As growers embrace no-till and stubble retention systems glyphosate is being used more often to control summer weeds. This has inevitably resulted in the evolution of herbicide resistance in summer weed species
Glyphosate-resistant populations of flaxleaf fleabane and feathertop Rhodes grass have been found in eastern Australia
The best strategy for managing these summer weeds is a double-knock with glyphosate plus an effective alternative herbicide in the first knock and a robust rate of paraquat in the second knock
Glyphosate is the most important herbicide for the control of weeds in fallows. While numerous weeds of summer fallows have evolved resistance to glyphosate in northern Australian systems, two of these weeds have also evolved resistance to glyphosate in the south: flaxleaf fleabane and feathertop Rhodes grass (FTR). Glyphosate-resistant populations are found in paddocks and on roadsides.
Both flaxleaf fleabane and FTR have some tolerance to glyphosate normally. However, research with GRDC investment. has identified resistant populations that require much more glyphosate for control.
Glyphosate-resistant flaxleaf fleabane is recorded at 65 sites across Queensland, New South Wales, South Australia and Victoria. Glyphosate-resistant FTR is recorded at four sites in Queensland, NSW and SA.
In FTR, the resistant populations have three to 10-fold resistance compared with the susceptible populations (Figure 1). The University of Adelaide research showed that the differences between populations are caused by different target site mutations within populations.
In flaxleaf fleabane, intermediate and high levels of resistance were found by researchers at the University of Adelaide and Charles Sturt University (Figure 2). The intermediate resistant populations have two to threefold resistance to glyphosate while the highly resistant populations have 10 to 15-fold resistance to glyphosate. Resistance to glyphosate in fleabane is complex and the mechanisms are not yet understood.
For both species, control of glyphosate-resistant populations by alternative herbicides in summer is difficult. The best strategy is a double-knock approach with glyphosate plus an effective alternative herbicide in the first knock and a robust rate of paraquat in the second knock. FTR seed can move short distances (30 metres) by wind and further on vehicles. It is most likely to appear on the edges of paddocks adjacent to roadsides and railway lines and also near trafficked areas on the farm. It has a short-lived seedbank, so controlling it early can stop populations increasing in size.
Flaxleaf fleabane is much more mobile by wind. While most seed moves less than 100 from the parent plant, a small amount of seed can move long distances. This means flaxleaf fleabane can appear anywhere in paddocks.
Both of these weed species start germinating from late winter/early spring, so control in winter crops can reduce their numbers in summer. Competitive cereal crops can greatly reduce weed establishment in spring.
As glyphosate-resistant populations of both species are present on roadsides, this is a potential source of glyphosate-resistant seed in paddocks. If glyphosate is relied on as the only control tactic in summer fallows, these resistant plants will survive and set seed. If fleabane or FTR survive a glyphosate application in summer then resistance is a possible reason. In this situation another practice should be used to control the surviving plants before they set seed.