How frosts can affect your spraying results.
If you’re like most growers, you get a bit anxious spraying around a frost. It’s always a difficult question of whether we should be out spraying or not. There are a lot of things to take into account, such as; how long it takes the weed to be actively growing again, the length and severity of the frost, and how the herbicide will affect the weed.
It is important to remember that the weeds we are targeting are winter weeds. These are generally more adapted to frosts and unlikely to be hurt by them. But, like other plants, it will take time for the weed to recover from a frost event. So, the most important thing to remember when doing a post-emergent spray is to apply when the weeds are actively growing.
A general rule to follow is this: for every frost, wait 24 hours before going out to spray, and if there are two frosts in a row, then wait 48 hours, and so on.
Plants affected by frosts will affect your spraying results, even if rates are kept high. For example, research has shown that when clethodim is used to control resistant ryegrass before or after a frost, results can be reduced or have nil effect even at rates as high as 4L/ha. This is why you may sometimes see varied results in a paddock. Remember in the picture below the ryegrass has resistance to clethodim.
(Photo: Maximize Clethodim Performance: Impact of Frost – by )
To help manage resistance to clethodim, we advise that you only apply the product to smaller annual ryegrass plants under warm and good growing conditions. Avoid applying it during frosty periods, and use higher rates when spraying days after frosts. You should be looking to use a cold front to your advantage and wait for cloud cover. This will help reduce the chance of frosty conditions overnight.
Dr Chris Preston advises “As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures.”*
*Quote from WeedSmart article ‘Does ambient temperature really affect herbicide performance?’ July 1, 2017 by Cindy