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Controlling Group 2/9-resistant kochia

GR kochia was first confirmed in southern Alberta on chemfallow fields. Photo courtesy of AAFC.

Whether you have it or not, you better get used to dealing with kochia resistant to both Group 2 and Group 9 herbicides. Researchers assume that all kochia on the Prairies is now resistant to Group 2 herbicides, and the incidence of Group 9 kochia resistance is only going to grow in time and space.

According to Bob Blackshaw, research scientist with Agriculture and Agri-Food Canada (AAFC) at Lethbridge, Alta., this doesn’t come as a surprise.

“Where you have heavy infestations, wide distribution and intensive use of herbicides, you will get resistance development. It is almost a given, and whether it will happen in 10 or 20 or 30 years, it will happen at some time,” he says.

Blackshaw and AAFC Saskatoon colleague Hugh Beckie have been following the development of glyphosate resistant (GR) kochia over the last several years. They have participated in surveys and are looking at herbicide control options.

Beckie has looked at GR kochia control with some common herbicides in greenhouse trials to assess weed control. Blackshaw has conducted small plot herbicide trials at Lethbridge and Coalhurst, Alta. over the last two years.

In the greenhouse
As part of Nikki Burton’s undergrad research at the University of Saskatchewan (U of S), Beckie looked at GR kochia control with Banvel II (dicamba, Group 4), Pulsar (dicamba+fluroxypyr, Group 4), Distinct (dicamba+diflufenzopyr, Group 4+19), Buctril M (MCPA+bromoxynil, Group 4+6) and Liberty (glufosinate, Group 10). The herbicides were applied to two GR kochia biotypes and one non-GR biotype.

Beckie explains both GR and non-GR biotypes responded similarly to each of the herbicide treatments. Although both GR biotypes were sensitive to the herbicides, MCPA+bromoxynil was the most effective treatment in reducing shoot biomass three weeks after application (99 per cent), followed by glufosinate (91 per cent) then the dicamba mixtures (82 per cent). Dicamba alone only suppressed kochia biomass (76 per cent reduction).

 “Control of glyphosate resistant kochia with dicamba is very rate dependent, especially if the kochia is larger – we only got suppression at the 140 g ai/ha rate,” Beckie notes.

GR control on chemfallow
Blackshaw looked at some registered and experimental herbicides to assess their potential for controlling GR kochia on chemfallow. All treatments were tank-mixed with glyphosate at the 450 g ai/ha, as would be commonly done on chemfallow fields. The kochia was at a fairly large growth stage. At Lethbridge, the kochia was 15 cm tall and 10 to 15 cm in diameter, while it was 10 to 15 cm tall at Coalhurst.

“It was a tough test, but kochia is usually larger in chemfallow fields than in-crop treatments so we wanted to have a realistic test,” Blackshaw says.

Fluthiacet, an unregistered herbicide, provided marginal control on chemfallow. 2,4-D did not control kochia very well.

“2,4-D never has controlled kochia. If you look at the data going back 30 years, 2,4-D by itself is not great. You have to mix it with Banvel or bromoxynil to get better control. Do not rely on 2,4-D by itself,” Blackshaw cautions.

Optica Trio (MCPA+mecoprop+dichlorprop, Group 4), dicamba (Group 4) and Distinct (dicamba+diflufenzopryr, Group 4+19) herbicides all provided fairly good control as well.

“Dicamba could be a good choice for many people to use but you need higher rates,” Blackshaw says. Best control with dicamba came with 580 g ai/ha rate.

A batch of Group 14 herbicides all worked quite well, too, Blackshaw notes. Aim, Valtera+2,4-D, Heat, and Aim+Authority herbicides provided a consistently high level of GR kochia control.

“It is nice to have Group 14 products as an option. Group 14 isn’t really a new herbicide group in the world, but it is relatively new to us in Western Canada,” Blackshaw explains. “The good news is there are other products to use in chemfallow.”

GR kochia control in spring wheat
In spring wheat, Blackshaw looked at 15 different herbicides and rate structures. Dicamba+2,4-D, bromoxynil+2,4-D, estaprop and Target (dicamba+mecroprop+MCPA) provided marginal control. Dyvel Dsp (dicamba+mecoprop+2,4-D) looked better at the higher rate. Optica Trio provided good control although not as good as some of the other herbicides.

“Probably the best products to use on spring wheat are those containing fluroxypyr. Fluroxypyr is dynamite on kochia and is probably the one you want to use,” Blackshaw says.

Fluroxypyr is a Group 4 herbicide. It is found in Attain (with 2,4-D), Prestige (with clopyralid+MCPA), Pulsar (with dicamba) and Stellar (with florasulam). Blackshaw combined Stellar with MCPA to boost up GR kochia control.

Enforcer D also contains fluroxypyr along with bromoxynil and 2,4-D. Blackshaw needed to use the higher rate to get better control.

Infinity (bromoxynil+pyrasulfotole, Group 4 and 27) also gave very good control. “I really didn’t know if it would have much control, but it did with more than 90 per cent control. We don’t use Group 27 herbicides as much so that is nice for managing herbicide resistance,” Blackshaw notes.

The final product tried for GR kochia control in spring wheat was Authority (sulfentrazone, Group 14). It is applied to the soil prior to crop emergence, but is not registered on wheat.

“We know Authority is great on kochia.  It looks like there may be enough safety in wheat to see if there is interest in moving Authority forward for registration prior to wheat,” Blackshaw says.

GR kochia control in field pea
Blackshaw looked at a number of fall-applied herbicides on field pea, and none provided very good control. These included Edge, Kerb, pyroxasulfone and Valtera alone and in some tank-mixes.

Aim (carfentrazone, Group 14) plus Authority applied preplant provided excellent control from 90 to 100 per cent. Heat (saflufenacil, Group 14) preplant followed by Viper ADV also provided excellent control. Fluthiacet alone or with Viper ADV showed promise as well, but fluthiacet caused some injury to pea.

“There are some good options. Authority will probably be the choice in field pea,” Blackshaw says.

Control in canola more difficult
In Roundup Ready canola, GR kochia would not be controlled with glyphosate. Similarly, in Clearfield canola, kochia would not be controlled because the herbicides used in this system are Group 2, and kochia is assumed to be resistant to Group 2 herbicides.

“Control in those crops will be a challenge if you have GR kochia,” Blackshaw notes.

In Liberty Link canola, though, GR kochia control has some good options. Liberty (glufonsinate, Group 10) herbicide alone provided excellent control. Fall-applied Edge, similar to field pea treatments, did not provide control.

Blackshaw also looked at Aim plus Authority pre-emerge at various rates. The products are not registered on canola. At rates used in field pea, this combination killed the canola. At reduced rates, some control was achieved, but Blackshaw isn’t sure crop tolerance is good enough. If crop tolerance was okay, it might be a registration worthwhile pursuing for Roundup Ready canola. But, “That might be a long shot,” Blackshaw says.

Ongoing studies
One more year of funding is providing opportunity for further research on GR kochia control. Blackshaw is continuing with the herbicide research; while he and Beckie, along with Julia Leeson with AAFC in Saskatoon and Sara Martin with AAFC in Ottawa, are looking at the mechanisms and genetics of resistance. They are also looking at seed germination, dormancy and persistence in the soil. This could provide guidance on when to spray kochia on chemfallow.

“One of the questions we have is, if kochia germinates in August do we have to worry about it or will frost take care of it?” Blackshaw says. “We’re hoping to find out more to help manage herbicide resistant kochia.”

Glyphosate resistance quick test
A quick bioassay for determining glyphosate resistance in kochia was developed by Hugh Beckie, along with U of S graduate student Jessica Pratchler and AAFC research technician Scott Shirriff. Seeds of six kochia populations, three GR and three non-GR (susceptible), were placed on filter paper in dishes treated with varying doses of glyphosate. After a seven-day incubation period, seedlings were assessed for growth.  If a true leaf (not cotyledon) was present after seven days, the seedling was GR resistant.

Farmers can try a version of this experiment at home by using common household items. Materials needed:

  • Two round plastic 500mL yogurt or sour cream containers
  • Four basket-style coffee filters
  • 500mL water
  • Glyphosate
  • One medicine dropper
  • Saran wrap
  • Minimum 50 kochia seeds

Method:

  • Place two coffee filters in each of the two plastic containers
  • Using the medicine dropper, add two drops of glyphosate to the 500mL of water
  • Shake the solution
  • Put 5mL of the mixed solution onto the coffee filters in one container
  • Put 5mL of plain water onto the coffee filters in the other container
  • Spread 25 kochia seeds evenly into each of the containers, and cover both with a transparent lid or Saran wrap
  • Poke a few pinholes into the lids/wrap to prevent potential anaerobic conditions
  • Place the containers on a windowsill at about 25 C, but out of direct sunlight, for seven days

If the herbicide-treated seeds grow true leaves, the kochia is GR.

The typical way to screen for GR kochia is to grow seedlings in soil-filled flats and spray with glyphosate to assess weed control 21 days after treatment. Beckie says this approach still provides the most accurate confirmation of weed resistance.

“If growers or agronomists want confirmation, we do testing for glyphosate resistant kochia for no cost here [at AAFC Saskatoon]. If you really suspect you have glyphosate resistant kochia, it will probably be fairly obvious if glyphosate isn’t controlling it, but we can do the screening.”