Pulse Beat, Soybean Weed Control

Herbicide Resistance in Manitoba

Tammy Jones, Industry Development Specialist – Weeds, Manitoba Agriculture

PALMER AMARANTH, TALL waterhemp, Canada fleabane… the scientific community continues to document the development of herbicide-resistant weeds with confirmation of resistance to new modes of action or multiple mechanisms within one weed. Weedscience.org collects data on the global number of unique cases of resistance, with the current total at 499 unique cases involving 23 herbicide modes of action. That is quite startling since there are only 26 known modes of action in herbicides.

It seems that most of agriculture is aware of the threat of prolific, competitive weeds like Palmer amaranth, and the significant impact it is having on crop production in the U.S. That being said, Palmer amaranth has not been reported in Manitoba to date. So while we wait for emerging threats, there are some local herbicide resistance issues that may not be getting the attention they deserve.

There is no denying that weeds continue to evolve and adapt to current crop production practices. Not only does that result in shifts in the predominant species in Manitoba, but it also means an increase in the mechanisms of herbicide resistance and fewer herbicide options to effectively manage those populations. Wild oat has been a consistent concern for Manitoba. Group 1 resistance was first confirmed in Manitoba back in the early 1990s, and over time, levels of Group 1 resistance have climbed. Based on the most recent herbicide-resistant weed survey done in 2016, in fields where there are wild oats, over 75% of those fields have Group 1 resistance. In 2015, prairie researchers confirmed a Manitoba wild oat population was resistant to five modes of action (Groups 1, 2, 14, 15, and 8). In fact, there is resistance to six known sites of action in wild oat in Manitoba, making us a global leader in herbicide- resistant wild oat. It has been predicted that western Canada is very likely to develop glyphosate resistance in wild oat, which would add even greater complexity to this already challenging weed.

I’ve spent the winter of 2018/19 talking about glyphosate-resistant kochia. After initial confirmation in Manitoba in 2014, there were five municipalities with confirmed resistant populations in 2016 and 12 municipalities by 2018. This is actually under-reporting the extent of the issue, due to a lack of testing and a limited data set. Not only does this weed tumble and spread resistant seeds, it is an out-crossing species, so pollen flow contributes to the spread of this problem.

Recent herbicide screenings of suspected Group 2 resistant redroot pigweed have confirmed a substantial increase in occurrence. Group 2 resistant redroot pigweed was first confirmed in the province in about 2002. In 2016, the weed survey only detected Group 2 resistant pigweed in one of 22 fields that were tested based on random selection. This past summer, pigweed was challenging to control and subsequent sampling of problem fields confirmed resistance in four of the five fields. Anecdotally, there was more than one area struggling with this issue, which warrants further investigation. There were also two types of pigweed in the fields sampled, redroot pigweed and then what has been tentatively identified as Powell amaranth. While there are a number of reasons for difficulties in controlling pigweed (environment, staging, and so on) herbicide resistance seems to be another key factor to consider.


A common definition of herbicide resistance is the inherited ability of a plant to survive and set seed after an application of a normally lethal dose of herbicide. At a purely scientific level, resistance can be any statistical difference between a susceptible population and a resistant population, but at the field level that may still mean that a typical use rate kills the weed. So in the field, “normal” use rates are more relevant, but can vary from region to region based on the herbicide label. It makes it challenging to draw a line in the sand on what level of resistance actually determines that a weed should be classed as “resistant”.

With many weeds, the level of resistance is so high, that these arguments are irrelevant, but it does add complexity to the issue. When a scientist is identifying a new type of herbicide resistance, a susceptible population is grown in comparison to the suspect population, a dose-response curve is generated to demonstrate the level of resistance and the genetics are usually investigated.

That dose-response curve allows scientists to determine a rate of herbicide that differentiates susceptible biotypes from resistant biotypes, which is useful when screening populations in the future. Resistance testing involves collecting seed, growing it and exposing the seeds/plants to the differentiating rate of herbicide. There are many methods; petri dishes of herbicide solution, agar plates infused with herbicide or growing out plants in pots and spraying them. Just like assessing control in a field situation, a visual assessment is the ultimate judgement.


Herbicide resistance is a numbers game. A one in a million occurrence seems rare until you think about the number of weeds in a patch and the number of times that you spray those weeds. If only one or two weeds survive, it is unlikely that anyone would notice. Typically, a patch in a field that won’t die is the first sign of herbicide resistance. And scouting is so important. After determining that weeds are at the right stage, and a herbicide is applied at the right rate under the right conditions, assessing that herbicide application 14–21 days is the first opportunity to notice herbicide resistant weeds (or rule out other factors). A general indication of herbicide resistance is a significant variation in herbicide effect within a small area of similarly sized weeds that does not have a pattern (pictured above) — i.e., not a sprayer miss, not a nozzle issue, nor an environmental influence.

Check out the “Weeds to Watch for” in Manitoba here.


Herbicides continue to be an important tool for weed management, even when there is herbicide resistance in a field. However, there are many cropping practices that help with the efficacy of the herbicide. A competitive crop, established in narrow rows with good plant densities, and appropriately-placed fertilizer will favour the crop rather than weed growth. Timing herbicides when weeds are small and more readily controlled is important, and this typically means more than one spray application for flushing weeds like kochia (or someday waterhemp). Hand rogueing, inter-row tillage, tillage of patches and silaging a crop can be effective in minimizing seed set, when herbicide- resistant weeds are no longer effectively managed with herbicides. Harvest weed seed management is another option for minimizing contributions to the weed seed bank.

As herbicide resistance issues develop, we need to incorporate more of these tools into our arsenal.