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The safest way to manage pre-em herbicides at seeding?

with Sam Kleemann, Research Associate, University of Adelaide

Achieving effective pre-emergent weed control while protecting crop safety at seeding is no mean feat, with plenty of variables to manage.

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Dr Sam Kleemann, research associate at the University of Adelaide, has investigated seeding systems and pre-emergent herbicide combinations that give the best results under different circumstances.

“Effective early weed control is more important than ever, and pre-emergent herbicides have a big role to play, often delivering multi-generation weed control while the crop gets established,” he says. “The difficulty is that these herbicide products vary significantly in their solubility and their tendency to bind to organic matter, and this can lead to considerable variability in weed control efficacy and crop safety.”

Dr Kleemann says that understanding the way different products behave will influence the decisions growers and advisors make when selecting products and setting up their equipment for seeding.

Dr Sam Kleemann says seeder set up and product choice are pivotal decisions when using pre-emergent herbicides to control weeds early in the crop cycle. Photo: Alistair Lawson

Dr Sam Kleemann says seeder set up and product choice are pivotal decisions when using pre-emergent herbicides to control weeds early in the crop cycle. Photo: Alistair Lawson

“There are a few rules of thumb that can help at seeding,” he says. “The first is to minimise soil disturbance so that weed seeds remain on the soil surface as much as possible.”

“The second is to remember that pre-emergent herbicides can cause crop damage. It is essential to separate the product from crop seed.”

“The third rule is to choose the right herbicide for the job and follow label recommendations closely. There are many differences in the properties of pre-emergent products such as their volatility and their rate of degradation in sunlight.”

Tined seeding systems fitted with knife-points generally provide for better crop safety when using pre-emergent herbicide than low disturbance discs. Triple discs are safer than single disc seeders provided adequate soil throw is achieved.

Why is minimising soil disturbance at seeding so important?

Short answer: Weed seeds on or very near the soil surface have the most exposure to the concentrated band of herbicide.

Longer answer: If weed seeds are buried and mixed through the soil they have a greater chance of avoiding the herbicide than when they are concentrated on or near the surface. If a weed seed germinates and establishes a root system before intercepting the pre-emergent herbicide, the weed is often able to grow through the herbicide layer. While it may be suppressed, it probably will not die.

What is the best way to keep herbicide away from the crop seed?

Short answer: Follow the label instructions carefully. Pre-emergent herbicides are mostly non-selective and will cause crop damage if they are thrown or washed back into the furrow.

Longer answer: Seeding systems that throw soil away from the furrow provide the greatest protection for the crop. Knife-point seeders generally provide sufficient herbicide incorporation while maintaining separation from the crop seed. Avoid excessive soil throw from one furrow to the next. Pre-emergent herbicides are not recommended for use with low soil disturbance disc seeders due to the unreliability of herbicide separation from crop seed and increased risk of crop damage.

Why do the label instructions for pre-emergent herbicide products vary so much?

Short answer: Each product has different properties that impact on efficacy and crop safety.

Longer answer: Properties such as solubility, need for incorporation, binding on organic matter, rate of breakdown in the environment and so on make a big difference to how a herbicide is used. Use this information to make product choice decisions that take into account the stubble load, short-range rainfall forecast, sowing date and seeding equipment and set-up.

How to ask a WeedSmart question

Ask your questions about protecting emerging crops for pre-emergent herbicides at seeding on Facebook or Twitter @WeedSmartAU or leave a comment below.

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