Dave’s Concise Spray Drift Management Guide

There has been a lot already written about spray drift by some very smart people. In an attempt to keep it brief and easy to follow, here is my take on the subject;

How do we define drift?

When spray goes where we don’t want it!

How do we control it and still do a decent spray job?

Boom Height

Boom height determines the amount of overlap of nozzles subject to speed considerations above. Double overlap is the objective in order to maintain the optimum Coefficient of Variation (CV).

The overriding factor though is that the higher the boom above the target the greater the potential drift simply because the smaller droplets lose momentum quickly and are prone to wander off on the breeze.

Nozzle Choice

Choosing the coarsest possible nozzle is usually the way to go if it’s only about drift. Generally, that means air-induction type nozzles. They are unequalled in regards to drift reduction; however, nozzles that are rated Coarse (C) and more particularly Extra Coarse (XC) perform at their worst with;

1. Contact herbicides,
2. Small target weeds,
3. Heavy stubble or crop canopy,
4. Low application volumes, and/or
5. High speeds,

On the other hand, Medium (M) rated Low Drift Nozzles perform at their best under these very same conditions except for 5, high speeds.

# Note that the combination of bigger droplets AND heavy stubble can result in droplets being captured on the upright surfaces.  Part of the cure for this is to angle nozzles backwards (to help compensate for forward speed) and go more slowly!

Nozzle Spacing

Though not strictly a contributing factor regarding drift, it is worth noting that 25 cm nozzle spacing (on the same or twin boom lines) is also quite common.  In this case, the overlap is somewhat more than double depending on boom height and fan angle.  The great opportunity that presents here is the ability to mix up the nozzle types alternately to allow you to deal with different targets in the same pass.

Nozzles may also be angled alternatively forward and backward for other potential benefits to do more homogeneous deposition, particularly in stubble.

Speed

Going too fast causes the edges of a fan’s spray pattern to peel off fold back, so the Coefficient of Variation (CV) increases meaning that the evenness of your spray pattern takes a beating.

As to what is too fast, that is a matter for debate, however staying under 20Km/h would cause less of this specific problem than spraying at say 25 or even 30 Km/h, which happens!

Spray Pressure

High spray pressures result in a higher percentage volume of smaller droplets.  No mystery there, it is a simple matter of checking the technical specifications for the nozzles you are considering. The information is usually available in table and graphical format and is easy to follow. Your nozzle supplier will have all that information for you.

Wake Effect

Basically, this is about your machine displacing air as it moves along and causing small droplets to be transported into the air, possibly several meters (meets our definition of drift ;) Basically, the faster you go, the greater the potential effect which is said (by Consultant Bill Gordon – see reference below) to kick in at 15 or 16 Km/h for most sprayer types.

Wheel Tracks

Spinning wheels displace air too.  The result is usually to displace smaller droplets away from the wheel tracks; we have all seen this!  That’s where nozzles with bigger droplets with more momentum are a good choice behind the wheels, regardless of what else is on the boom.

Summary

I have deliberately not gone into the meteorological considerations of wind and weather, adjuvants or chemical choices, they are broad and complex topics covered way better by others.

What we are looking for is to strike the balance between the environmental aspects of drift control and the financial (and potential herbicide resistance) aspects of weed control and to minimize the potential to harm your own, or even worse, your neighbor’s sensitive crops.

Ultimately, the label will determine what droplet category to select, and therefore the type of nozzle to choose from the wide variety available. How effectively they are used and how you account for all of the variables outlined above as well as weather, adjuvants, and chemical options; well better you than me on that score and that’s what you pay your agronomist for ;)

Finally, however, you put it all together, even if you think of nothing else at all, it can be argued in almost every circumstance that keeping application rates up and speeds down is a winning combination ;)

#Note that the opinions contained in this article are based on my own observations, a quick read of the GRDC piece, (referenced below) which I fully endorse and a few common sense ideas of my own. You can make your own mind up about those!

Regards, Dave

Reference

GRDC publication dated June 2016 entitled “Ground Cover Supplement, Spray Application” with contributing authors including Bill Gordon (referred to above).

Article by David Young

As for products, what do we recommend?  This nozzle will allow you to reduce drift to a bare minimum for COARSE COMPLIANT APPLICATIONS, without having to go to VC, XC Spray Quality - Dave