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Australian Herbicide Resistance Initiative (AHRI)

Too cold for glyphosate resistance

Glasshouse with plants

Most of us are a bit slow out of bed on a cold morning and take a little while to get moving. Weeds are no different. Some glyphosate resistant weeds become less resistant in cool weather.

One of the mechanisms of resistance to glyphosate is to reduce the movement of the chemical through the plant (known as translocation). Researcher Dr Martin Vila-Aiub and others from AHRI tested annual ryegrass and Johnsongrass with this resistance mechanism. They found that the plants were much less resistant to glyphosate when the plants were grown in cool conditions.

The good news? If you know that the resistant weeds you are spraying have the reduced translocation mechanism, then spraying in cool weather may help.

The bad news? There are multiple mechanisms of glyphosate resistance so it is difficult to know which one you are dealing with.

However, it may be possible to improve control of glyphosate resistant weeds by spraying glyphosate in the cooler months of the year and avoiding glyphosate in the warmer months. Similarly, this approach may reduce the evolution of glyposate resistance.

Annual ryegrass (Lolium rigidum), a C3 plant, is a common winter weed of Australian winter grain crops. Johnsongrass (Sorghum halapense), a C4 plant, is a common crop weed in sub-tropical areas of Argentina. Given these differences, both weeds have different optimal growing temperatures. In this study, two glyphosate resistant populations of Johnsongrass and annual ryegrass from Argentina and from Australia, respectively, were tested.

The annual ryegrass was grown in controlled conditions at either 19oC or 8oC and the Johnsongrass was grown at either 30oC or 19oC. The higher temperature was considered optimal for each weed, and the lower temperature sub-optimal. The plants were grown at these temperatures from when they were 2cm tall to four leaf stage of the Johnsongrass and the two leaf stage of ryegrass, at which time they were sprayed with glyphosate. After spraying they were returned to the corresponding temperature regime.

The graph below shows the dose response curve of glyphosate applied to annual ryegrass. Similar results were observed for Johnsongrass.

Annual ryegrass survival and biomass – top graph 19o C, bottom graph 8o C for two known resistant populations compared to a known susceptible.

4 graphs showing the dose response curve of glyphosate applied to annual ryegrass

Figure 3. Plant survival (A, B) and above-ground biomass (C, D) in glyphosate-resistant (NLR70 black square; WALR50 black triangle) and glyphosate-susceptible (VLR1 white circle) Lolium rigidum populations in response to increasing glyphosate rates when grown at 19 and 8 ◦C. Dashed lines represent predicted values derived from non-linear regression analysis. Symbols denote mean (n=3) ± standard error of the mean.

What does this mean for resistance evolution?

This research suggests that it is less likely for weeds to evolve the reduce translocation mechanism of glyphosate resistance in cool climates than in warm climates where optimal growing temperatures are experienced. This may partially explain why many Johnsongrass populations in Argentina have evolved resistance to glyphosate in the sub-tropical agricultural areas but not in the temperate-cold cropping areas.

What about other weeds?

Some previous research by Doug Sammons’ research team at Monsanto demonstrated similar results in Fleabane / Horseweed (Conyza canadensis). In this research, the reduced glyphosate translocation mechanism was due to increased vacuolar glyphosate sequestration. A vacuole is an organelle that can be found in all plant cells. Its main functions include disposal of waste products and isolating materials that may be harmful to the cell. In this case, the resistant plant moves the glyphosate into the vacuole where it is not active. This resistance mechanism was also found to be less efficient at low (<10oC) temperatures.

What are the implications?

As previously mentioned, it is difficult to use this information to improve the control of glyphosate resistant weeds unless the specific resistance mechanism is known. In this study, plants were grown at the various temperatures for the duration of their life, so it may not be a simple as recommending spraying on a cool morning. However, it may be possible to improve control of glyphosate resistant weeds by spraying glyphosate in the cooler months of the year and avoiding glyphosate in the warmer months. Similarly, this approach may reduce the evolution of glyposate resistance.

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