Pre-emergence herbicides: the does and don’ts

Getting seedlings off to a good start means taking care of a whole lot of variables.

There’s choosing the right seed, planning fertiliser programs, soil moisture, and closely linked to cultivation, getting the pre-emergent herbicides right.

ADAMA NZ portfolio manager Hamish Mulcock says pre-emergent herbicides provide effective, selective short to medium-term weed control. “Why that’s critical to maximising 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 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 use in fodder beet), Acierto®, Atranex® and Mesoflex® (for use in maize) and Cyclone® (for use in 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’re 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.”

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, 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. “10-20mm is generally adequate 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 residual herbicides take 2-3 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.”

Hamish 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-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 provide contact activity on existing weeds, performance will often be lower and herbicide ends up being used controlling 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 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 information on ADAMA products and spray programmes, contact ADAMA or visit www.adama.com.

®Ethosat, Goltix, Acierto, Atranex, Cyclone, Polaris and Mesoflex are all registered trademarks of an ADAMA Group Company