May 2021 - 5

DRIFT
Spraying during the day may mean
CONTINUED FROM PAGE 1
produce a range of droplet sizes at any
single pressure.
" A coarse nozzle still produces some
small droplets, " Deveau said. Even
the low drift nozzles recommended
for volatile products, like dicamba,
produce a few small droplets.
The spray volume produced by
a nozzle and
arranged by
droplet sizes
from smallest
to largest would
create a classic,
bell-shaped
curve.
Most of the
JASON DEVEAU
spray a nozzle
produces falls
within a range
of droplet sizes
described by the volume median
diameter. This is the middle of the
bell-shaped curve. There is less spray
volume tied up in the smallest and the
largest droplets, which is why the curve
tapers off on each end.
However, the relationship between
spray volume, droplet size and droplet
count is tricky. Even though there
is very little volume tied up in the
smallest droplets, they far outnumber
the larger droplets.
spraying when it's hot and dry, which
maximizes vaporization. There's also
the concern of the pesticide's effect on
pollinators. Spraying at night means
spraying when it's calm, but then
there's the risk of a thermal inversion,
which may carry vapor drift for miles.
Everyone wants to get done spraying
quickly, but only use travel speeds that
still allow the spray boom to be kept
low.
" The faster the speed, the higher
Growers are encouraged to use travel speeds that reduce the amount of spray drift.
To help growers navigate all of this,
the American Society of Agricultural
and Biological Engineers created a
protocol that compares nozzles to a
known standard, which allows them to
be ranked from Very Fine up to Ultra
Coarse.
This information can be used to
predict the spray's potential for drift,
and the spray's potential for providing
adequate coverage. Some pesticide
labels specify a given nozzle category
to reduce drift potential.
If there is no guidance, growers
should use the coarsest nozzle they can
that still provides adequate coverage.
" Droplet movement is less
predictable with small droplets, and
they are more likely to drift, " Deveau
said.
Volatile products like dicamba
are a special challenge because they
can vaporize long after application,
depending on the weather conditions.
Hot days with low humidity
speed the conversion to vapor. If
this conversion occurs during a
thermal inversion, the vapor can
accumulate under the inversion layer
before moving great distances in
unpredictable directions.
Spraying in cooler weather or
on humid or cloudy days are good
strategies to manage the problem
since vaporization is slower in these
conditions. Watching the forecast
for predictions of the clear, calm,
cool evenings conducive for the
development of thermal inversions is
also important.
Choosing the right time of the day to
spray can be complicated.
the boom, and the more the spray can
drift, " Deveau said. " Driving fast isn't
the way to get done fast. Streamlining
mixing and loading is the way to speed
spraying. "
Adjuvants can improve coverage and
reduce drift, but can also cause crop
injury in less than ideal conditions.
Compatibility is always an issue, too.
Test your sprayer by spraying on
water-sensitive papers to get a picture
of your spray pattern. " The paper is
cheap and gives immediate feedback, "
Deveau said.
More information on managing
spray drift can be found at
https://sprayers101.com. This site
is a non-profit resource maintained
in Canada by Deveau and other
technology specialists.
" There are a lot of ways pesticides
can go where we don't want them to
go, " Deveau said, but there are best
practices in agricultural spraying to
reduce the risk. VGN
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VGN | MAY 2021 | 5
https://www.sprayers101.com http://www.bioworksinc.com/
May 2021

Table of Contents for the Digital Edition of May 2021
