If growers know they have glyphosate-resistant kochia in their fields, it is recommended to grow Liberty Link canola, as glufosinate provides a consistent and high level of kochia control, and to consider using ethalfluralin or carfentrazone to get other herbicide modes of action in their weed control program.
Kochia was identified as the first glyphosate-resistant weed in western Canada in 2011 and is now present in seven counties in Alberta, 14 municipalities in Saskatchewan, and two municipalities in Manitoba. This development of glyphosate-resistant kochia sparked many questions from farmers and the agricultural industry about how to minimize its spread and what best management practices could be employed for its control. This research study (2013-2016) was conducted to determine the timing of seed maturity, seed production potential, and seed dormancy characteristics of glyphosate-resistant kochia and identify effective alternative herbicides to control glyphosate-resistant kochia in pre-seed/chemfallow and in-crop applications.
The results from Blackshaw’s study, which wrapped in 2016, indicate that kochia readily emerges in early spring and it is those early emerging plants that produce the most seed. In the absence of crop competition and at densities ranging from 75-150 plants m-2, kochia emerging in April and May produced up to 2.4 million seeds m-2 in Edmonton and up to 5.2 million seeds m-2 in Lethbridge. Kochia emerging in mid-to-late July in Edmonton and mid-August in Lethbridge were capable of producing viable seed before a killing fall frost. Thus producers must be diligent in controlling kochia that emerges as late in the growing season as late July or early August to stop seed production. Kochia seed was found to possess short-term dormancy (a few weeks) after maturity but subsequently germinated at high levels. Overall results indicate that kochia will likely germinate or die within 1-2 years so growers can quickly reduce the soil seedbank if they also prevent new introductions.
Glyphosate-resistant and glyphosate-susceptible kochia were both included in the herbicide experiments to determine if they responded differently to herbicides other than glyphosate. Results indicated that the glyphosate resistance trait did not confer resistance to any other class of herbicides. This is good news for farmers; if the herbicide they are using is currently effective on kochia then it should remain so on glyphosate-resistant kochia.
The majority of glyphosate-resistant kochia populations in western Canada were selected for in chemfallow fields due to the repeated sole use of glyphosate. Thus farmers required immediate advice on effective herbicides that could be tank-mixed with glyphosate for use on fallow. Results indicated that dicamba (Banvel) at higher rates of 290-580 g ai ha-1, dicamba/diflufenzopyr (Distinct), saflufenacil (Heat), MCPA/dichlorprop-p/mecoprop-p (Optica Trio), and carfentrazone (Aim) are the best tank-mix partners with glyphosate to control kochia in chemfallow.
In wheat, any herbicide product containing fluroxypyr (e.g. Pulsar, OcTTain, Enforcer D) gave consistent kochia control with excellent crop tolerance. Pyrasulfotole/bromoxynil (Infinity), dicamba/2,4-D/mecoprop (Dyvel Dsp), and MCPA/dichlorprop-p/mecoprop-p (Optica Trio) also provided a high level of kochia control. In field peas, the combination of carfentrazone/sulfentrazone (Authority Charge) applied pre-plant provided superior kochia control. Imazamox/bentazon (Viper ADV) also provided reasonable in-crop kochia control and this was improved when saflufenacil (Heat) was applied pre-plant to control early germinating plants. Kochia control will be problematic in Roundup Ready or Clearfield canola but this study found that glufosinate (Liberty) applied once or twice in Liberty Link canola consistently controlled kochia over sites and years. Other effective treatments were pre-plant ethalfluralin (Edge) followed by postemergence glufosinate and the three-way combination of pre-plant ethalfluralin, pre-emergence carfentrazone (Aim), and postemergence glufosinate. Farmers were advised of these results and they were also provided information on how best to rotate herbicides from different herbicide groups to manage existing herbicide-resistant kochia and prevent development of additional herbicide resistance.
Several unregistered herbicides were evaluated and a few showed good potential to control glyphosate-resistant kochia. Fluthiacet (Cadet) and pyroxasulfone (Focus) both provided selective control in field peas. Sulfentrazone (Authority) is not currently registered for use in spring wheat but this research indicated that wheat tolerance is acceptable and it was among the very best treatments in terms of kochia control. The respective companies selling these herbicides have been informed of these results and are proceeding with new herbicide registrations. These Group 14 and 15 herbicides will be beneficial in terms of rotating with the widely used Group 2 and Group 4 herbicides in many of the Prairie field crops.
Management of glyphosate-resistant kochia will be problematic in canola. Obviously, using glyphosate in Roundup Ready canola will not be effective and Clearfield canola is not a very viable option as greater than 90% of kochia in western Canada is resistant to Group 2 herbicides. Thus, the research team decided to focus on Liberty Link canola in these field experiments to determine glufosinate (Liberty) efficacy when applied once or twice postemergence. Additionally, the research team evaluated some other potential herbicides for use in canola that could be used alone or in combination with glufosinate. Study findings clearly indicate that glufosinate applied early postemergence (2-3 leaf stage of canola) at either 500 or 590 g ai ha-1 consistently controlled kochia over sites and years. Repeated glufosinate applications (2-3 and 5-6 leaf stage) resulted in slightly higher levels of kochia control and would have the biggest benefit in years where multiple kochia flushes occurred. Glufosinate providing a consistent and high level of kochia control is very good news for farmers and the entire canola industry.
Ethalfluralin (Edge) can be used in all canola types but this study found that kochia control was generally inadequate; ranging from 40-70%. However, ethalfluralin could still be a useful treatment if used in conjunction with other herbicides. For example, fall-applied ethalfluralin followed by postemergence glufosinate was quite effective and the three-way combination of fall-applied ethalfluralin, pre-emergence carfentrazone (Aim), and postemergence glufosinate provided a high and consistent level of kochia control over sites and years. Postemergence carfentrazone was evaluated in 2013 but canola injury was very high and this treatment was not tested in subsequent years. Sulfentrazone is registered at rates of 105 and 140 g ai ha-1 in field peas and sunflower and at those rates it causes unacceptable canola injury. However, the research team decided to evaluate low sulfentrazone rates (27 and 53 g ai ha-1 ) to determine if there was any potential to suppress or control kochia with minimal canola injury. Results indicated that there may be potential to use pre-emerge sulfentrazone at the lowest rate of 27 g ai ha-1 especially when combined with pre-plant ethalfluralin, pre-emerge carfentrazone, or pre-emerge carfentrazone followed by postemergence glufosinate. Visual canola injury was usually <10% and canola yield was not reduced.
The researcher’s overall recommendation based on this 2013-2016 study would be for farmers to grow Liberty Link canola if they know they have glyphosate-resistant kochia in their fields and to also consider using ethalfluralin or carfentrazone to get other herbicide modes of action in their weed control program.