Weed Control in Pulses - PulseResearch
April 24, 2017
New research offers better options to control weeds
Ask most pulse growers in Saskatchewan what some of their biggest challenges are for growing pulses and it is likely you will hear the same response over and over again — weed control.
Unlike cereal and oilseed crops, pulse crops are generally not competitive with weeds and are highly susceptible to yield loss as a result of weed competition.
Furthermore, the development of herbicide resistance in weed populations is a growing concern for growers in Saskatchewan, as resistance has developed rapidly here and in other major pulse growing regions of the world.
Currently there are limited options for herbicides on the market for Saskatchewan growers, and that is not likely to change anytime soon, says Allison Fletcher, Research Project Manager with Saskatchewan Pulse Growers (SPG).
“Ag chemical companies do not often register chemistries solely for pulses, because globally it is a small market, so we end up screening chemistries that have initially been tested on other crops, in the hopes of finding something that is suitable for use on pulses,” she says. “There is not a lot of research done, relatively speaking, in this area by private companies, so we have to step up in that area to fill in the gaps.”
As a result of the lack of control options, the impact of weeds in pulses can be devastating. Research shows that pulse crops, the most susceptible crops to weed interference, commonly suffer yield losses of 20 to 40 per cent, but that number can climb up to 80 per cent in a bad year.
For all these reasons, weed research has long been a priority for SPG not only through research projects but also through the Weed Research Program, which is specifically targeted to developing Saskatchewan-focused, integrated management techniques for weed control. Earlier this year, SPG renewed its commitment to the program with a more than $2 million investment.
Previous investment in weed control research has already yielded some significant developments, Fletcher says. For example, in the past five years the industry has been able to collect a significant amount of data for submission to the Pest Management Regulatory Agency (PMRA) for approval of minor-use herbicides. Researchers have also developed imidazolinone (IMI) tolerant chickpea varieties, and improved the tolerance of field pea to Odyssey®. Furthermore, research has allowed for the reduction of the sulfentrazone (Authority®) re-cropping interval for canola to 12 months and for lentils to 24 months after application.
There have also been small developments in managing cleavers in high organic matter soils by herbicide layering, which refers to the act of combining preseed, short-term soil residual herbicides, with post-emergence, in-crop treatments.
But there is still lots of work left to do. For the next five years SPG-funded weed research will focus on diversifying growers’ options for weed management and moving towards a more integrated approach.
This work will include:
- Developing control and management guidelines for weeds that are problematic right now,
- screening candidate herbicides for use, especially in faba beans and soybeans,
- aiming to understand more about the ecology of the weeds in order to come up with management strategies, and
- developing re-cropping guidelines to be used following application.
Some other newer-to-Saskatchewan alternatives might also be explored, Fletcher says.
Focusing on More Integrated Methods For Weed Control
Managing weeds without herbicides is becoming an increasingly popular aspiration amongst the Western Canadian pulse industry, as levels of herbicide resistance continue to grow.
Globally more than 400 biotypes of resistant weeds have been discovered, and in Canada alone there have been approximately 100 species of resistant weeds confirmed. The majority of these cases have been discovered in the past 30 years, in which time only one new control option has become available.
Furthermore, weed resistance appears to be rapidly increasing, says Dr. Chris Willenborg, Assistant Professor at the University of Saskatchewan’s College of Agriculture and Bioresources.
“We have seen a marked increase in the spread of glyphosate-resistant weeds across the globe,” he says, adding that approximately three new biotypes of glyphosate-resistant weeds are discovered every year. “Based on these trends, I do not believe relying exclusively on herbicides is in our best interest over the long term.”
Dr. Willenborg is currently leading research which aims to determine and develop the most effective management techniques for weeds, including integrated methods used in combination with one another.
“Using multiple tactics, also known as ‘many little hammers,’ creates synergies that result in better weed control than when any single weed control method is utilized alone,” he says. “Combining two tactics that work similarly should yield twice the yield benefits, but when you have synergism, you may end up with two or three times greater weed control and crop yield.”
The methods can include row spacing, seeding rates, and weed seed management, which refers to managing the weed seeds leftover in the fields to ultimately decrease the number of weeds during production. Currently growers do this through the use of dry-down products or machines like the Harrington Seed Destructor, but Dr. Willenborg’s research looks at biocontrol options such as beneficial insects and their role in weed seed predation, to attack the problem at a different level.
“Weed seed predators destroy significant proportions of weed seeds and if we can better understand this process and how best to create environments that favour these processes, we can improve our ability to manage weed seeds and reduce the level of germinating weed seeds we encounter each spring,” he says.
“And the best part is that these predators work for free, since they require the nutrients in the seeds as part of their diets.”
But despite the desire to move away from chemical control options, Dr. Willenborg cautions that a completely chemical-free approach is not possible for most pulse crops. Instead his research will focus on using chemicals in a more judicious, integrated manner instead of as a single solution.
“Much of the work we are doing actually still involves herbicides because this is the major tool on which we depend for weed control, especially in pulse crops. Herbicides are often the most cost-effective and efficacious way to control weeds and thus, they are still integral to any cropping system.”
For example, the idea of a two-pass herbicide system or herbicide layering is an option that shows promise, he says.
“Our previous work has shown excellent weed control if we use both a pre-emergence herbicide followed by a post-emergence herbicide. We are building on that and trying to develop this system for use in pulse crops with hard-to-control weeds like herbicide-resistant kochia and cleavers.”
This technique can also help manage resistance, as applying a pre-emergence herbicide can help reduce the size of the weed population that is exposed to a post-emergence herbicide application, which reduces the selection pressure applied by the post-emergence product.
Overall Dr. Willenborg estimates his research will yield solid information about a more integrated approach to weed control that growers may start putting to use in the next 5 to 10 years. In the meantime, he recommends growers stick to good agronomy and proper timing of weed control tactics, and perhaps consider combining their current approaches to find an integrated system of their own. There are also some mechanical options for weed control currently available, such as post-emergent harrowing, weed wicking, inter-row tillage, and rotary hoes, but these are not always as cost-effective as herbicides.
Glyphosate, first introduced in Canada in 1974, is the most widely used herbicide in the world and is a key herbicide for weed control in pulses in Western Canada, as well as for a number of other crops. To give an idea of the scope, glyphosate usage in Western Canada surpasses that of the next top 12 herbicides combined.
Therefore it was only a matter of time before weeds in Western Canada developed resistance to this popular herbicide. In 2011, a case of glyphosateresistant kochia was confirmed in Alberta, and by the next year it had spread to Saskatchewan.
Once the resistant weeds start spreading, it is important to be able to monitor their distribution and abundance, as this type of information is critical in developing and adopting prevention and control measures to reduce the evolution and spread of the resistant biotype. This is why SPG invested in research led by Dr. Hugh Beckie, a weed scientist at Agriculture and Agri-Food Canada, which aimed to determine the presence of glyphosate-resistant kochia in the province.
Dr. Beckie’s research surveyed crops in southern and central Saskatchewan, as well as kochia plants from a minimum of 300 populations, to determine the level of glyphosate resistance.
The takeaway from this research was that proper herbicide rotation and mixtures are key to avoid selecting for resistant weeds in such systems, Dr. Beckie says.
“This research reinforces the importance of judicious use of glyphosate in crop and non-crop situations, as well as the potential for glyphosate resistance to evolve in weeds other than kochia,” he says. “Tank mixing glyphosate with another herbicide mode of action, either pre-emergence or post-emergence, is important to delay glyphosate resistance.”
For the next phase of research, Dr. Beckie is currently investigating all the aspects of glyphosate resistance in kochia, from pollen and seed dispersal to best management practices. This work will include a new round of field surveys, to determine the occurrence and spread of glyphosate-resistant kochia, which will take place in Alberta in 2017, Manitoba in 2018, and Saskatchewan in 2019.
Going forward there will undoubtedly be new resistant weeds that develop in Western Canada, Dr. Beckie says, with high risk especially for wild oat, green foxtail, cleavers, and wild buckwheat. “Glyphosate-resistant weeds are continuing to evolve rapidly in other parts of the world, so it is unrealistic to think we are immune to this phenomenon.”
What we can do, however, is aim to stay on top of these new developments by better understanding the weeds as they evolve.
Since the 1970s, weed surveys have regularly been conducted in each of the Prairie provinces in order to understand which weeds are growing where, and to document changes between surveys. This data is ultimately used by researchers, industry members, and extension groups to develop weed management recommendations for growers and to set research and education priorities.
However, the last weed survey in Saskatchewan was conducted 10 years ago, which is why SPG invested in a new one in 2014/15, led by Julia Leeson, Weed Monitoring Biologist with Agriculture and Agri-Food Canada.
“Quantitative field surveys of weed populations are used to reveal the current size, extent, and order of importance of species,” she says. “Tracking the increase or decrease in weed populations and the changes in the composition and structure of weed communities indicates the extent by which various weeds are spreading or being controlled and thus the effectiveness of weed management programs.”
The results of this survey will be combined with results from a farm management questionnaire survey, which will provide detailed information on what growers are doing to produce a crop. This information can also help identify particular weed management practices that are important determinants of distinctive weed communities, Leeson says.
“Predicting shifts in weed populations and communities that might occur because of anticipated changes in agronomic practices, weed control management, and agricultural policy will allow agricultural agencies to develop weed management strategies that meet the future needs of growers.”
The results of Leeson’s 2014/15 survey so far show that the number one weed in lentils and peas is volunteer canola, and of the species currently in the top 20, narrow-leaved hawk’s-beard, spiny annual sow-thistle, and shepherd’s purse have increased in rank in both crops. Perennial sow-thistle and round-leaved mallow also increased in rank in lentils, while wild mustard and barnyard grass species increased in peas. Black medick and false cleavers also are currently ranked amongst the top 20 weeds in peas, and have increased in both crops.
“Most of the other species found in the top 20 have not changed much in rank since 2003 including green foxtail, wild oats, wild buckwheat, stinkweed, Canada thistle, kochia, and dandelion,” Leeson says.
Other completed and ongoing SPG-funded research in the area of weed control includes looking at combinations of chemicals and other management techniques for better control of herbicide-resistant broadleaf weeds in pulses, developing measurement tools for Group 1 and 2 herbicide resistance in weeds, reducing weed seed production in herbicide resistant weeds, and more.
For a listing of ongoing research in this area, download the magazine.