Herbicide resistance in summer grasses and fleabanes – results from a national survey

Abstract

Herbicide resistance continues to be a major challenge in the effective control of weeds in Australian broadacre cropping. As part of a national survey, seeds of awnless barnyard grass (Echinochloa colona), feathertop Rhodes grass (Chloris virgata), sweet summer grass (Brachiaria eruciformis) and fleabane (Conyza bonariensis and C. sumatrensis) were collected from survivors in crop and fallows and screened for resistance against commonly used herbicides to quantify the extent of resistance. These summer growing weeds were predominantly found in the northern cropping region of Queensland and New South Wales.
All species were screened for resistance to glyphosate (Group 9) and paraquat or paraquat + diquat (Group 22). In addition, grass weeds were screened against haloxyfop (Group 1) and fleabane was screened against 2,4-D (Group 4). Treatments were applied at the upper label recommended rate and weed growth stage using a motorised spray cabinet. Populations were characterised for resistance according to percent survival where susceptible = 0%, developing resistance = 1 – 19% and resistant >19% survival.
Results have shown glyphosate resistance is common in these species with 100% of fleabanes, 99% of feathertop Rhodes grass, 58% of sweet summer grass and 48% of awnless barnyard grass populations resistant to glyphosate. Paraquat resistance was found in 8% of feathertop Rhodes grass populations and in 2 populations of tall fleabane. Favourably, no resistance has been found to Group 1 herbicides in summer grass weeds and 2,4-D in fleabane.
The discovery of Group 22 resistance in both feathertop Rhodes grass and tall fleabane is of particular concern as these herbicides are commonly used alone or as a second knock partner for double knock of these weeds. Alternative control tactics are required to manage these weeds.
Ongoing surveillance for herbicide resistance is required so the grains industry can make informed weed management decisions.