Managing the tough, adaptable weed takes all the tools that are available in farmers’ integrated pest management toolkit
It is possible to reclaim fields where herbicide-resistant kochia has the upper hand, but it takes long-term planning for crop and herbicide rotations, as well as patch management.
Group 2 resistant kochia was first found on the Prairies in the late 1980s and within two decades it spread across the region.
“That type of resistance spread to the point where we consider all kochia populations found in the Prairies Group 2 resistant, to the point where it’s not even worth testing for resistance anymore,” said Charles Geddes, research scientist at Agriculture Canada during a virtual presentation at a Farming Smarter digital event held at the end of June.
Geddes leads the Weed Ecology and Cropping Systems research program, which focuses on discovery, monitoring, and management of herbicide-resistant weeds in Western Canada.
He said the first case of glyphosate-resistant kochia was first found in Alberta in 2011, then a 2012 survey found about five percent of the kochia population at the time contained glyphosate-resistant biotypes.
“This survey was repeated five years later and after only five years, glyphosate resistance had spread from five percent of the populations to now 50 percent of the populations. This is a very rapid spread of herbicide resistance,” Geddes said.
“Also in 2017, dicamba resistance was documented as well in about 18 percent of those populations. So that made about 10 percent of the populations triple resistant, to Group 2 herbicides, as well as Group 4 and Group 9.”
He said kochia with triple-herbicide resistance limits herbicide options available for a chemical management program, including the herbicide layering approach.
Another herbicide-resistant survey is being conducted this summer in Alberta.
Some of the more recent work Geddes is involved with examined kochia resistance to synthetic auxins in Alberta, which found that dicamba resistance is typically found in southern Alberta while kochia with fluroxypyr resistance is more common in western Alberta.
Geddes said a goal of the research is to document whether the populations were broadly cross-resistant to multiple synthetic auxins.
Dicamba and fluroxypyr are the most commonly used synthetic-auxin active ingredients used in small grain cereal crops for kochia management.
“What we found in our additional screening was that about 13 percent of the populations in 2017 were fluroxypyr resistant. But interestingly, only four percent of the population actually overlapped. Meaning that many of the populations were actually resistant to either dicamba or fluroxypyr, but only four percent of the populations had both biotypes.”
Therefore, if dicamba-resistant kochia is present in your fields, fluroxypyr may still be effective, and vice versa, he said.
To help understand what options producers have left to control herbicide-resistant kochia, Geddes led a study in Lethbridge that examined how herbicide layering throughout the rotation combined with increased seeding rates and narrow row spacing can affect yield loss due to herbicide-resistant kochia.
“We actually planted glyphosate-resistant kochia into the plots at the beginning of the experiment. And then basically, our challenge is to try and manage the herbicide-resistant weed, using both a chemical management program but then also adding in cultural management to help get an additional edge.”
Auxinic-resistant kochia plants were not planted, however the use of auxinic herbicides were limited in the study because of the resistance to this chemistry is building in the kochia population.
The study used a four-year rotation of wheat, canola, wheat and lentils.
In both wheat crops, a glyphosate pre-plant was used with carfentrazone, and for the in-crop herbicides pinoxaden, pyrasulfotole, and bromoxynil were used.
For the canola rotation ethalfluralin was used the fall before, glyphosate was used pre-plant, and glufosinate and clethodim were used in-crop.
For lentils, glyphosate and saflufenacil and pyroxasulfone were used pre-plant, and imazomox was used in-crop.
Liberty Link canola was used because Geddes said glufosinate has good activity on kochia.
“We had all of these phases present within every year, and then in addition we also have this entire rotation using either wide-row spacing versus narrow spacing, as well as either seeding these crops at the recommended densities versus double the recommended densities.”
He said it was easy to see plots with cultural management of decreased row spacing and increased seeding rates because they had better success competing with the kochia.
In the first two years of the study, 2018 and 2019, crop density did not have an effect but in 2020 crop density had a significant effect with a 74 percent reduction in kochia biomass when a higher seeding rate was used.
When it comes to row spacing, this practice had a consistent effect throughout each year of the study.
“Growing the crops with a wide-row spacing versus the narrow spacing, we have about 60 percent reduction in the (kochia) biomass,” Geddes said.
“We’re really seeing better performance of those herbicides, and then also increased competition with the kochia plants that remained unmanaged.”
In 2018, in the plots where herbicide-resistant kochia was present, there was a 26 percent increase in crop yield in all crops and crop rotation phases in the plots with narrow-row spacing.
A separate study published in 2021 that was conducted by Geddes and others examined ways to control glyphosate-resistant kochia.
Of the 20 treatments examined, the three most effective herbicide treatments were: sulfentrazone applied pre-emergence had 99 percent control, fluroxypyr/bromoxynil/2,4-D applied post-emergent had 94 percent control, and pyrasulfotole/bromoxynil applied post-emergent had 92 percent control of the glyphosate-resistant kochia.
The study suggests that layering the pre-emergence sulfentrazone with either the fluroxypyr/bromoxynil/2,4-D or the pyrasulfotole/bromoxynil post-emergent application will give excellent control of glyphosate-resistant kochia.
Geddes said a common site in the fall is kochia patches that have been harvested around because they are too green to put through the combine.
So he designed a study to find the best way to manage these patches.
“In order to do this, we wanted to focus on seed production because essentially seed entering the soil seed bank is going to be the source from which the population establishes in subsequent years,” Geddes said.
He said kochia can emerge late in the growing season and still produce viable seed because it needs about 2,000 growing degree days, which typically happens in the middle of August.
Viable seed production from kochia plants that emerged earlier in the year also starts in mid August.
Geddes said if these two timings are put together an optimal management time can be found.
There is a period of time in the middle of August where the kochia patches can be managed before they produce viable seeds and there is not enough time for new plants to emerge and seed out.
“If we came in and managed those patches in mid-August, say cut those patches and maybe perhaps also treated the re-growth with a herbicide. That would go a long way to preventing the production of seed back into the soil seed bank,” Geddes said.
“This is really important with a weed species like kochia that has very short seed longevity in the soil seed bank.”
He said some of the research he’s conducting focuses on the impact of pre- versus post-harvest herbicides applied the previous fall when it comes to the best time to manage kochia patches.
“Preliminary results are showing that we can extend this period a bit later in growing season as well if we decide to use either pre- or post-harvest herbicide,” Geddes said.