Can using a drift-reducing nozzle improve coverage?

I have seen sooo many statements in adverts, articles in the popular press, and social medial posts suggesting that drift-reducing nozzles give better coverage, and I really wonder where this idea has come from (apart from the marketing department, obviously). So let’s be clear, the answer is emphatically no!

Firstly, larger droplets put less spray on plants, and the smaller and more vertical the plant, the bigger this effect.

Secondly, the area of leaf covered by spray liquid, for a given retained spray volume, will be reduced with bigger droplets. So you have a double whammy – less spray that covers less well.

However, good formulation can compensate for these to a huge extent, so products with modern formulations might not give the drop-off in performance that we expect from larger droplets. So an important question to ask your supplier is – ‘how does this product perform with drift-reducing nozzles?’

Also, there is some evidence that modest levels of drift control - what we would call a small-droplet air induction nozzle, like the Guardian Air or Billericay BubbleJet (other makes are available etc) - give better penetration into a cereal crop canopy (as measured by the quantity on the lower stem) than traditional flat fan nozzles, so there might be a small improvement in distribution over the plant in some circumstances.

But what we usually mean by coverage relates to spray on an individual leaf surface, and we measure the percentage of that area covered with spray liquid by putting a fluorescent tracer in the tank, photographing under UV light and then conducting image analysis.

For the same tank mix, a the greater the nozzle star rating (or drift reduction), the lower the coverage.

Some people talk about coverage relating to soil surface, and this is an altogether more difficult concept, with, as far as I can tell, zero underpinning science.

The soil surface is complex, the real surface area is huge, and the whole thing is also porous, so the notion that the same ideas and terminology we use for foliage-applied sprays are relevant to soils is questionable. My feeling is that spray characteristics will be very very much less important than soil characteristics in determining the performance of a soil-applied herbicide. While bigger droplets might give a different distribution of active substance over the soil, there is no evidence that this affects the level of control.

All the evidence suggests that a high level of drift reduction should work well for pre-emergence herbicide applications, but not because of any effects on 'coverage'.

So when you read something that suggests you can get drift control and better coverage, be sceptical and ask difficult questions!

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