NovaChem > Industry News > 2023 > Pre-emergence herbicides: practical advice for optimal results

Pre-emergence herbicides: practical advice for optimal results

Published on 23/11/2023

There’s choosing the right seed, planning fertiliser programs, soil moisture, and closely linked to cultivation, getting the pre-emergence herbicides right.
Adama NZ portfolio manager
Hamish Mulcock says pre-emergence herbicides provide effective, selective short to medium-term weed control.

“Why that’s critical to maxim
ising yields is 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’re at their most susceptible – at the point that they’re germinating.  That saves a lot of extra effort, labour, and costs later on.”

Residual herbicides are com
monly 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 her
bicides are Ethosat (MOA Group15) and Goltix Gold (Group 5), for use in fodder beet; Acierto (Group 15), Atranex (Group 5) and Mesoflex (Group 27), for use in maize; and Cyclone (Group15), for use in forage brassicas.

Hamish Mulcock 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-emergence herbicides you need a good seed bed. 
What you’re aiming to do, when spraying pre-emergence 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.”
He says 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’s just a waste of herbicide. It also creates pockets where weeds can get a foothold, undoing much of the hard work that’s already gone into the crop.”

In terms of soil moisture, Mul
cock says a light rain is generally required to incorporate and ‘activate’ pre-emergence herbicides in the soil.
Irrigation could also be a use
ful tool for incorporation where available.

“10-20mm is generally ad
equate to wash pre-emergent herbicides into the top layer of the soil profile and allow binding to soil and organic matter. This binding to soil and organic matter is critical to subsequent residual activity.”

Following incorporation resid
ual herbicides take two to three days to bind to soil and organic matter, and to reach a balance between what’s bound to the soil and what's 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 discuss delaying with your local agronomist. In some instances it will be well worth waiting.”

Mulcock says just because
a paddock looks clean, that doesn’t mean there’s not a problem just below the surface.
"You might be surprised. There
are often multiple weed strikes and heavy weed germination as buried weed seeds can delay their germination until soil moisture and temperature are more favourable. Then, there are some weed species that can last for decades in the soil.”

He says pre-emergence herbi
cides rely on controlling weeds as they germinate and shouldn’t be used on weeds that are already well established.

While some pre-emergence
herbicides provide contact activity on existing weeds, performance will often be lower and 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-emergence sprays through cultivation or non-selective herbicides including 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.”

For more detail, contact Adama or visit

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