Wild oats are the most economically detrimental weed species in the Canadian Prairies, according to Agriculture and Agri-Food Canada. It takes very few wild oat plants to cause a significant reduction in the yield of wheat or cultivated oat fields. Wild oat’s delayed germination makes it’s extremely competitive and difficult to control.

60% of wild oats across the Canadian Prairies display herbicide resistance. Leaving these plants uncontrolled means the potential for increasing their lifespan on the land they are growing and decreasing the crop’s yield and profits.

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Group 1-resistant wild oats have developed a target-site mutation. Group 1 herbicides work by attaching themselves to the acetyl-CoA carboxylase enzyme in the weed and inhibiting cell growth. Group 1 resistant wild oats have mutated that targeted enzyme, so ACCase inhibiting herbicides can no longer attach there, making them ineffective.

Wild oats were the first weeds noticed in Western Canada to have resistance to Group 1 herbicides, so the answer for many farmers t o control wild oats was to switch to a Group 2 herbicide. This solution did work for many farmers for many years, until wild oats’ resistance to Group 2 herbicides was noticed on these same farms years later.

Where the complete switch was made to Group 2 wild oats control, selection pressure was increased resulting in the development of Group 2 herbicide resistance in only a few years. As of 2020, surveys indicated that 69% of in-field wild oats have herbicide resistance. This reinforces the need to keep using all the tools in proper rotation and in combination with other practices, even if one mode of action becomes less effective.

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Wild oats became resistant to Group 2 herbicides by developing a target-site mutation of the acetolactate synthase (ALS) enzyme, which is a key enzyme in the bio-synthesis of the branched-chain amino acids isoleucine, leucine, and valine. The absence of these amino acids block the effect of the Group 2 herbicide, allowing normal plant growth.

Repeated use of highly effective Group 1 and 2 graminicides has resulted in the development of wild oat resistance. With an increased awareness of how repetitive selection exacerbates the development of resistance, growers moved to new chemistries. Unfortunately, the problem has reared its ugly head again as reliance on Group 15 herbicides has resulted in the confirmation of resistance to this mode of action in several geographies in Western Canada.

It should be noted that Group 8 herbicides have been reclassified as Group 15.

Group 15 herbicides work as shoot-growth inhibitors. A shoot-growth inhibitor prevents the formation of ver y-long-chain fatty acids in the cell membranes preventing growth of the seedling shoot , which affects the weed prior to its emergence. It is believed that the resistance mechanism for Group 15 for wild oats is metabolic but may also be due in par t to cross-resistance with Groups 1 and 2 herbicides.

Additional to herbicides, including alternative management practices for wild oats will help with control, such as: increasing seeding rate to boost crop competition, seeding early to reduce yield losses, the use of precision fertilizer techniques, and strategic crop rotation.

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With kochia, it ’s important to prevent seed production which will cause spreading. Kochia seeds are short-lived and preventing seed production will dramatically decrease next year’s infestation. An effective herbicide program combined with patch mowing are effective ways of preventing kochia from setting seed.

Research has also determined that increasing the rate of glyphosate applied, to overcome enzyme (EPSP synthase) overproduction is not a long-term strategy. Increasing the rate of glyphosate is more likely to simply increase the level of resistance due to increased selection pressure.

A 2021 study of kochia indicated 78% of kochia populations are showing resistance to Group 9 herbicides and 44% showing resistance to Group 4 herbicides. Compared to a survey from 2017 where only 18% of the kochia samples were resistant to Group 4 herbicides, the increase over 4 years is notable, and it is believed by researchers that it will show the same rapid spread as Groups 2 and 9.

Because of kochia’s short seedbank longevity, delayed and reduced germination, and slower seedling development , additional strategies should be considered, such as, delayed pre-seed weed control, or early seeding to enhance crop competitiveness. Controlling kochia weed escapes to minimize gene flow is also crucial.

In 2021, Agriculture and Agri-Food Canada researchers in Saskatchewan confirmed a population of kochia with resistance to Group 14 herbicides, calling into questions the longevity of another valuable tool. At this time, testing has been done on saflufenacil, carfentrazone, and sulfentrazone, but there may be other actives in the group that kochia is not showing resistance to yet.

Just because there has been a control escape, doesn’t mean your whole field is resistant, so be aware of the possibility, but don’t discount the herbicide right away. It will be important for farmers to continue to watch how kochia responds to herbicide treatments.

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Group 14 herbicides are protoporphyrinogen oxidase (PPO) inhibitors, which are light dependent. It hasn’t been specifically determined for kochia how the resistance mechanism works, but it is likely, as with other weeds, that there is a mutation to the target site.

Wild mustard is an aggressive annual weed and can present a serious problem in canola and spring cereals. Its early germination under cool spring temperatures allow wild mustard to take over quickly. Some Group 2 herbicide resistance was found in w ild mustard in Alberta in 1993 and Manitoba in t he early 1990s, i t wasn’t again no ted until 2000 in Manitoba. In a year where growing conditions allowed cool weather weeds to thrive, and persistent wild mustard infestations were seen surviving up to two applications of a Group 2 herbicide. Group 2-resistant wild mustard is also now prevalent in western Saskatchewan.

Wild mustard is a weed that outcrosses readily, creating the potential f or rapid spread of herbicide resistance via pollen as well as through seed movement. At t his time, no tall wild mustard has Group 2 herbicide resistance, but any grower who notices wild mustard that was difficult to control with a Group 2 herbicide or saw variable control of wild mustard but good control of other broadleaf weeds, should consider the possibility that those wild mustard populations are herbicide resistant.

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Wild mustard’s herbicide resistance is metabolic, which is a mor e variable and unpr edictable level of resistance. Herbicide metabolism is a t hree-phase process the plant undergoes to naturally dispose of the toxin being administered. To begin, the plant must slightly modify the herbicide molecule, predisposing it to further modification. Then it will combine the modified herbicide with another molecule (sugar, glu tathione, etc.) to facilitate the final step of moving the herbicide outside the cell, isolating it from the target site. When metabolic resistance occurs, the rate of herbicide metabolism is altered and t he herbicide is rendered ineffective at controlling the plant.

Cleavers, also known to some as sticky willy, are an extremely competitive broadleaf weed that grow as annuals or winter annuals and are particularly tricky when growing in a canola field, as their seeds are similar in shape and size. Cleavers grow well in cool conditions, often found in Western Canadian springs. Even though little else may be growing in a typical cool spring, cleavers could be digging in their heels and the pre-seed burn off could be too late.

Cleavers are now also showing a resistance to Group 2 herbicides. As cleavers are a cool weather-germinating plant, and often over-wintering, the most important part of cleaver control is catching them early. A fall burn down with a non-ALS inhibiting herbicide treatment is recommended, on top of early spring management. It’s also important to rotate herbicide groups that are effective on cleavers and use multi-mode of action products to delay the development of resistant plants.

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Like other Group 2-resistant weeds, cleavers has adapted to ALS inhibitors by mutating its ALS enzyme, stopping the ability of Group 2 herbicides from causing plant death.

Pale smartweed, also known as lady’s thumb, can grow to be a large plant with deep roots. It can grow to three feet tall and almost as wide, which is why this weed is a serious threat. It is also difficult to fully control as, some studies have shown, it can take over 1 6 years to completely rid the soil of germinating seed. It has been noted in parts of Western Canada that pale smartweed is showing resistance to Group 2 herbicides and is having an effect on pulse and cereal crops.

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Pale smartweed i s resistant to Group 2 herbicides through its target-site mutation of the ALS enzyme rendering Group 2 herbicides ineffective in causing plant death.

Redroot pigweed is a dicot weed in the Amaranthaceae family. Each plant ’s ability to produce up to 100,000 seeds if left alone through the growing season, means it can produce large infestations in a short period of time. Their 100,000 seeds can also lay dormant for up to five years, creating potential for future infestations when you aren’t expecting it.

Group 2-resistant redroot pigweed was first confirmed in Manitoba in 2002. While the 2016 weed survey only detected one of 22 fields with Group 2-resistant redroot pigweed, focused sampling of suspicious patches has indicated that there are many more fields across Manitoba that have Group 2-resistant pigweed. Group 2-resistant redroot pigweed has also been confirmed in Saskatchewan. This is particularly concerning due to the lack of non-Group 2 control options in pulse crops.

Redroot pigweed germinates when the crop is up, so a spring burnoff application won’t help control them. However, pigweed germinates when the soil starts to warm, so darkening the soil and warming it early using tillage equipment can be an effective tool in encouraging early emergence allowing for an effective pre seed herbicide application. It will also ensure that most of the weed seeds have germinated and emerged in time for an in- crop herbicide application.

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Redroot pigweed developed a resistance to Group 2 herbicides through a mutation of the ALS enzyme which blocks the effect of Group 2 herbicides on the plant, allowing it to survive the treatment.

Stinkweed is an economically-important weed in the Canadian prairies, ranking on average 7th in abundance across Alberta, Saskatchewan, and Manitoba, with the most seen in Alberta. Stinkweed is a hardy winter annual with a persistent seed bank. In 2000, its resistance to Group 2 herbicides was first noticed near Lethbridge, Alberta. Known for its unpleasant aroma, stinkweed can produce up to 15,000 seeds per plant that can live up to six years in the tillage zone. Densities of 750 plants per square metre can reduce wheat yields by 20%. With its resistance to Group 2 herbicides on the rise, early control of stinkweed is crucial.

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Stinkweed’s Group 2 resistance was caused by the target-site mutation of the acetolactate synthase (ALS) enzyme, rendering Group 2 herbicides ineffective in causing plant death.

Lamb’s quarters is a dicot weed in the Chenopodiaceae family (now considered a subfamily of the Amaranth family) and has been ranked as one of the five most widely distributed weeds in the world. An average lamb’s quarters plant can produce more than 70,000 seeds, ensuring the weed will return the following spring. Seeds can also remain dormant in the soil for several decades and it will take at least 12 years to reduce seed in the soil seedbank by 50% and 78 years to reach 99%. Lamb’s quarters is one of the first braodleaf weed to emerge in the spring and often before any spring tillage or burndown application.

Group 9-herbicide resistance hasn’t been determined in lamb’s quarters yet, but it’s being looked for and farmers should be wary of over- using glyphosate. There are still herbicides available for use on lamb’s quarters (see the Defy Resistance Guide); however, they need to be used efficiently.

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Lamb’s quarters has shown resistance to Group 2 herbicides in Western Canada affecting spring barley and wheat. Group 2 herbicides are ALS inhibitors and it has not been determined what the mechanism for resistance of these herbicides is in lamb’s quarters, specifically.

In addition, environmental conditions can have a significant impact on the ability of lamb’s quarters to tolerate normally lethal doses of herbicide.

Yellow fox tail has been an underestimated weed for many years, rated 30th in relative abundance in a 2002 AAFC weed survey. More recently, however, in a 2016 survey it made a surprising jump to 6th place.

Yellow fox tail is a heat-loving plant which may be increasing in abundance due to higher temperatures across the Prairies. It is an annual grassy weed that reproduces by seed with a longevity in the seed bank of up to 10 years, compared to most grasses which last three to six years. Yellow fox tail is now showing resistance to Group 1 and 2 herbicides.

Unlike yellow fox tail, green fox tail has been seen as a troublesome weed for a long time, ranking in the top three in all Agriculture and Agri-Food Canada weed surveys since the 1970s.

Green fox tail has a late spring germination peaking with temperatures above 20 degrees Celsius. This timing means it is often missed by pre- and post-emergent herbicide applications, so a herbicide application during the one to three leaf stage is critical to minimize yield reductions.

Because green fox tail is a poor competitor, it can be controlled by a strong crop stand produced by early seeding and a healthy crop to shade out the weed. Barley and canola are good candidates for this job, but the suppressed plants may still produce enough seed to infest the field in subsequent years.

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As of 2014, green foxtail has shown resistance to Groups 1 and 2 herbicides and have likely become more prevalent since. Which mechanism the plant uses for resistance for each group hasn’t yet been determined.

The likely mechanism for Group 9 resistance in downy brome is 5-enolpyruvylshimate-3-phosphate synthase (EPSPS) gene amplification, which is similar to the mechanism of glyphosate resistance in kochia. Downy brome is a weed that has invaded North American crops since its introduction to the continent back in 1861. It is abundant in durum, barley, oats, lentils, canola, and spring wheat , but it is mainly an issue for winter wheat since their life cycles are very similar (fall germination and overwintering). Downy brome has the potential of reducing yields of winter wheat by up to 68%.

In 2021, resistance to Group 9 herbicide glyphosate was suspected and later confirmed when a lack of control was observed in a downy brome population in Alberta. This occurrence represents the first confirmation of a glyphosate resistant grass weed in Canada.

Downy brome plants reproduce by seed and can produce up to 6,000 seeds per plant with no competition, and while on average 98% of the seeds germinate within one year, the seed will only last up to 3 years in the soil seedbank . Ensuring those seeds don’t return to the soil seedbank can be an effective long-term strategy for downy brome populations. Cleaning of equipment and seed products will be critical to limiting the spread of glyphosate-resistant downy brome.

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The likely mechanism for Group 9 resistance in downy brome is 5-enolpyruvylshimate- 3 phosphate synthase (EPSPS) gene amplification, which is similar to the mechanism of glyphosate resistance in kochia.

Waterhemp is a problematic resistant weed in the midwestern United States, and in 2014 was found in Ontario with herbicide resistance. Waterhemp populations have now been detected in Manitoba and have been confirmed with resistance to Group 2, 9, and 14 herbicides. Since Waterhemp is not native to Manitoba it is classified as a Tier 1 noxious weed and there is a concerted effort to eliminate the weed in that geography. It has not yet been detected in Saskatchewan or Alberta. Waterhemp is a weed species with separate male and female plants (dioecious). As a result, it provides variability within populations due to cross pollination, which is evident from differences in plant characteristics. Waterhemp can produce up to one million seeds per plant, which can rapidly shift the response of a population to a herbicide, increasing the risk of herbicide resistance.

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Because of waterhemp’s genetic variability, it has a relatively high frequency of mechanisms that can impart resistance to herbicides, allowing waterhemp’s resistance to develop more rapidly than most weeds. In the U.S. there are many different biotypes, but single plants have been identified with resistance to Groups 2, 5, 9, 14 and 27, with several other groups suspected and waiting to be confirmed.

Palmer amaranth, a noxious weed known in the U.S. to be detrimental to yields, was confirmed to be present for the first time in September 2021. Seeds are able to travel through foot traffic, wildlife, or waterways, so transmission to Manitoba was expected once cases were found in North Dakota. Palmer amaranth is described in the U.S. as one of their largest weed problems due to fast growth, large size, and significant yield impact – it can drop soybean production by 79 percent or corn production up to 91 percent. The plant can grow up to three inches a day to a maximum of eight feet tall and a single plant can produce up to a million seeds. Herbicide resistance is also a common struggle in the U.S. with palmer amaranth.

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As of January 2021, a U.S. study showed palmer amaranth had a six-way metabolic resistance in a single population, exhibiting possible resistance to 8 different modes of action.

Canola has become a staple crop in Western Canada, but volunteer canola is a different story. Volunteer canola in a canola field does not make a positive contribution to yield. Volunteer plants do not have a seed treatment and can introduce seedling diseases and increase flea beetle pressure. In non-canola fields, volunteer canola can provide a host for blackleg and clubroot , reducing the effectiveness of crop rotation for managing these issues in canola the following year. Volunteer canola can reduce crop yield by 10%.

As glyphosate-tolerant traits in canola became the norm for canola growers across Western Canada, the instances of volunteer glyphosate-resistant canola have risen, creating an issue for volunteer canola management.

The best management practice for volunteer glyphosate-tolerant canola may be a two-pass strategy – a pre-emergent herbicide application and a post-emergent herbicide application. Crop rotation is also key, providing the abilit y to use a canola-controlling herbicide to clean the site before going back to a canola crop.

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Group 9 herbicides, or glyphosate, inhibit 5-enolpyruvylshimate-3-phosphate synthase which leads to depletion of the aromatic amino acids: tryptophan, tyrosine, and phenylalanine, all amino acids needed for protein synthesis leading to growth. Group 9 volunteer glyphosate-resistant canola contains a modified EPSPS enzyme to ensure the production of the essential amino acids and further growth.

As the volunteer glyphosate-tolerant canola issue has been growing, the Group 10 volunteer glufosinate canola issue is growing as well.

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Group 10 herbicides, or phosphinic acids, inhibit activity of glutamine synthetase, the enzyme that converts glutamate and ammonia to glutamine. If ammonia is left unconverted, its accumulation in the plant will destroy cells. A Group 10 resistant plant will possess a mutation to the glutamine synthetase enzyme to ensure the ammonia gets converted and the plant will live.

Clear field herbicides (Group 2) are in the Imidazolinone (IMI) class of herbicides which function by binding strongly to the active site of the ALS enzyme, thereby inhibiting the branched-chain amino acid biosynthesis pathway.

To help control volunteer Clear field canola in cereals, Group 2 products may be applied with Group 4 products.