Published on 03/10/2023
Adama NZ portfolio manager Hamish Mulcock says pre-emergent herbicides provide effective, selective short to medium-term weed control.
That’s critical to maximising yields because it removes the threat of early season weeds. That means weeds are not competing with crops for essential nutrients, moisture, and even light.
“It makes sound commercial sense to tackle weeds when they are at their most susceptible – at the point that they are germinating. That saves a lot of extra effort, labour, and costs later on,” Hamish says.
Residual herbicides are commonly used for early-season weed control in high-value and time-critical crops including maize, fodder beet, cereals and forage brassicas. Among Adama’s residual herbicide range are Ethosat and Goltix Gold (for fodder beet), Acierto, Atranex and Mesoflex (for maize) and Cyclone (for forage brassicas).
Hamish says getting the application of the residual herbicides right is about good planning and attention to detail.
“To really get the best out of pre-emergent herbicides you need a good seed bed. What you are aiming to do, when spraying pre-emergent herbicides, is to create a herbicide ‘film’ on the soil that is then incorporated by rainfall. You need an even cover right across the paddock. It’s an investment in the crop.”
Anything that prevents an even application or intercepts herbicides at spraying can reduce the efficacy of the spray and/or the length of residual activity.
“You definitely don’t want clods or large amounts of previous crop residues, stubble or trash. That can waste herbicide. It also creates pockets where weeds can get a foothold and undo much of the hard work that’s already gone into the crop.”
In terms of soil moisture, Hamish says a light rain is generally required to incorporate and activate pre-emergent herbicides in the soil. Irrigation could also be a useful tool for incorporation where available.
“An application of 10-20 mm is generally adequate to wash pre-emergent herbicides into the top layer of the soil where it can bind to soil and organic matter. Binding to soil and organic matter is critical to subsequent residual activity.”
Following incorporation, it takes two or three days for residual herbicide to bind to soil and organic matter and to reach a balance between what’s bound to the soil and what is in the soil water available for uptake by germinating weeds.
Heavy rain, however, can be an issue, especially if it occurs before the herbicide has had a chance to bind effectively to the soil.
“If that happens, you can get the herbicide leaching through the soil profile or run-off if the soil is already at field capacity. If heavy torrential rain is forecast immediately following spraying, you should discuss delaying the application with your local agronomist. In some instances it will be well worth waiting.”
Hamish says just because a paddock looks clean doesn’t mean there are not problems below the surface.
“You might be surprised. There are often multiple weed strikes and heavy weed germination because buried weed seeds can delay their germination until soil moisture and temperature are more favourable. And some weed species can last for decades in the soil.”
Pre-emergent herbicides rely on controlling weeds as they germinate and shouldn’t be used on weeds that are already well established.
While some pre-emergent herbicides will provide contact activity on existing weeds, their performance will often be lower and the herbicide ends up being used to control existing weeds rather than on the soil for longer term residual activity.
“Ideally existing weeds should be controlled prior to pre-emergent sprays through cultivation or with non-selective herbicides such as Polaris glyphosate. Residual herbicides should be part of a robust, carefully planned spray programme.”
Adama has specialist programmes for fodder beet and for maize, particularly.
Ethosat, Goltix, Acierto, Atranex, Cyclone, Polaris and Mesoflex are all registered trade-marks of the Adama Group. For more information on Adama products and spray programmes, contact Adama or visit the website adama.com.
Rural Contractor article supplies by Adama.