Keep an eye on the mercury when spraying in-crop

Tim Bartimote | Senior Land Services Officer - Cropping

As we move forward in the growing season it is important to be vigilant and make the most of the in-crop weed control options which are available. Getting on top of weeds early can pay dividends into the silo due to a lack of competition for valuable nutrients and moisture. Part and parcel of cropping is that the majority of costs are incurred up front. Hence throwing money at weed control when conditions are not ideal can seem a bit harsh on the bottom line. My encouragement to you is not to see consistent weed control as an unnecessary expense but as a wise investment. To make the most out of our crops it is important we give it the best chance of success.

Just like any investment, it is important that every step is taken to maximise the chance for each herbicide application to succeed. This involves getting a couple of things right. Such as spraying when weeds are small or more susceptible, using high water rates, incorporating appropriate nozzles for the situation, spraying in appropriate conditions, rotating chemistry and using the correct herbicides for your weed spectrum.

As we move into the winter months it is important to remember the impact cold temperature and frosts can have on our in-crop sprays. Generally, as temperature increases, the rate of uptake of herbicides increases as long as the conditions do not limit respiration or photosynthesis of plants.

Frosts can shut down plants for extended periods of time and applying herbicides while this occurs can lead to increased crop damage (Figure 1). This is because crops rely on quickly metabolising herbicides to avoid incurring damage. Due to reduced crop growth, frost events impact the production of P450 enzymes. These enzymes are responsible for the breakdown of toxic compounds. Which means, when they are compromised, herbicides are deactivated more slowly within crop plants and damage occurs.

Frost reduces a crops ability to metabolise herbicides. Image courtesy of GRDC

Figure 1. Frost reduces a crops ability to metabolise herbicides. Image courtesy of GRDC

On the other hand, herbicides applied close to frost events can have reduced efficacy due to poor absorption rates by targeted weeds. Environmental conditions slow down the initial absorption of herbicides into the plant, meaning less chemical is taken in. Cold temperatures are particularly impactful on glyphosate, group A and group G herbicides as they require foliar uptake.

For example, a collaborative project by the University of Adelaide and the GRDC observed the impact of frost on the treatment of annual ryegrass with clethodim.  They found that frost events reduced control but particularly when clethodim was applied after a frost. This was further exacerbated if the ryegrass was known to be resistant to clethodim.

Clethodim is most active when temperatures are over 20 degrees C. It is recommended to apply this herbicide when weeds are small and temperatures are more likely to be warmer. If you know a cold front is coming through, wait until there is cloud cover as this reduces the likelihood of a frost occurring. If a frost has occurred, ensure to wait a couple of days before spraying.

For more of what this looks like in your patch, contact your local agronomist or for more information, contact your local LLS Ag Advisor.


Benjamin C, ‘Does ambient temperature really affect herbicide performance?’, 2017, Weed Smart, viewed 24 May 2019,

Saini R, Boutsalis P, Dolman F, Malone J, Gill G & Preston C, ‘Maximise Clethodim Performance: Impact of Frost’, 2015, Weed Smart, 24 May, p. 1-3.

Xie HS, Hsiao AI, Quick WA, 1994, ‘Impact of temperature on the phytotoxicity of imazamethabenz and fenoxaprop to wild oat (Avena fatua)’, Crop Protection, vol. 13, pp. 376-380

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