Seed & Chemical
Assessing droplet size and coverage
By Bruce Barker
Making the nozzle choice.
In the drive to reduce spray drift, herbicide applicators have moved to coarse
sprays. While that may cut down on drift, the result is fewer droplets reaching
the leaf surface. At some point, weed control will diminish.
"The movement to coarser spray has been led by high clearance sprayers
which travel faster, have higher booms, and need to cover more acres per day.
As a result, they need to control drift. In addition, lower water volumes are
popular because they increase sprayer capacity. The question is, where do we
draw the line to reduce drift and reduce the amount of water?" asks research
scientist, Tom Wolf with Agriculture and Agri-Food Canada at the Saskatoon Research
Based on field and laboratory research, Wolf says there are several considerations
in the balancing act between drift control, increased coverage and canopy penetration.
Target type and mode of action
Grassy weeds and some broadleaf weeds (lamb's quarters, kochia, cleavers) can
be difficult-to-wet, meaning that larger droplets tend not to stick to their
surfaces. Therefore, these require somewhat finer sprays or higher carrier volumes.
Contact modes of action require higher droplet densities than systemic products.
Higher droplet densities are most easily achieved with higher water volumes
or finer sprays.
At eight to 10 gallons per acre, any nozzle can be used successfully provided
the correct pressure is chosen. Air-induced nozzles require higher pressures
than conventional nozzles, but the ideal pressure depends on the specific nozzle.
At four gallons per acre and lower, nozzle choice will be limited to ASAE 'Coarse'
sprays. Nozzle manufacturers now provide charts that tell the user what spray
quality is achieved with each nozzle and pressure. Conventional flat fan, pre-orifice
and low-pressure air induced nozzles would work with the selection of the correct
nozzle size. Pressure and boom height must be sufficient to generate good overlapping
At slow travel speeds, orient nozzles forward. At fast travel speeds (15 to
20mph), Wolf does not have enough information to make a firm recommendation,
but says that coarse sprays oriented forward had the best deposition on vertical
targets in his research. Coarse sprays oriented backwards may improve spray
patterns at fast speeds.
Double nozzles (one pointed forward and the other backwards, such as the Lurmark
TwinCap) are a good idea to improve coverage on vertical targets such as wheat
heads (for fusarium head blight control) or grassy weeds. In laboratory tests,
using coarser sprays with these double nozzles significantly improved coverage,
but in field tests, this did not result in a yield advantage with fungicides
or better weed control with herbicides.
Fast travel speeds have both advantages and disadvantages, but the most important
advantage is greater work rates. Disadvantages are greater dust generation,
less uniform deposition (especially behind the tractor unit) and less canopy
Lower boom heights are almost always preferable over higher heights to reduce
drift. For conventional nozzles with 80 degree fan angles, heights can be as
low as 18 inches above target; 110 degree fan angles as low as 14 inches. Lower
heights reduce drift and improve overall targetting. Low-drift nozzles require
higher heights (an additional six inches) to obtain good uniformity.
High booms increase nozzle overlap, which can be useful for low-drift nozzles
and when pressures are low and patterns begin to collapse. But high booms can
increase drift potential significantly. Use the lowest boom height that still
offers sufficient overlap given the boom movement.
Automatic boom levellers are available and have been useful for suspended booms
on uneven terrain.
Droplet size does not have as much impact on canopy penetration as
carrier volume. Penetration improves with slower travel speeds and higher carrier
volumes for any nozzle. Air assist is one of the best ways to improve penetration
and is more effective than the other methods.
Determine your spray coverage
Wolf has plotted out the effect of water volume and droplet size on spray coverage.
He analyzed the spray coverage on water-sensitive paper and found that the number
of droplets per square centimetre decreased when water volumes were decreased
or droplet size was increased. Combining low volumes with coarse sprays resulted
in the largest droplet density reduction.
|Figure 1. Effect of water volume
and droplet size on spray coverage as shown on water-sensitive paper. Source:
Tom Wolf, AAFC Saskatoon.
Instructions for assessing spray coverage
- Place a strip of water-sensitive paper on the ground (available from TeeJet
- Make one pass with the sprayer.
- Where spray droplets land, the water-sensitive paper will turn colour.
- Compare the field results with Wolf's chart (see Figure 1).
Spray coverage that falls in the lower right hand portion of the chart has
a very coarse spray with low drift and would be pushing herbicide performance,
especially for grassy weeds and contact pesticides that require good coverage.