Seed & Chemical
Fighting against resistance
By Carolyn King
The discovery of glyphosate-resistant kochia is a startling reminder that herbicide resistance is a serious concern on the Canadian Prairies and the bordering U.S. states. It also underlines the crucial importance of taking steps to slow the development of resistance.
Kochia is the first glyphosate-resistant weed confirmed on the Canadian Prairies. But kochia is not the only Prairie weed with herbicide resistance, and glyphosate is not the only herbicide that Prairie weed species have overcome. With the increasing spread of herbicide resistance, crop growers could face higher weed control costs and, if they run out of herbicide options, the possibility of yield losses, quality losses and loss of preferred crop options.
“I believe herbicide resistance is a major threat to crop production, not just here but around the world. I think it deserves more attention than it has been getting,” says Dr. Bill Dyer, a professor and researcher specializing in weed physiology at Montana State University (MSU).
Glyphosate-resistant kochia spreading
In the U.S., the first documented case of glyphosate-resistant kochia was in Kansas in 2007. Since then it has been found in nearby states such as South Dakota in 2009, Nebraska in 2011, Montana, North Dakota and Colorado in 2012, and Oklahoma in 2013.
“In North Dakota, we had an inkling there were some glyphosate-resistant kochia populations in 2011. But in 2012, after normal spring moisture, the weather turned dry and hot, and kochia just loves those conditions. We expected kochia would increase, but it exploded,” explains Dr. Rich Zollinger, a professor and extension specialist in weed control at North Dakota State University (NDSU).
“So in the fall, Dr. Kirk Howatt, a weed scientist at NDSU, asked people across the state to collect kochia seed in fields and field borders, and send the samples to him. He germinated the seeds in the lab and tested the plants for resistance to glyphosate. More than half were resistant.”
On the Canadian Prairies, the first cases were found in 2011. “The three original cases were discovered in chemfallow fields in southern Alberta in 2011. By 2012, we had about 50 cases documented in southern Alberta, all the way from the foothills to the Saskatchewan border, and in west central and southwestern Saskatchewan,” says Dr. Hugh Beckie, a research scientist at Agriculture and Agri-Food Canada (AAFC) who specializes in herbicide-resistant plants.
In the fall of 2013 Beckie led surveys for glyphosate-resistant kochia in Saskatchewan and Manitoba. The collected samples will be tested for resistance in the coming months. “We’ll have to see what the survey results tell us, but I expect that we’ll have considerably more than the 50 documented cases that we officially have,” he notes.
Even though glyphosate-resistant kochia was first documented in the U.S., that doesn’t necessarily mean the resistance spread from the U.S. to Canada. Beckie is leading some research to determine the genotypes of the various glyphosate-resistant populations to see if individual populations spread from the same source or arose independently.
Kochia has a history of developing resistance to herbicides in various groups. Herbicide groups are based on their mode of action – the way they attack the plant. For example, Group 4 herbicides disrupt plant cell growth. This group includes herbicides with various active ingredients, such as dicamba, fluroxypyr, MCPA, 2,4-D and others. Group 9 herbicides block amino acid synthesis. This group contains only one active ingredient, glyphosate, which is in products such as Roundup.
When you apply a herbicide to control a weed population, you select for those few individual plants within the population that are able to resist the herbicide’s mode of action. Those survivors go on to produce seeds that carry the genetics to resist that mode of action. If you use herbicides with that same mode of action year after year, eventually the only weeds remaining in the field will be ones that are resistant to the mode of action.
“Farmers need to know the herbicide group so they can rotate modes of action,” emphasizes Dyer. “Just knowing the trade name, like Roundup, or the active ingredient, like glyphosate, doesn’t help.”
Beckie explains that simply switching to a herbicide that has a different active ingredient but is still in the same herbicide group is at best a short-term option. “Usually the weed can quickly develop resistance to other herbicides within the same group.”
In North Dakota, kochia has developed resistance to several groups. “In the United States, 2,4-D, a Group 4 herbicide, was registered in 1945. It was about the only tool farmers had to kill weeds, so everybody used it,” says Zollinger. “Over the next 10 to 25 years, we killed all the wimpy 2,4-D-susceptible kochia. So then we had only 2,4-D-resistant kochia. Next, within about three years of the release of the Group 2 herbicides in the United States [in the mid-1980s], we got Group 2-resistant kochia.”
In the mid-1990s, Zollinger says they found kochia populations resistant to dicamba, another Group 4. “But dicamba-resistant soybean has not [become very widespread], and dicamba will still hurt kochia. We think the hurt from the dicamba and crop competition from wheat have kept that biotype pretty much under control.”
He adds, “When Dr. Howatt tested the 2012 kochia samples for glyphosate resistance, he also tested them with the best kochia herbicide that we know of in small grains. In the United States we call it Starane, and the active ingredient is fluroxypyr. A number of the samples had some level of resistance to fluroxypyr.” (In Canada, fluroxypyr is found in products like Attain.)
Similarly in Montana, kochia populations have developed resistance to herbicides in Groups 2, 4 and 9. The Group 4-resistant types haven’t been as troublesome as the other types, according to Dyer. “That is probably because nobody uses dicamba alone or 2,4-D alone. It’s always mixed with another herbicide. So chances are that the other herbicide would still be effective on the kochia.”
On the Prairies, Group 2-resistant kochia populations started to develop shortly after these herbicides became available, with the first confirmed cases in southern Saskatchewan and southern Manitoba in 1988, and in southern Alberta in 1989. “By 2007, our surveys throughout the Prairies indicated that about 90 per cent of the kochia populations were Group 2-resistant,” says Beckie.
AAFC is also testing glyphosate-resistant kochia samples for resistance to other herbicides. All of the 50 glyphosate-resistant populations are also Group 2-resistant, but none are resistant to dicamba. Beckie notes, “Based on previous research, if it is resistant to dicamba, it’s likely also resistant to fluroxypyr and vice versa. But just to make sure, in our next stage screening, we will screen those 50 samples for fluroxypyr resistance, because it is a herbicide that is often used in Western Canada.”
Born to be wild
Beckie and his colleagues are also watching for glyphosate resistance in other weeds. “Based on our predictive modelling, we think some other weeds such as wild oats, green foxtail and cleavers, in particular, appear to be at high risk of developing glyphosate resistance. So we’re keeping a close eye on any [suspect weed] populations reported by growers or found in our surveys; those are species that we’re paying particular attention to.”
Beckie says two strong indicators that a weed species might be prone to glyphosate resistance are a history of resistance to other herbicide modes of action and a large population size. Kochia is a good example of a weed with both characteristics.
“A single kochia plant produces at least 20,000 seeds. And if conditions favour the weed, it can produce two, three or even four times that number of seeds. It also spreads its seeds easily – the plants blow as tumbleweeds across fields, dropping their seeds for miles in long, winding trails as they bounce and roll,” explains Zollinger.
Dyer adds, “Kochia is also cross-pollinated, meaning that it is mostly pollinated from another plant [rather than being self-pollinated]. Its pollen lives for longer than most pollen, and it can blow a long way. So it can spread its genes for resistance through pollen and through seed.”
As well, Zollinger notes that kochia loves salty soil and dry conditions. Under such conditions, large populations of the weed can develop whereas most field crops struggle.
Serious economic impacts
Herbicide resistance can have significant economic impacts for farmers. Beckie explains that the impacts will depend on the weed and the cost of the alternative herbicides for controlling that weed. “If that weed population has developed resistance to a number of herbicide modes of action, then you may run into a situation where you don’t have the herbicides available to control it,” he says. “Then the cost gets into yield loss and crop quality losses and so forth.”
Dyer gives an example of such a situation in the Fairfield Bench area of Montana. “Almost all of the farmers there grow malt barley every year. It’s irrigated ground so they can crop every year. And there is really nothing else they can grow that has as good an income.” With this cropping system, the growers have very limited herbicide choices, so they’re at high risk of developing resistant weeds.
One of the farmers in this area now has wild oats that are resistant to five different herbicide modes of action. “That farmer had to quit growing malt barley because he cannot kill the wild oats. He put the fields in alfalfa, which is not nearly as high value. Because of [the relatively long] seed dormancy in wild oat, as soon as he takes those fields out of alfalfa and goes back to malt barley, the resistant wild oats will be right back,” says Dyer.
“So resistance can be an extremely serious problem if the weeds are resistant to multiple herbicides and if the farmer is constrained by the kinds of crops he can grow.”
And if you do develop weed populations with multiple resistances, it could be many years before a new mode of resistance comes along to help you kill that weed. “According to the experts, new modes of action will be few and far between. It will certainly not be like it was in the 1960s, 1970s or even the 1980s, because a lot of companies are out of herbicide discovery totally. Those that are still investing in herbicide discovery are having to spend more and more money to find a promising candidate,” says Beckie.
Preventing and managing resistance
According to Zollinger, growers have been spoiled by the low cost of weed control with a glyphosate-based weed control system. “If you have glyphosate-resistant weeds, then it will take two or even three times the money to control that weed than if you were to take some preventive action in the first place,” he notes.
Dyer says the best prevention is to change the selection pressure that is put on weeds every year. That includes key practices like rotating crops and rotating herbicide modes of action.
“Crop rotation is always a good idea for many, many reasons,” he says. “For herbicide resistance, crop rotation allows farmers to use different suites of herbicide action to change the selection pressure year by year. Rotating from a grass crop to a broadleaf crop can help a lot because there are very different modes of action that can be used in the other crop. Of course we always recommend if it’s possible to not use a herbicide, then don’t use one.”
Beckie advises focusing on good agronomy. “Crop health is the number 1 thing farmers can do. Make sure you have a competitive crop, and try to diversify your crop rotation as much as possible.” Along with rotating herbicide groups, Beckie also suggests considering herbicide mixtures with two modes of action.
“Hopefully we’ll have some new tools to manage weeds in the near future; not just herbicides, but other technologies that might be five or 10 years away that may help growers,” he says. For example, during the next three or four years, Beckie will be investigating technologies for capturing or destroying weed seeds at harvest time to minimize the amount of weed seeds that go into the seed bank.
Dyer says regular scouting is also essential to catch herbicide resistance before it escalates into a major problem. “Usually the population of resistant plants gets up to 20 to 25 per cent of the whole field before the farmer takes notice. Then he will start to realize that the problem isn’t just due to a misapplication or a sprayer skip or a plugged nozzle; that it’s probably resistance. As a result, he has already a lot of resistant weeds before he even notices the problem,” he notes.
Dyer recommends scouting about a week after spraying. “Look for a dead weed next to a living weed. The dead weed tells you the herbicide was put on correctly and it did what it was supposed to do. If a metre away, there’s an uninjured weed by itself, that is a pretty good indication the individual weed is resistant.”
If you find weeds that you suspect might be resistant, you can collect some seeds and send them for testing to see if they are indeed resistant.
Zollinger says if you see a weed you suspect might be resistant, the safest course is to treat it as though it is resistant. “If it is resistant and you don’t do anything about it, then you’re in trouble.”
Zollinger also offers some tips for growers who are dealing with glyphosate-resistant kochia. “Our number 1 practice we’re asking our growers to consider is to use a pre-emergence herbicide. There aren’t many labelled in wheat, but many are available for crops like chickpeas, lentils, field peas and soybeans. These herbicides do cost some money, and they do need to be activated by rain, so if you don’t get timely rain, then they don’t work as well. But they are a first line of defence in kochia control.”
He also suggests field perimeter management. “Most of the resistance seems to start on the field perimeter and then travel into the field. If farmers could kill the resistant plants in the field perimeter, then that would take care of a lot of the resistance.” Perimeter management options include using a different herbicide or cultivation around the field edges, or planting a strip of corn around the field edges to create a barrier to stop kochia tumbleweeds from rolling across the field.
So, to slow the development of herbicide-resistant weeds – and their serious economic impacts – change the selection pressure on weeds every year. That includes practices like rotating herbicide groups, rotating crops, using herbicide mixtures with two modes of action, and using cultural control measures. And scout after you spray to look for weeds that might be resistant so you can tackle the problem before it explodes.
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