes says.
“This is an area that can be improved upon. Know which modes of action target which weed species.”
Indeed, farmers should apply multiple modes of action and an effective burndown product to control problematic weeds.
“You need overlapping effective modes of action on all the weed species present. That is the number 1 strategy to achieve three things: Clean fields, high yields and reduce the risk of weeds developing resistance,” Bent says.
And “use a good burndown product in conjunction with the residual product. I see a lot of farmers use a multi-mode residual product, but they need something to knock down the weeds that have already emerged. Residuals will not take them down.”
In addition, follow the label and be informed about the herbicide you apply – know what chemistry classes you use and how they work.
“Make sure you’re using the right rate of herbicides. In the past, farmers considered using lower rates of herbicide because it helped save on herbicide costs. But lower rates are not the recommendation,” Geddes explains.
For example, Aim EC from FMC Corporation has a rate range to allow growers to target their specific weed spectrum. If a grower is concerned about Eastern black nightshade, the 30 millilitre/acre rate is required for control of nightshade greater than five centimetres tall.
“Research has shown that frequent applications of lower rates of herbicide can select for herbicide resistance,” says Geddes.
Also, some herbicides are more effective with different water volumes and droplet sizes.
“Contact herbicides, for example, work better when sprayed with higher water volumes in the middle of the day with a medium or fine droplet size,” Miller says.
Overall, a diversified approach is the most effective and responsible approach to in-field weed management.
Farmers must use an integrated pest management approach, including cultural, mechanical or physical, biological and chemical methods to control weeds.
Cultural approaches include “crop rotation, different row widths, and other practices that help the crop canopy as quickly as possible. These strategies effectively shade out weeds and increase competition,” Miller says.
Mechanical or physical techniques include “physical disturbance of the weed’s life cycle, such as tillage, mowing and using weed seed destructors after harvest,” Geddes says.
Biological controls include living organisms. One example is grazing cattle on a crop field to reduce the presence of weed seeds, he adds.
Ideally, farmers use a combination of all these strategies to develop an integrated weed management plan.
“Solutions will come from integrating non-chemical tools with chemical tools,” Geddes says.
“This approach will reduce selection pressures for resistance and prolong the efficacy of modes of action to maintain viable tools for growers.” BF
Questions to consider when managing Glyphosate-resistant weeds
When growers find glyphosate-resistant weeds in their fields, Dr. Peter Sikkema poses eight questions to identify areas for improvement in their field management strategies. Sikkema is a professor of field crop weed management at the University of Guelph’s Ridgetown Campus.
These questions include:
- Would you consider introducing a diversified crop rotation? I think the number 1 way to address herbicide resistance in Ontario is to introduce more diversity into our crop rotations. Ideally, farmers would have a minimum of three crops in rotation: corn-soybean-winter wheat. If you can add more crops, that is even better.
- Would you consider strategically incorporating alternate or new technologies when they become available? For example, we are very reliant on Roundup Ready corn and soybeans; however, there are LibertyLink corn and soybeans, Enlist corn, Xtend soybeans, E3 soybeans and HT3 soybeans.
- Would you consider applying multiple herbicide modes of action on every acre, every year? Growers can use a two-pass weed control program – spray a soil-applied or pre-plant herbicide prior to crop emergence and then come back with a post-emergence herbicide to control any escapes. Growers can also apply multiple modes of action by using herbicide tank mixes post emergence.
- Would you consider tillage at strategic points in your diversified crop rotation?
- Would you consider planting a cover crop after winter wheat harvest to reduce weed emergence?
- Would you consider making perfect weed control an objective in your diversified crop rotation to minimize weed seed return to the soil?
- Would you consider maintaining detailed weed management records?
- Would you consider developing a multi-year herbicide-resistance management plan for each field on your farm to minimize the repeated use of herbicides with the same modes of action in the future? I think farmers need to look back to see the herbicides they have used and then look forward to develop a five-year management program suited to their crop rotation.
“At the end of the day, we need to diversify integrated weed management programs where we incorporate all the aspects of weed management – cultural through crop rotations, biological through increasing microbial activity in the soil, mechanical through cultivation, and chemical control,” Sikkema says. BF
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