Agronomy Update Features Herbicides Weed Management Weeds
Agronomy update: Controlling glyphosate-resistant kochia in chemical fallow

In 2011, the first cases of Group 9 glyphosate-resistant (GR) kochia in Canada were confirmed in chemical fallow fields located in Warner County, Alta. Previously, all populations were considered resistant to Group 2 herbicides. Since then, GR kochia has rapidly spread across Alberta, increasing from an estimated five per cent of kochia populations in 2012 to 50 per cent in 2017. 

In chemfallow, glyphosate is heavily relied upon for weed control, but with GR kochia spreading across the Prairies, alternative herbicide control options are required. A study was recently conducted in southern Alberta to assess herbicide mixtures with multiple modes of action to manage GR and glyphosate-susceptible (GS) kochia in chemfallow fields. 

Field experiments were conducted near Lethbridge and Coalhurst, Alta. Plots were split between GR kochia and GS kochia. Kochia was seeded in early spring at a rate of 30 seeds per square foot (300 viable seeds/m²) in all environments, with the exception of Lethbridge in 2015, where it was seeded at 40 seeds/ft² (400 viable seeds/m²). 

The herbicide treatments tested included an untreated control and glyphosate applied alone or in mixture with 13 other herbicide combinations, which were either registered for kochia management in chemfallow, or to determine whether they would be effective for this usage. In all glyphosate mixtures, the rate was equivalent to the 0.33 litres per acre (L/ac) of Roundup Weathermax (450 g ae/ha). Herbicide treatments were applied post-emergence when kochia plants reached four inches (10 centimetres (cm)) in height. 

The best glyphosate mixture treatments that resulted in acceptable (greater than or equal to 80 per cent) control and biomass reduction of GR kochia were Roundup + Banvel II (Groups 9+4), Roundup + Distinct (Groups 9+4/14), Roundup + Heat (Groups 9+14), and Roundup + Aim + Authority (Groups 9+14). 

The label rate of Banvel II plus glyphosate suppressed GR kochia with less than 80 per cent control at the Lethbridge location, but had excellent GR kochia control (94 per cent visual control) at Coalhurst in both years. Two times the label rate of Banvel II provided excellent control of GR Kochia, averaging 91 per cent visual control at all four site years. The GR Kochia did not have any Group 4-resistant biotypes.

The label rate of Distinct plus glyphosate provided acceptable control at two of four site years. Two times the label rate of Distinct showed excellent control with an average of 90 per cent, resulting in an up to 90 per cent reduction of GR kochia biomass at Coalhurst. 

The low label rate of Heat plus glyphosate showed acceptable (greater than or equal to 80 per cent) visual control in three out of four environments, and reduced GR kochia biomass by 84 per cent. The high label rate of Heat plus glyphosate showed excellent GR kochia control with 91 per cent control among environments.  This herbicide tank mixture provided an excellent, effective option for control of GR kochia in chemical fallow. 

Glyphosate + Aim + Authority at the label rates resulted in an average of 90 per cent visual control but only a 72 per cent reduction in biomass in 2014. Doubling the rate of Authority in this mixture resulted in excellent visual control of GR kochia at an average of 96 per cent control and a 98 per cent reduction in kochia biomass in 2014. This combination was among the best mixture options for controlling GR kochia, in part  because it included a quick contact herbicide resulting in rapid necrosis and plant cell death, in addition to extended residual activity to help control subsequent emergence of kochia seedlings. However, Authority is not registered for chemfallow application.

Glyphosate + 2,4-D (Group 4) did not provide acceptable control of GR or GS kochia in Lethbridge. 

Glyphosate + Optica Trio (Group 4) provided acceptable control at three of four site years, with suppression rated at 79 per cent control for the fourth site year. This resulted in a 90 per cent biomass reduction in 2014. Optica Trio was applied at the label rate for post-emergent application in cereals, but is not registered as a chemfallow treatment. 

Blackhawk (Group 14/4) plus glyphosate did not achieve commercially acceptable control of GR kochia at either site in 2015. 

Due to the confirmation of triple-resistant kochia in Alberta (Group 2+4+9), glyphosate mixtures utilizing a Group 14 mode of action are required for successful and sustainable kochia management. 

For this reason, farmers are urged to adopt a proactive approach to integrated weed management, with herbicides playing an important role supported by several other non-chemical tools. The use of cover crops, strategic spot tillage, mowing and patch management are all tools that could help prolong the efficacy of these herbicide mixtures by mitigating seed production and limiting the number of kochia seeds returned to the soil seedbank.  

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.