Does ambient temperature affect herbicide performance?

Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance.

Dr Chris Preston, Associate Professor, Weed Management at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds.

“Applying herbicides outside the optimal temperature range may contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have low-level resistance to that herbicide.”

Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action.

“Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.”

As a general rule of thumb, Group 1 (fops), paraquat (Group 22) and glyphosate (Group 9) are more effective at lower temperatures, while Group 1 (dims), atrazine (Group 5) and glufosinate (Group 10) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures.

Other factors also affect the efficacy of these herbicides, including humidity, water rates and adjuvants. Glyphosate and particularly glufosinate work better at higher humidity and also benefit from the addition of ammonium sulfate. Glufosinate and paraquat require adequate water rates. Clethodim works better with a methylated seed oil adjuvant. Finally, all of these herbicides simply work better on smaller weeds.

“The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing. However, the Quick-Test can only be used to test for resistance to post-emergent herbicides.”

Maximising herbicide efficacy is a key tactic in the WeedSmart Big 6 integrated weed management toolbox.

To get the best performance out of clethodim, avoid applying it during frosty periods.

Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied following three days of frost. Even higher rates are required if the plants have resistance to clethodim.

Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C.

Herbicide resistance testing for clethodim during summer may return false negative results, which could translate into spray failure in the field the next season.

A new GRDC factsheet – Optimise Group 1 herbicides in canola – provides detailed information about achieving success with Group 1 herbicides, like clethodim.

For glyphosate, efficacy is much better at 20 degrees C than at 30 degrees C.

Spraying glyphosate-resistant barnyard grass at lower temperatures is more effective than under hotter conditions. Also, if barnyard grass is tested for herbicide resistance during the cooler parts of the year, it may appear susceptible to the field rate of glyphosate. But then, when this rate is applied to the population in summer, there may be many survivors.

When glyphosate causes symptoms rapidly, it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable. When using glyphosate under warmer conditions, it is important to get everything else correct: product choice, rate, weed size, and droplet size. Doing this will help overcome any negative effects of temperature.

If you notice weeds that appear to have survived in-crop herbicide treatment, consider collecting samples for a herbicide resistance Quick-Test for post-emergent herbicides.

The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied, so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for most pre-emergent herbicides.

The Quick-Test is available through Plant Science Consulting, and results are normally available after four weeks.

More resources

• Optimise Group 1 herbicides in canola –GRDC fact sheet
• Controlling herbicide-resistant ryegrass in canola crops
• Plant Science Consulting herbicide resistance testing – Quick-Test

The timing of herbicide application influences the efficacy of herbicides—in this case, applying clethodim to ryegrass in frost-free conditions increased the efficacy of the herbicide on clethodim-resistant plants.

Article first published in July 2017. Reviewed and updated June 2026.