What's the best nozzle for pre-emergence herbicides?
As ever, there is no single, simple answer to this question, as you need to take account of your own circumstances. I am also very wary of recommending anything that directly contradicts the label, because I don’t want to get anyone into trouble.
I started with a check through some common pre-emergence product labels to find out what the legal requirements are – thanks to our old friend and NIAB TAG agronomist David Parish, who pointed me towards what you might be using this year.
Most are ‘LERAP B’, which means that if you have a water body within 6 m of the field, you will need to either use some drift reduction, or not spray within 6 m. A few products (e.g. Hurricane, Sempra) have much greater restrictions – 6 m no-spray zones and 3 star nozzles up to 30 m from the water. We’ll come back to what to do in these situations later.
If you don’t have any water near the field, you can pretty much spray how you want. The labels all say ‘Medium’ quality spray, unfortunately, which is not the most helpful suggestion. I also found two that indicate you could spray with a ‘Fine’ spray. I think it would be worth pointing out that this would be a stupid thing to do, and I am really surprised that this is still allowed on the label. It directly contradicts the Code of Practice, which clearly states that you must not ‘apply pesticides in a way which may lead to drift’. Fine sprays will lead to drift. Don’t use them outdoors for anything.
There are no star-rated nozzles in the Medium category, and thus we have a problem. But confining your spray to the area to be treated is important, and drift reduction is a crucial part of this. For many years (and strongly encouraged by at least one product manufacturer) drift reduction has been used for pre-emergence applications and clearly the labels are lagging behind best practice.
So, which nozzle to use? There are no legal requirements for a star-rating if you have no water nearby, so your choice is quite broad. Don’t ignore the basic requirements for a good nozzle just so you can achieve drift reduction: make sure they are not worn, and they are calibrated, and operated within their rated pressure range. Nozzles tend to give their best ‘patternation’ (or uniform distribution across the boom) at pressures of 3 bar and above, so this should be your minimum target pressure. There are not so many nozzles that can give high levels of drift reduction at 3 bar, but three examples are:
all of which come in a range of sizes so that you can match them to the forward speed and volume and will have at least 3 stars at 3.0 bar. There will also be some Hypro nozzles in this category, but we haven’t done the testing and they are not on the official ‘star-rating’ list, so they are more difficult to identify.
These recommendations are relevant only for soil-applied residual herbicides. If there is also significant foliage action required, then the large droplets produced by these nozzles will probably be less effective, and a lower level of drift reduction might be wiser.
Nozzles that give a better balance between drift reduction and performance for foliage applications at 3.0 bar include:
Hypro Guardian Air
So whoever your favourite nozzle manufacturer, you should be able to find something that does the right job for you. (Other nozzles and manufacturers are available – we do not get to test them all!!)
You will note I haven’t recommended 90% drift reduction explicitly. I think it is important to recognise there is no such thing as a 90% drift-reducing nozzle (or any other percentage, for that matter), just a 90% drift-reducing treatment, which includes restrictions on forward speed, pressure and boom height. If the pressure has to drop to achieve 90% drift reduction, it might not be the best option for uniformity of your application. Go for something like 50% plus (or 2 stars) where application to foliage is also important, and 75% plus (or 3 stars) for soil only.
So what if you have nearby water bodies and therefore have legal restrictions? And what about angling nozzles? I’ll look at these next time.