What producers want to know

By Tom Wolf

Here are some of the most common producer questions dealing with field sprayer equipment and pesticide efficacy.

1) Which low-drift nozzle should I put on my sprayer?

That question is best answered with more questions:

What pressure is your sprayer capable of? (There are low- and high-pressure versions)
What carrier volume do you use? (Use low-pressure air-induced or pre-orifice nozzles for ‹ 6gpa)
What is your travel speed range? (Use low pressure nozzles for greater range of speeds)
What is your tolerance to drift? (High pressure nozzles give better drift control, even at high pressures)
Do you use Group 1 or 10 products, contact modes of action, or consider grassy weeds a serious problem? (use higher pressures and volumes with any nozzle)

2) If lower pressure reduces drift, why do I need to have high pressure with air-induced nozzles?

Low pressures are a good way to reduce drift with conventional nozzles, but this can be dangerous with air-induced nozzles. High pressures are needed with these nozzles so that patterns are fully developed. They also provide flexibility in travel speed with rate controllers. Too low a pressure is the number one reason for poor performance with air-induced nozzles.Fortunately, drift reduction with air-induced nozzles at high pressures is still superior to conventional nozzles at low pressures. Use a pressure that is in the middle of a nozzle's recommended range. For example, if a nozzzle is rated for 30 to 60 psi, aim for 45 psi. For an air-induced nozzle rated for 30 to 100 psi, aim for 60 to 70 psi. This approach gives you flexibility in travel speed when using a rate controller.

3) Do the air-vents on venturi nozzles ever get plugged, and if they do, what happens?

They rarely plug. If they do, the spray pattern gets a bit narrower and the spray gets a bit finer. Neither change is dramatic and spraying can continue. Normal nozzle inspection and cleaning is sufficient.

4) Many of the new nozzles are plastic and ceramic. How do these compare to steel for wear?

There are many different types of plastic materials used in nozzles. They wear at least as well and usually significantly better than steel. Care should be taken when cleaning, as they can deform irreversibly. Ceramic is the most wear-resistant material available, and can last 10-times longer than steel.

5) Do I need air-induction or can I just use low-pressure conventional sprays?

Air-induction provides much more drift reduction than any comparable technology, even at high pressures. A low pressure conventional spray is not as coarse as an air-induced spray and many of the pre-orifice sprays. Nonetheless, lower pressures are still a good way to reduce drift. Air induction seems to improve spray retention on grassy weeds compared to same-sized droplets without air.

6) Should I get shrouds or low-drift nozzles to control drift?

Nozzles are less expensive than shrouds per foot of boom (about half). With shrouds, finer conventional nozzles can be used if the user is nervous about the efficacy of pesticides with coarser sprays. Air-induced nozzles provide similar drift control, depending on the shroud and the nozzle. Both can be used together.

7) Don't coarser droplets reduce coverage?

Yes, they can, but it's not important at reasonable water volumes. In most cases, coverage is very similar, and efficacy is the same for broadleaf herbicides and fungicides at 10 gpa. Grassy herbicides require some caution - don't use a combination of very coarse sprays and low carrier volumes, and do maintain higher pressures. Droplet number per square inch is more important than droplet size for determining efficacy. Pressure and volume should be adjusted to maintain at least 300 - 500 drops/square inch on water-sensitive paper, or 10-15% coverage.

8) Some nozzle manufacturers advertise that their nozzle gives larger and more uniform droplets, better coverage, and can be used at lower volumes than other nozzles. Is this true?

No. Low-drift nozzles are designed to reduce drift, and they do this by eliminating many of the finer droplets and adding some larger ones. Therefore, a low-drift spray reduces the number of droplets available for coverage, even if they are 'more uniform'. In practice, this is offset by maintaining a higher carrier volume and optimizing pressure for the specific nozzle. If you want to reduce water volumes, you need to use finer sprays to keep droplet number up. But you can compromise. Use a reasonable water volume (5 to 7 gpa) and an intermediate low-drift nozzle such as a low-pressure air-induced.

9) Is air assistance any good?

Yes, air assistance is great for increasing dense canopy penetration. This means it can be useful for fungicides and insecticides, but is not necessary for herbicide application. Although some claim that air-assistance can reduce herbicide rates, most of this effect is attributable to lower carrier volumes and very fine sprays. Older air-assist technologies can increase spray drift significantly. Newer technologies such as the Hardi Twin reduce drift and improve coverage.

10) If aerial application can get away with 2 to 4 gpa for fungicides, why do I have to use 10 to 15 gpa with a ground sprayer?

For fungicides, more water is usually better. Both aerial and ground sprays use as much water as economically and practically justifiable in each case. Aircraft can apply sprays in a more timely manner under some conditions, which can be more important than carrier volume.

11) How do I get rid of sprayer tracks?

Sprayer tracks are hard to get rid of. Fast travel speeds, heavy sprayers, and dusty conditions appear to be the culprits. Some strategies involve placing higher flow-rate nozzles behind the wheels, moving the boom back away from the wheels, adding nozzles behind the wheels, or traveling slower.

12) How far can spray drift move?

Fine spray droplets can move for many miles under the right conditions. They move farthest during temperature inversions (night or early morning) because high humidity keeps them from evaporating and calm air keeps them from dispersing. Topography is also important, as drift will follow low-lying areas. Windy conditions actually help disperse the spray. When it's windy, a greater proportion of the spray will drift, but it also gets diluted rapidly.

13) How low can I go with water volumes and still get good results?

The secret to using low water volumes is that coverage is maintained. Since lower volumes result in less water available per square inch, applying this water in smaller droplets compensates. That is fine as long as drift can be managed. Low water volumes typically reduce the effect of hard water. Unfortunately, canopy penetration and overall consistency can be reduced when water volumes are reduced too low. I would not recommend that less than 5 gpa be used for any product other than glyphosate. Remember, water is a relatively cheap input and it offers significant returns in terms of a quality job.

14) Can I use low drift nozzles with all my chemicals?

Yes, as long as all the other guidelines (appropriate water volume and spray pressure) are followed. Among herbicides and weeds, broadleaf weeds and Group 2 and 4 herbicides can actually work better with coarser sprays. Grassy weeds and Group 1 herbicides prefer finer sprays. A Group 1 and Group 2 tank-mix can be applied with a Coarse to Very Coarse spray but water volume should be kept above 7 gpa.

15) Should I point my nozzles forward or backward or both?

Canopy penetration is best when nozzles are pointed backwards. Coverage of vertical targets such as wheat heads of grassy weeds is best when nozzles are pointed forwards. Using a double nozzle provides the best of both worlds, but they work best with a coarse spray. Using a double nozzle with a fine spray mostly increased drift potential with few other benefits. Double nozzles are available in conventional sprays (TeeJet TwinJet), as an air-induced spray (Albuz AVI Twin), or can be custom configured using the Lurmark TwinCap or swivel nozzles.

16) How do I know what droplet size my nozzles are producing?

All nozzle manufacturers now publish spray quality charts that identify the ASAE spray quality. These charts are available in catalogues or on-line. For examples, see

Dr. Tom Wolf is researcher with Agriculture and Agri-Food Canada, based at the Saskatoon Research Centre.

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