Vigilance needed to manage glyphosate-resistant summer weeds

GroundCover Live and online, stay up to date with daily grains industry news online, click here to read more

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

KEY POINTS

  • 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.

Resistant populations

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.

Graph showing survival rate of populations of Feathertop Rhodes grass to glyphosate
FIGURE 1 Feathertop Rhodes grass: survival rate of a susceptible (CV7) and four resistant (V14.2, V14.16, V12 and V15) populations to glyphosate. Error bars represent the standard error of the mean for 4 replicates. SOURCE: The University of Adelaide

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.

Control options

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.

Graph showing the effect of glyphosate on Flaxleaf fleabane populations
FIGURE 2 Flaxleaf fleabane: survival rate of three susceptible (DL4, HV11 and IR7), three intermediate resistant (DL3, DL13 and HV14) and three resistant (DL19, HV5 and IR10) populations to glyphosate. SOURCE: The University of Adelaide

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.

More information

Dr Christopher Preston, University of Adelaide
08 8313 7237
christopher.preston@adelaide.edu.au