Group 14 resistant kochia confirmed in Saskatchewan
Dose response bioassays confirmed high resistance to foliar applied Group 14 active ingredients saflufenacil and carfentrazone in a kochia population from Kindersley and two populations from North Dakota.
Herbicide-resistant kochia has become a serious problem for Prairie farmers over the last two decades. Currently, all kochia is considered resistant to Group 2 ALS inhibitors, and since then, the number of herbicide-resistant kochia biotypes has continued to grow. In Alberta, a 2017 survey found that all populations were resistant to ALS inhibitors, while a 2021 survey found that 78% were resistant to glyphosate, 28% were resistant to dicamba, 44% were resistant to fluroxypyr, and 45% were triple-resistant to ALS inhibitors (assumed), glyphosate, and at least one auxin mimic (dicamba or fluroxypyr).
In Saskatchewan, a 2019 survey found glyphosate resistance in 87% of kochia samples, while 45% were dicamba-resistant. Populations resistant to both glyphosate and dicamba were found in 40% of samples, resulting in an assumed three way multiple resistance to Group 2 + Group 4 + Group 9.
Because of evolving kochia resistance to different herbicide Groups, farmers have turned to Group 14 mode of action herbicides applied as a pre-plant/pre-emergence in a layering strategy with Group 6 and 27 post-emergent products that are still effective on multiple resistant Group 2 + 4 + 9 kochia. However, poor control of kochia with Group 14 herbicides was seen in a brown mustard field near Kindersley, SK in 2021, in a sunflower field near Mandan, ND and research plots near Minot, ND in 2022.
The objectives of this research were to determine if the kochia accessions collected from Saskatchewan and North Dakota were resistant to Group 14 foliar-applied PPO-inhibiting herbicides saflufenacil and carfentrazone, and if so the level of resistance observed.
Seeds from at least 20 mature, uncontrolled kochia plants were collected at random from the fields with suspected Group 14-resistant kochia. The field near Kindersley was sampled in October 2021 and designated ‘KindersleyR’. The North Dakota fields were sampled in October 2022 were designated ‘MandanR’ and ‘MinotR’.
For control comparisons, a field from near Eastend, SK with known susceptibility was sampled in 2021 (EastendS), and a second control from a lab-maintained Group 2 ALS inhibitor-resistant population collected near Rosetown, SK (RosetownS) was also compared. Two previously collected susceptible populations in North Dakota from Fargo (FargoS) and Minot (MinotS) were also used as controls.
Separate dose-response bioassays were conducted for the Saskatchewan populations at Agriculture and Agri-Food Canada’s Lethbridge Research and Development Centre, and for the North Dakota populations at North Dakota State University. Kochia was grown in greenhouse pots and were treated with saflufenacil or carfentrazone at 2 to 3 inches (5 to 8 cm) height in the Lethbridge trials, and about 1 inch (2 to 3 cm) in the North Dakota trials.
The herbicide dose rates were 0 (untreated), 0.01, 0.1, 1, 3.16, 10, 31.6, and 100x the US field rate for each active ingredient (25 g ai/ha for saflufenacil and 17.5 g ai/ha for carfentrazone). This is equivalent to the western Canadian mid-saflufenacil rate for wheat and the kochia rate for carfentrazone.
Visible kochia control was estimated at 7 and 21 days after treatment (DAT), and shoot biomass fresh weight (FW) and dry weight (DW) were measured 21 DAT, except for the North Dakota saflufenacil experiments that included visible control, FW, and DW measurements 28 DAT. Visible control was estimated as a percentage from 0% to 100% control relative to the untreated control.
Confirmed Group 14 resistance
The dose response bioassays confirmed high resistance to foliar applied saflufenacil and carfentrazone for the suspected kochia populations from Kindersley and the two populations from North Dakota.
The Kindersley population exhibited 57- to 87-fold resistance to saflufenacil based on biomass DW 21 days after treatment (DAT). The estimated rate of saflufenacil causing 80% reduction in biomass DW was well-above the high field rate registered in western Canada, and was >125-fold more than the saflufenacil rate causing 80% decline in biomass DW for the susceptible controls.
The Kindersley population also showed a 97- to 121-fold resistance to carfentrazone based on biomass DW 21 DAT. The estimated carfentrazone rate causing 80% reduction in biomass DW was also well above that for the susceptible controls, and was 6 to 18 times the registered burndown field rates for carfentrazone in western Canada.
Similarly, the Mandan accession exhibited 204- to 321-fold resistance to saflufenacil and 111- to 330-fold resistance to carfentrazone based on biomass DW, while the Minot accession exhibited 45- to 71-fold resistance to saflufenacil and 88- to 264-fold resistance to carfentrazone.
Substantial differences in visible control 7 and 21/28 DAT were also observed between the resistant and susceptible populations. For example, the Saskatchewan saflufenacil treatment found visible control indices ranged from 46.9- to 47.4-fold resistance at 7 DAT and increased to 56.5- to 101.1-fold resistance by 21 DAT.
The response of one putative-resistant (KindersleyR) and two susceptible (RosetownS and EastendS) kochia accessions from Saskatchewan to a range of foliar-applied carfentrazone rates based on visible control at [A] 7 and [B] 21 days after treatment (DAT) and shoot biomass [C] fresh weight (FW) and [D] dry weight (DW) 21 DAT.
Source: Geddes et al. 2025
Management implications
While confirmation of Group 14 kochia resistance was from three isolated cases on the Northern Great Plains, past experience with kochia resistance to glyphosate (Group 9) has shown that resistance can develop and spread rapidly.
With confirmed Group 2 + 4 + 9 stacked resistance, and the confirmation of Group 14 resistance, available herbicide control options will rely on, and put selection pressure on, herbicides like glufosinate (Liberty; Group 10), and Group 6 (bentazon and bromoxynil) and Group 27 (pyrasulfotole, topramezone, tolpyralate) herbicides
Herbicide options registered for kochia control or suppression in western Canada assuming blanket resistance to all active ingredients within Groups 2, 4, 9, and 14a
aThe Blue Book: Alberta’s Crop Protection Guide 2024. Alberta Grains; Guide to Crop Protection 2024. Saskatchewan Ministry of Agriculture; C indicates control (≥80% control), S indicates suppression (60-79% control)
bScientific names
cPreplant incorporated or late fall-applied
dYellow mustard only
eMixed with glyphosate
fGlufosinate-resistant varieties
gMust be applied with tank mix partner
Source: Geddes et al. 2025
Efforts to control the spread of herbicide resistant kochia will rely on integrated weed management practices such as growing competitive crop with alternative crop life cycles, higher seeding rates, strategic tillage, cutting kochia for animal feed, and cutting or mowing kochia prior to seed set.
Funding for the research conducted at the Lethbridge Research and Development Centre was provided by the Prairie Oat Growers Association, Manitoba Crop Alliance, Saskatchewan Canola Development Commission, Saskatchewan Pulse Growers, Saskatchewan Wheat Development Commission, Western Grains Research Foundation, ADAMA Agricultural Solutions Canada Ltd., BASF Canada Inc., Bayer CropScience Inc., FMC of Canada Ltd., Gowan Agro Canada, Nufarm Agriculture Inc., Valent Canada Inc., and the Saskatchewan Ministry of Agriculture – Agriculture Development Fund. Research conducted at North Dakota State University was partially funded by the North Dakota Agricultural Experiment Station and USDA-NIFA Hatch Project.
Geddes CM, Law QD, Jenks BM, et al. Protoporphyrinogen oxidase (PPO) inhibitor resistance in kochia (Bassia scoparia). Weed Science. Published online 2025:1-38.
Open access: https://doi.org/10.1017/wsc.2025.4
Photo courtesy Charles Geddes
