Control glyphosate-resistant downy brome with soil-applied herbicides

Fierce provided 99% visible control and biomass reduction of glyphosate-resistant downy brome 8 weeks after treatment. High rates of Focus and Heat Complete, and two rates of Authority Supreme provided excellent control greater than 90%.

Glyphosate-resistant downy brome was confirmed in a glyphosate-resistant canola field in southern Alberta in 2021. This was the first known glyphosate-resistant grass weed confirmed in Canada. Previous research found post-emergent treatments of quizalofop alone or in combination with imazamox, imazamox + bentazon, or imazamox/imazethapyr, and glufosinate mixed with clethodim resulted in more than 80% visible control.

A research study that focussed on soil-applied herbicides was conducted at Lethbridge’s Agriculture and Agri-Food Research and Development Centre. The objectives were to determine a) which pre-emergence soil-applied residual herbicides control glyphosate-resistant downy brome; b) if control was different between resistant and susceptible populations; and c) if control was different between a sandy loam and clay loam soil. The study was repeated in two different greenhouses at AAFC Lethbridge in October and December 2022.

Seed was collected from the glyphosate-resistant population in southern Alberta in 2021, along with a susceptible population from southern Alberta. Two soil types were collected from  grassland fields near Lethbridge, AB where no soil-applied herbicides had been applied previously. The clay loam soil was a dark brown Chernozem, and the sandy loam soil was a brown Chernozem.  They were chosen because the soils are common to the area where resistance was confirmed, but also because soil properties can impact soil residual herbicide control.

Fourteen different herbicide and rate combinations that were registered or being considered for registration were applied immediately after greenhouse pots were seeded with the resistant or susceptible seeds. The rates applied were typical labelled rates. The soil-applied herbicides were not incorporated into the soil to mimic the common practice in no-till fields in southern Alberta. An untreated control was also included for comparison.

Source: Geddes and Pittman, 2024

The density of downy brome plants was determined at 2, 4 and 8 weeds after treatment (WAT). Visible control was estimated at 4 and 8 WAT and downy brome aboveground biomass was measured at 8 WAT.

High levels of control

Herbicide combinations containing pyroxasulfone (Group 15) at 89 g ai/ha or above controlled downy brome by more than 80% when average across soils and populations 8 WAT. Of these, Fierce provided 99% visible control and biomass reduction 8 WAT, while the high rates of Focus and Heat Complete, and both rates of Authority Supreme provided excellent control of glyphosate-resistant downy brome. Health Canada’s Pest Management Regulatory Agency considers greater than 80%  control as commercially acceptable and over 90% control as excellent control. Sixty to 80% control is considered suppression.

The researchers note that while Treflan and Ethalfluralin did not achieve acceptable control, the herbicides were not incorporated after application, which could have impacted their efficacy.

Visible control (P < 0.0001) of downy brome 8 WAT in combined analyses among sandy loam and clay loam soils and glyphosate-resistant and glyphosate-susceptible downy brome populations.

Source: Geddes and Pittman, 2024

Numbers in parentheses indicate the amount of each active ingredient in g ai/ha.

Soil type impacted visible control for several herbicides. Valtera applied alone had 82% control 8 WAT on the sandy loam soil compared to 13% visible control on the clay loam soil. The low rate of Heat Complete had poor visible control at 51% on the clay loam soil and adequate control at 80% on the sandy loam soil.

The glyphosate-resistant population emerged and grew more vigorously than the glyphosate-susceptible population resulting in greater plant densities in the untreated control. This resulted in a difference in downy brome plant densities but not visible control or biomass. Because the current study only tested one population of each biotype, any differences in control between biotypes cannot be  conclusively related to the glyphosate resistance trait.

Overall, the study found that combining pyroxasulfone (Group 15) at a rate of 89 g ai/ha or greater with a PPO-inhibitor (Group 14) could be an effective pre-emergence component of an herbicide layering strategy targeting both resistant and susceptible downy brome.

Funding for this research was provided by the governments of Manitoba and Canada through the Canadian Agricultural Partnership, the Alberta Wheat Commission, Manitoba Canola Growers Association, Manitoba Crop Alliance, Manitoba Pulse & Soybean Growers, Saskatchewan Ministry of Agriculture, Saskatchewan Wheat Development Commission, and Western Grains Research Foundation. The authors also thank Agriculture and Agri-Food Canada for supporting the ongoing efforts of the Prairie Biovigilance Network.

Geddes CM, Pittman MM. Glyphosate-resistant and susceptible downy brome (Bromus tectorum) management with soil-applied residual herbicides. Weed Technology. 2024;38:e36. OPEN ACCESS:

Photo Credit RYosha


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