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Improving pre-emergent herbicide efficacy in high stubble situations

with Catherine Borger, Researcher, Department of Agriculture and Food, Western Australia

Maintaining high stubble has many benefits but there is a downside when it comes to applying pre-emergent herbicides, which must be applied to the soil surface to be effective.

An application of 2.5 L/ha of trifluralin or the full label rate of Sakura® would usually be expected to achieve 70–90% ryegrass control in crop, however Department of Agriculture and Food (DAFWA) researcher, Dr Catherine Borger, has shown that the carrier volume has a large effect on the level of control achieved.

Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high crop residue ground cover situations.

“The good news is that the effect was consistent, regardless of droplet size—from medium to extremely coarse,” says Dr Borger. “Surprisingly, Sakura® responded similarly to trifluralin even though these two herbicides have quite different properties.”

“Our four trial sites had stubble covering 50–90 per cent of the ground surface, a factor known to influence pre-emergent herbicide efficacy,” she says. “At all sites the average ryegrass control achieved increased as the carrier volume increased.”

Expert boom spray set-up may be able to improve efficacy of pre-emergent herbicides at lower water rates but since this is hard to achieve, simply adding more water or slowing application speeds could increase ryegrass control levels with pre-emergent herbicides applied to paddocks with high levels of stubble cover.

Is increasing the water volume the best way to improve ryegrass control with pre-em herbicides?

Short answer: Yes, in high stubble situations high water volumes are the best option.

Longer answer: To increase ryegrass control when spraying trifluralin in high crop residue situations the only options are to increase the herbicide rate, or increase the carrier volume (water rate). Since the top label rate of trifluralin is 2.9 L/ha pre-sowing of wheat, there is little scope to increase the herbicide rate.

Why did such different products show the same response to water volume?

Short answer: All pre-emergent herbicides work best when the product is applied to the soil.

Longer answer: Trifluralin and Sakura® herbicides have different solubility and adsorption properties. Trifluralin has low solubility and is highly adsorbed to organic matter and Sakura® is the opposite, with higher solubility and low adsorption to organic matter. In these trials ryegrass control was similar for both herbicides and increasing the water rate also gave a similar response with both products.

If droplet size didn’t make any difference to weed control rates, does that mean it isn’t important?

Short answer: No, droplet size is important for several reasons and label instructions must be followed.

Longer answer: Set the droplet size to suit factors other than stubble load e.g. drift risk, delta T (bigger droplets for higher Delta T), mixing partner (e.g. medium droplets for paraquat). To evaluate spray jobs, place some water sensitive paper on the ground (in and between old stubble rows) and some vertically on standing stubble. For best results keep ground speeds below 25 km/hr when applying pre-emergent herbicides.

Dr Catherine Borger from DAFWA has demonstrated the clear benefits of using high water volumes when applying pre-emergent herbicides in high stubble load situations.

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