Controlling glyphosate-resistant kochia in field pea

Layering Group 14 herbicides saflufenacil or carfentrazone + sulfentrazone pre-plant with a Group 2/6 imazamox/bentazon post-emergent treatment can effectively control glyphosate- and ALS inhibitor-resistant (Group 9 + 2) kochia in field pea. Layering will also help to reduce herbicide selection pressure through the use of multiple effective herbicide modes-of-action.

In the most recent surveys, glyphosate-resistant (Group 9) kochia has been found across the Prairies. The 2017 Alberta survey found 50% of kochia populations were glyphosate-resistant. The 2018 Manitoba survey found 58% were glyphosate-resistant, and populations in Saskatchewan are expected to show similar levels in the next herbicide-resistance survey. All kochia is also considered resistant to Group 2 (ALS inhibitor) herbicides on the Prairies.

Field experiments were conducted near Lethbridge, Alberta in 2013-2015 and Coalhurst, Alberta in 2013-2014 to determine which herbicides applied in the fall, pre-plant and post-emergent could effectively control glyphosate- and ALS inhibitor-resistant kochia in field pea. At Lethbridge, field pea was established following an oat cover crop in 2013 and silage barley in 2014 and 2015. At Coalhurst, field pea was seeded into chemical fallow plots in both years.

Ten herbicide treatments were compared:

Active ingredient	Trade Name	Application timing*	Herbicide Group
Ethalfluralin	Edge Microactive	Fall	3
Pyroxasulfone	Pyroxasulfone**	Fall	15
Flumioxazin (low rate)	Valtera	Fall	14
Flumioxazin (high rate)	Valtera	Fall	14
Ethalfluralin + pyroxasulfone	Edge Microactive + pyroxasulfone	Fall	3 + 15
Ethalfluralin + flumioxazin	Edge Microactve + Valtera	Fall	3 + 14
Carfentrazone + sulfentrazone	Aim EC + Authority 480	PP + PP	14 + 14
Imazamox + Bentazon	Viper ADV	Post	2 + 6
Ethalfluralin fb. Imazamox/Bentazon	Edge Microactive fb. Viper ADV	Fall fb. Post	3 fb. 2/6
Saflufenacil fb Imazamox/Bentazon	Heat fb. Viper ADV	PP fb. Post	14 fb. 2/6

*Fall-applied herbicides were applied in the spring in 2013, otherwise in the fall in 2014 and 2015

**Pyroxasulfone is currently sold in a pre-mix with other active ingredients.

Legend: fb., followed by; PP, pre-plant 7-10 days before seeding POST, post-emergence at field pea 3-6 node stage;

Source: Adapted from Torbiak et al. 2021.

Layering herbicides will provide the most effective kochia control

In 2013, the low rate of flumioxazin pre-plant, carfentrazone + sulfentrazone pre-plant, or imazamox/bentazon post-emergent alone or preceded by spring-applied ethalfluralin or saflufenacil, consistently provided greater than 80% control of kochia.

In 2014 and 2015, carfentrazone + sulfentrazone pre-plant, imazamox/bentazon post-emergent alone or preceded by saflufenacil pre-plant resulted in greater than 80% kochia control or more than 80% kochia biomass reduction.

Over the three years, the only herbicide to consistently reduce kochia density was carfentrazone + sulfentrazone pre-plant. In addition, the imazamox/bentazon alone post-emergent treatment was the only treatment  to provide at least an 80% reduction in kochia biomass across the sites.

In terms of visible control, carfentrazone + sulfentrazone pre-plant and saflufenacil pre-plant followed by  imazamox/bentazon post-emergent were the only treatments that provided more than 80% control of kochia across the 3 years.

The researchers reported that while a combination of carfentrazone + sulfentrazone pre-plant followed by imazamox/bentazon post-emergent was not applied in this study, based on these results, this combination suggests that excellent control could be achieve by layering these herbicides for kochia control in field pea.

Based on the experiments, the researchers came to several conclusions. The first was that while the Group 14 herbicides were effective in controlling glyphosate-resistant kochia in this, and other studies, over-use of these herbicides should be avoided and they should be used strategically in crops like field pea where few effective herbicide options exist.

Another conclusion was that while the imazamox/bentazon post-emergent application alone was effective in controlling the Group 9/2 biotypes, that control relied on only Group 6 Bentazon for control, as the Group 2 imazamox would not have been effective on this biotype. As a result, this combination should be applied in a layering program with other herbicide Groups to reduce the selection pressure on Group 6 Bentazon.

Finally, the researchers recognize that as herbicide resistance continues to reduce kochia control options, such as the confirmation of Group 4- (dicamba and fluroxypyr) resistant kochia, other non-herbicide strategies will be necessary to decrease herbicide selection pressure.  These can include strategic crop rotations to include other crops targeting the kochia critical period for weed seed control, and improving the competitiveness of less-competitive field pea through the use of leafy pea cultivars, and higher seeding rates.

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This research was supported by the Alberta Barley Commission, Alberta Canola Producers Commission, Alberta Crop Industry Development Fund, Alberta Wheat Commission, BASF Canada, Dow AgroSciences, Nufarm Canada, Valent Canada, and Western Grains Research Foundation.

Alysha T. Torbiak, Robert Blackshaw, Randall N. Brandt, Bill Hamman, and Charles M. Geddes. Glyphosate- and acetolactate synthase inhibitor-resistant kochia (Bassia scoparia) control in field pea. Canadian Journal of Plant Science. Just-IN  https://doi.org/10.1139/CJPS-2021-0178