Features Weeds
Herbicide resistance makes kochia management challenging

Kochia is a significant weed problem for many growers and is now widespread across Western Canada. Along with its ability to thrive in challenging conditions such as heat, drought and high-saline soils, increasing herbicide resistance to glyphosate and dicamba across the Prairies is making kochia management more challenging. Researchers are investigating cultural strategies that may help to reduce kochia populations while reducing the burden on herbicides.

“Kochia is fairly widespread across all three prairie provinces, and herbicide resistance to glyphosate and also dicamba is on the rise,” says Shaun Sharpe, research scientist with Agriculture and Agri-Food Canada (AAFC) in Saskatoon, Sask. “Although a lot of kochia is found in field margins, ditches, railways and oil and gas sites, the populations in crop fields are increasing. We recently completed a field survey in 2019 across Saskatchewan that found 87 per cent of kochia had glyphosate resistance within 137 rural municipalities, and dicamba was detected in 45 per cent of kochia samples in 87 rural municipalities. With the drier drought conditions in many areas over the past couple of years, those numbers are expected to increase as kochia spreads really quickly and populations can escalate very fast.”

Field surveys in Manitoba in 2018 detected 58 per cent of kochia samples with glyphosate resistance, and in 2021 in Alberta, 78 per cent of kochia had some level of glyphosate resistance.

In the study, resistance was expressed as low (one-20 per cent), moderate (21–60 per cent) and high (61–100 per cent), which corresponds to the percentage of resistant plants within each population. In Saskatchewan, over 60 per cent of the glyphosate-resistant populations sampled were at the high resistance level, where many of the kochia plants can live and continue to grow after three weeks. On the other hand, only one to 20 per cent of the dicamba-resistant samples were at the high level of resistance. With dicamba, the resistance overall was a lot lower and the plants were typically quite injured and few were able to continue growing. However, to reduce the risk of increasing the level of resistance to dicamba over time, growers should consider including other modes of action in their herbicide management.

“When managing kochia with herbicides, consider what other modes of action can be mixed, either in the tank or applied at a different time. Herbicide management for kochia usually includes early pre-seed burndown and other early applications, and it is really important for growers to use tank mixes with alternative modes of action,” adds Sharpe. “Talk to an agronomist about options that could be best for your area, and whether residual herbicides are a consideration. Group 14 herbicides can be an option for canola and pulse growers; however, some resistance concerns have been reported, and growers need to be careful not to overuse Group 14 pre-herbicides. The tumbleweed properties of kochia mean that one resistant plant can spread seed to multiple distant fields in a single year. Watch where the plants typically move into a field and monitor those areas and other areas such as low spots and field margins for higher kochia populations. With dicamba, the resistance issues are still fairly low; however, it is important to start looking at what other modes of action can be included to reduce the risk of increasing resistance.”

A camelina/pea intercrop plot, heavily dominated by camelina, with kochia plants growing next to the plot.

Sharpe was also interested in investigating other cultural practices that may help reduce kochia populations and seed production in fields. In 2021, a three-year project on six sites was launched to study kochia patch management. In collaboration with growers, established patches were identified where kochia persisted in their fields. Several different cultural treatments were compared at each site including mowing, black plastic mulch, hydromulch and piling chaff on weedy patches in field margins. The treatments were applied in late May when the kochia plants were about the two- to five-leaf stage. The mowing treatment included mowing very close to the soil at that stage and every three to four weeks throughout the growing season. In 2021, the plots averaged about 800 plants/m 2 and in 2022 populations spiked to over 10,000 plants/m 2 at times during the growing season. The 2023 data is still being finalized.

“Overall, the chaff treatments performed the best, and we were able to reduce kochia plant stands by up to 90 per cent,” says Sharpe. “Wheat chaff was piled about six centimetres deep on top of the kochia patches at four sites where chaff was available. On two other sites, either kochia plants were used or adjacent cattails were cut and used to bury the plants. The treatments using either kochia plants or cattails to bury the kochia patches also worked well. This treatment was pretty promising, and with chaff usually available in the field, it is fairly easy with no additional input costs to collect and spread on the kochia patches. We only had the one treatment, so it would be good to do some additional trials to determine the best chaff depth to optimize control. Within fields, we expect that crops would be able to grow through this chaff layer while keeping the kochia seedlings buried.”

Sharpe adds that the mowing treatment worked well in the first year, especially if the mowing was done close to the ground. In drought years, kochia plants continue to grow and produce seed even if they are stunted and short. Although the kochia continued to grow, repeated mowing usually stopped any growth by the end of the season. However, this treatment is more labour-intensive, with multiple mowing passes required. The black plastic mulch worked well and kept the kochia plants from growing through the plastic. The hydromulch treatment, typically used for erosion control, was applied as a slurry and water mix in a layer about one to two centimetres deep. Although it worked well in the first year, in subsequent years when the hydromulch didn’t form a hard mat, the kochia plants started growing through the treatment. One additional treatment included seeding forages to the salt-prone field margin areas; however, with the very dry conditions, the crop did not get established.

“We also trialled intercropping camelina and peas as a possible cultural treatment to manage kochia,” explains Sharpe. “In 2022, we seeded the intercrops in alternate rows in a plot that had some kochia plants, but we didn’t end up with a lot of kochia. So, in the next year, we added kochia seeds to the plots to provide a more even stand. The treatments included both an Edge pre-seed treatment followed by a pre-seed burndown on the plots, but no post-emergent applications were applied. Edge can be used safely with both crops and where it was applied kochia was controlled quite well. Although in the first year, the intercrop was mostly camelina, in 2023 we had more moisture, and the intercrop was about 60 per cent camelina and 40 per cent peas. The intercrop performed quite well, with the peas using the camelina as a substrate to grow on, keeping the canopy upright and closing over more quickly. The intercrop kept upright and was quite competitive, and with a fairly fast canopy closure, reducing weed pressure. However, in the single crop treatments with peas only, the crop lodged quite a bit.

“Herbicides will continue to be an important management tool, but growers must use as many modes of action as possible through the year to help keep those chemicals in use. Growers should also look at their cropping systems to find ways they may be able to incorporate additional cultural methods to help take the burden off the herbicides. Using all strategies available to reduce the risk of resistance and protect the herbicide tools available is key. Implementing patch management strategies such as burying weedy patches with chaff can help by reducing kochia emergence, reducing the areas that need spraying, and reducing the risk of developing herbicide resistance. Intercropping and other cropping systems that increase crop competitiveness and faster canopy closure can also help reduce kochia populations. In field margins or areas with significant kochia patches that are low-yielding or not productive, consider seeding a saline-tolerant perennial forage or cover crop to help compete against kochia and build carbon in the soil for a few years. Using all strategies available to reduce kochia biomass and seed production and the risk of herbicide resistance will help to protect available herbicide tools.”