The use of Group 2 herbicides for controlling broadleaf weeds in pulse crops has been widespread across the Prairies primarily due to the limited number of in-crop herbicides registered for use on pulses. As a result of the pressure placed on the weed spectrum by this reliance on Group 2 herbicides, weeds have developed resistance to this group at a faster rate than others, says Ken Sapsford, a research assistant in the department of plant sciences at the University of Saskatchewan’s College of Agriculture and Bioresources. Group 2-resistant weeds, in particular kochia, cleavers and wild mustard, are becoming a significant issue for Prairie farmers.
“What we’ve done by going to the very good but one mode of action is we’ve allowed weeds to mutate that much quicker,” says Sapsford. Compounding the problem is a steady decline in the discovery of new modes of action since the 90s. “We’re not seeing new products or modes of action coming into the market anymore… A new silver bullet isn’t coming down the pipe. We must work with what we have and the only way to keep those herbicides working effectively is by using multiple modes of action,” he says.
For the past few years, Sapsford and fellow researcher Eric Johnson, a weed biologist at Agriculture and Agri-Food Canada’s Scott Research Farm, have been evaluating different modes of action on herbicide-resistant weeds in pulse crops.
From those studies, Sapsford and Johnson have discovered a number of options for growers to prevent, manage and control the most problematic Group 2 herbicide-resistant weeds in pea and lentil crops.
Group 2-resistant cleavers have become a significant issue for producers growing peas in the black soil zone. Help is at hand, says Sapsford, in the form of the Group 14 herbicide Authority, which provides excellent control of Group 2-resistant cleavers in soils with less than six per cent organic matter, although it is not yet registered to control cleavers.
When applied to soils with higher organic matter content — greater than six per cent — in the black soil zone, Authority provides around 50 per cent control of cleavers at the recommended rate as it becomes tied up with the organic matter, says Sapsford.
Viper herbicide, a combination of Basagran (Group 6) and Solo (Group 2), has cleavers listed under suppression on the label and, depending on the year, says Sapsford, “does a very good job to an average job on cleavers.”
The answer for producers growing peas in the black soil zone with higher organic matter levels may be a combination of Authority followed by Viper. Studies indicate applying Authority in the spring, which controls about 50 per cent of the cleavers, followed by Viper in-crop, which provides suppression, should bring growers back into the control level, says Sapsford. “You have three modes of action working to control your broadleaf weeds. If we had started with that we probably wouldn’t have near the resistant weed populations we do now,” he says.
Another option to suppress cleavers in peas is to apply Edge, a Group 3 herbicide, along with heavy harrowing in the fall, says Johnson.
He also suggests increasing seeding rates as part of an integrated weed management plan for improved weed control. “For peas, we like to see 80 plants per square metre or higher, which will help by making the crop more competitive,” says Johnson.
Putting the kibosh on kochia
Group 2-resistant kochia continues to challenge farmers growing lentils, peas and chickpeas in brown and dark brown soil zones.
Authority is very effective for controlling Group 2-resistant kochia in pea and chickpea fields. However, controlling the weed in lentils is another matter. “At the present time, we do not have an in-crop control for Group 2-resistant kochia in lentils,” says Sapsford. He also warns Authority should not be applied to any lentil varieties because it will cause injury. Although the application of Authority on lentils may become reality in the future as work on the development of tolerant varieties is ongoing at the Crop Development Centre in Saskatoon, Sask.
The application of Edge in the fall to suppress kochia in lentils continues to be one control option, says Johnson. In addition, integrated weed management strategies, such as higher seeding rates, is another way growers can improve in-season weed control. Rate recommendations will be forthcoming, says Johnson, once studies are completed. For now, he says seeding at least 130 plants per square metre may improve weed control in lentils.
Research on Cadet, a Group 14 post-emergent herbicide, for control of Group 2-resistant kochia in lentils, and even peas, is promising, says Johnson. “We’re looking at it as, perhaps, a tank mix partner with Solo,” he says. Although yield is not affected when Cadet is applied to lentils in-crop, some crop injury, particularly to the leaves, occurs. Determination of formulation, rates and timing is ongoing, says Johnson, and the development of Cadet-tolerant lentil varieties could be on the horizon.
Wrestling wild mustard
Group 2-resistant wild mustard is proving troublesome for producers growing peas, chickpeas and lentils in brown and dark brown soil zones. Lentil growers, in particular, located near the Rosetown area of southern Saskatchewan are finding high populations of Group 2-resistant wild mustard in their fields a major challenge, says Johnson, also noting that control options are limited.
One of the few in-crop herbicides available to growers to control Group 2-resistant wild mustard in lentils is Sencor (Group 5). However, growers often express concern about the injury to their crops from its application, says Johnson. He recommends split applications at reduced rates to mitigate herbicide injury. Although still in the early stages, research to improve the tolerance of lentils to Sencor is ongoing, he adds.
A Group 2 herbicide, such as Solo or Odyssey, should also be applied, notes Sapsford, because Sencor will not control other broadleaf weeds present in the field, such as wild buckwheat. He also warns Sencor cannot be mixed with Solo or Odyssey because it will cause the Sencor to heat up, causing more injury to the lentils.
Once again, increasing lentil seeding rates to a minimum of 130 plants per square metre may help curb wild mustard populations, says Johnson.
It is also important to note that Authority may work well on Group 2-resistant kochia in peas and chickpeas, but it does not control wild mustard in these crops, says Sapsford.
A word on wild oats
Group 2-resistant cleavers, kochia and wild mustard may be the major broadleaf weed issues affecting pulse crops today. However, examination of major herbicide-resistant weeds would not be complete without mentioning Group 1- and/or Group 2-resistant wild oats. Producers applying Group 2 herbicides, such as Solo or Odyssey, must mix in a Group 1 product if Group 2-resistant wild oats are present in the field, warns Sapsford. In addition, at least two modes of action should be targeting each weed species.
“As a producer you must be aware of what weeds each mode of action is targeting so that you ensure you’ve got multiple modes of action hitting specific weeds,” says Sapsford. “It doesn’t help if you mix a wild oat herbicide with a broadleaf herbicide — you have two modes of action, but they’re not hitting the same weeds. It’s like one mode of action on each weed species.”
Researchers are close to adding another mode of action to control wild oats. Pyroxasulfone, a soil-applied Group 15 herbicide, inhibits germination by working on weed shoots. Peas are tolerant to the herbicide while lentils exhibit some tolerance. In addition, pyroxasulfone shows some broadleaf weed activity, controls cleavers in soils with lower organic matter content and controls Japanese and downy bromegrass. Pyroxasulfone is already registered for use on some crops in Eastern Canada and could be registered in cereals within the next year or two in the West with pulses to follow, says Johnson.
Changing it up: revisited
At one point, says Sapsford, researchers thought using one mode of action one year and another the next was a good resistance management practice. Opinions have since changed. “That was better than doing nothing at all, but it’s not as good as using multiple modes of action within the same year,” says Sapsford. “For a weed to develop resistance to one mode of action is much easier than developing resistance to two or three modes of action that same year.”
There are no new modes of action coming down the pipe in the near future. However, careful consideration of all options at hand will aid in the prevention or management of herbicide-resistant weeds.
“We can easily become creatures of habit, and we continue to use what has worked in the past until it quits working,” says Johnson. “I think the best thing producers can do is discuss their weed control programs with their agronomists or their retailers and (consider) options for dealing with their weed issues and what they can do to delay the onset of resistance.” †
Kari Belanger is a freelance writer writing from Winnipeg, Man.