A coarse is a coarse, isn’t it?

Feb. 19, 2023 | 5 Min read
To Mr Ed, 1960s TV’s famous talking horse, it may have been obvious that a horse was a horse, but when it comes to spraying crop protection products in the mid-2020s, being certain you have a coarse spray quality is not so straight forward.

To Mr Ed, 1960s TV’s famous talking horse, it may have been obvious that a horse was a horse, but when it comes to spraying crop protection products in the mid-2020s, being certain you have a coarse spray quality is not so straight forward, writes David Johnson*.

When it comes to spraying crop protection products, the drift risk a coarse spray quality presents is as fluid as the wind that carries it across the landscape.

But a coarse spray quality has a specific definition, doesn’t it? It does, but let’s take a closer look at how that works in the real spraying world.

It’s obvious that the droplets with the most potential to cause an off-target problem are the small ones – the ‘fines’. There’d be no off-target losses if every droplet that came out of the nozzle was large enough to get to ground under its own momentum. But because that may not get the weed control result we want, we compromise by spraying effectively with the fewest fines possible.

Fine droplets are those that are not under direct control from the nozzle because they need a co-operative wind to get them to ground.

It so happens that the droplet size affected in this way changes depending on the height from which it is released.

From 0.5m, droplets with a diameter less than 150µm (microns) struggle to reach the ground. But from 1m, larger droplets with a diameter between 150 and 200µm will also struggle.

A coarse spray quality has between 6%–10% of the spray volume finer than 150µm, and another 6%–10% between 150 and 200µm.

So the boom height alone – without changing the nozzle type, size or pressure; or the colour of this combination on a nozzle selection guide chart; or the conditions you’re spraying in – changes the downwind drift risk of a coarse spray quality.

This fundamental issue is starting to be addressed on some recently registered product labels which carry different no-spray buffer zones for release heights of 0.5m and 1m.

Stipulating a no-spray buffer zone for a specific spray quality is pointless without boom height also being stipulated. It is important to understand that the no-spray buffer zone of a chemical changes with the boom height from which it’s applied.

So – if you have your boom higher than the minimum needed to get a double overlap at the stubble or weed height, then it’s extra important to start with as coarse a spray quality (fewest fines) as possible.

Now, turning to tank mixes. Does adding a chemical have any effect we should be aware of? You’ve no doubt noticed that adding a wetter to a tank-mix makes more fines. And so it can go with a formulated pesticide too.

Depending on the formulation type, whether suspension concentrate (SC) or soluble liquid (SL), the end effect on fines can range from nil to huge.

But when the nozzle selection guide tells you a particular nozzle and size used at a particular pressure is coarse, then how can it produce more fines and still be a coarse according to the chart? A coarse is a coarse, isn’t it?

The answer lies in the fact that all testing behind the nozzle selection guides was with water only. The charts do not reflect what happens with a real tank mix. So, it is very likely, particularly when spraying soluble liquid formulations, which means most glyphosates, that you are using a spray quality finer than what you’d set up to apply.

How, as a spray operator, are you supposed to know what spray quality you’re really spraying?

Well FMC is working towards improving things by providing the market with a nozzle selection guide built from the results of a real tank mix. Our first test mixture was Roundup Ultramax plus Amicide Advance and the results are charted up in a special new nozzle selection guide available from the FMC website.

Turns out that a lot of the nozzles in the middle section of the GRDC nozzle guide spray are in fact ‘medium’ with that particular herbicide mix when the guide says they’re supposed to be ‘very coarse.’ That’s two whole spray qualities finer. A similar effect is had on the high-pressure air-induction nozzles, including the TTI, which can go from ultra-coarse to very coarse or extremely coarse to only coarse.

‘On Coarse DRA’ from FMC is an adjuvant which was developed to address these problems first and foremost – to build in some off-target margin for error for the spray operator.

As a herbicide adjuvant, On Coarse DRA was formulated around an ingredient – guar - that genuinely, consistently and reliably reduces driftable sized droplets.

It counteracts the effects of the tank mix so that more droplets have enough momentum to get the herbicide to ground. This means less horizontal movement too, which reduces stubble droplet capture.

Another way On Coarse DRA helps herbicide performance is by getting the larger droplets to actually stay on the weed. It does this by improving the extensional viscosity of the droplets so that instead of bouncing off the weed, impact shock is absorbed, and they hang on to the leaf.

If you still feel you need to be using a 1m boom height, with On Coarse DRA added you can effectively have the same downwind drift risk profile of a lower boom height and the nozzle spray quality you were aiming for.

With On Coarse DRA, Mr Ed’s certainty can now be applied to a coarse, of course.

*David Johnson is a Brisbane based technical extension specialist – northern region for FMC Australasia.

Categories Grain protection Merchandise

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