K. A. Langeland
Herbicide application rates may be given as 1)
pounds or gallons per acre (surface acre or bottom acre), 2) final concentration of herbicide
active
ingredient in the water, usually in parts per million (ppm), or 3) as percent (or other proportion)
solution applied to foliage.
When determining the amount of herbicide to be applied for a given surface area or for a
specific concentration in the water the first thing that must be done is to measure the area to be
treated. Dimensions of small ponds can be measured with a tape measure, by stepping off if you
know your step length or with a rangefinder. When using a rangefinder check frequently to make
sure it is accurate within the distances being measured. On water, distances can be measured by
dragging a floating rope of known length behind a boat and dropping buoys every time the end of
the
rope passes a buoy (so that the distance equivalent to the length of the rope can be measured
again),
with a rangefinder, or by using a map and some type of planimeter if a map with an adequate
legend
is available. Once accurate distances have been measured and marked, boat speed (in feet per
second
or other convenient measure), at a specific rpm can be determined by timing the boat through any
marked distance. Distances can then be estimated by operating the boat for a time that is
equivalent
to a desired distance. Boat speed should be measured in two different directions and averaged to
allow for differences caused by wind or current.
The acreage of small rectangular ponds and lakes is easily calculated by multiplying their
length times their width, measured in feet, and dividing by the number of square feet in 1 acre,
which
is 43,560, as follows (Example 1):
.
SURFACE ACRES =
43,560 (sq ft per acre)
.
Example 1. What is the acreage of a rectangular pond that measures 800 ft by
440 ft?
.
SURFACE ACRES =
43,560 (sq ft per acre)
.
SURFACE ACRES =
43,560 sq ft
.
SURFACE ACRES =
Many lakes and small coves are not rectangular but may be more or less triangular or circular.
The equations for determining the area of a triangle or circle are as follows (Example 2):
.
AREA OF A TRIANGLE (ACRES) =
43,560 sq ft per acre
.
.
AREA OF A CIRCLE (ACRES) =
43,560 sq ft per acre<
(radius = 1/2 diameter)
.
.
Example 2. Estimate the area, in acres, of the following cove.
.
.
AREA OF TRIANGLE (ACRES) =
43,560
.
AREA OF TRIANGLE (ACRES) =
43,560
.
AREA OF TRIANGLE (ACRES) =
Example 3. Estimate surface area, in acres, of the following water body.
DIAMETER = 610 ft
RADIUS = 1/2 x diameter
RADIUS = 1/2 x (610)
RADIUS = 305 ft
.
AREA OF CIRCLE (ACRES) =
43,560
.
AREA OF CIRCLE (ACRES) =
43,560
.
AREA OF CIRCLE (ACRES) =
The shapes of many lakes and coves are very irregular and do not conform to any geometric
shapes. In such cases, the areas can be estimated as follows 1) inscribe a
sketch of the lake inside a
rectangle, 2) measure the area of the rectangle, 3) estimate
the proportion of the rectangle that is
occupied by the lake (this step can be facilitated by using graph paper and dividing the number of
blocks occupied by the lake by the total number of blocks in the rectangle), 4)
multiply the
fraction
of the rectangle occupied by the lake times the area of the rectangle for the lakes acreage
(Example
4).
Example 4. Estimate the surface area, in acres, of the following irregularly
shaped lake.
Step 1
AREA OF RECTANGLE (ACRES) =
43,560
.
AREA OF RECTANGLE (ACRES) =
43,560
.
AREA OF RECTANGLE (ACRES) =
.
Step 2
The number of squares in the rectangle is 100 and the number of squares in the lake is 51.
Therefore
the fraction of the rectangle occupied by the lake is 51/100.
.
Step 3
AREA OF LAKE =
number of squares in rectangle
.
AREA OF LAKE =
100
.
AREA OF LAKE =
Measurement of areas for large herbicide applications is best done in rectangular shapes, and
plots should be marked as subplots of smaller areas. For example a 200-acre area should be
marked
in subplots of 25 acres or smaller so that the application rate can be checked continuously
(Example
5). In addition it may be helpful to lay plots out to conform to swath width so that a certain
number
of swaths will be equivalent to an acre (Example 6). This can be done using the AREA=Length
x
width relationship and knowing that there are 43,560 square ft per acre. By dividing swath width
into
43,560 we know how long a one acre swath would be. Using this as one dimension of the plot
we
can calculate the other dimension by dividing the length of a one acre swath into the area of the
entire
plot. In this way we can keep very close track of our application because we know that with
every
swath we should apply the rate prescribed per acre. Depending on the size of the plot we can
make
a dimension half swath, two swaths, etc. Also, we can assume that the herbicide diffuses
between
swaths and assume that "effective swath" is the distance between the centers of each actual
treated
swath (Example 6).
Example 5. A pellet herbicide formulation is to be applied to a 25 acre plot in
a large lake with a
spreader that has a swath width of 40 feet. Show the rectangular plot layout that could be used so
that each pass would treat one acre.
.
LENGTH OF ACRE SWATH =
swath (ft)
.
LENGTH OF ACRE SWATH =
40 ft
.
LENGTH OF ACRE SWATH =
.
Therefore the length of one side of the plot can be 1089 ft. Now determine the length of the
other
side of the plot as follows:
.
AREA OF PLOT (SQ FT) =
.
AREA OF PLOT (SQ FT) =
.
AREA OF PLOT (SQ FT) =
.
WIDTH (FT) =
length (ft)
.
WIDTH (FT) =
1089
.
WIDTH (ft) =
.
Note that the number of 1-acre swaths or 25 could have been multiplied times the swath width of
40
ft to arrive at the 1000 ft width. This can also be used to check your calculations.
Example 6. Swath width for boom applications to submersed aquatic weeds
can be considered
the
boom length or an approximation and often only alternate swaths are actually treated so that the
effective swath width becomes twice the boom length. It is assumed that the herbicide, with the
help
of turbulence caused by the sprayboat and environmental factors, will mix throughout the water
column.
Show the rectangular plot layout that could be used if 25 acres are to be treated with a liquid
herbicide, using an 8-ft boom treating alternate swaths, so that each pass would treat one-half
acre.
If every other 8 ft is actually covered then the effective swath width is 16 ft.
.
LENGTH OF ACRE SWATH (FT) =
length of effective swath (ft)
.
LENGTH OF 1/2 ACRE SWATH (FT) =
length of effective swath (ft)
.
LENGTH OF 1/2 ACRE SWATH (FT) =
16
.
LENGTH OF 1/2 ACRE SWATH (FT) =
.
The length of the plot can be 1,361 feet. Now determine the width of the plot as follows:
.
AREA OF PLOT (SQ FT) =
.
AREA OF PLOT (SQ FT) =
.
AREA OF PLOT (SQ FT) =
.
WIDTH OF PLOT (FT) =
length of plot (ft)
.
WIDTH OF PLOT (FT) =
1,361
.
WIDTH OF PLOT (FT) =
.
To determine the amount of herbicide needed for an application multiply
the recommended
rate per acre times the surface area.
Example 7. How much herbicide is needed to treat an 8.1 acre lake at the rate
of 120 pounds of
granular herbicide per acre?
.
HERBICIDE NEEDED =
.
HERBICIDE NEEDED =
.
HERBICIDE NEEDED =
Herbicide labels usually have tables from which the applicator can determine the appropriate
amount of herbicide formulation for a desired concentration and given water depth. However, it
may
be necessary to calculate the amount of herbicide to use without the use of a provided table. It
may
also be useful to calculate the (potential) concentration of herbicide in water for a given depth
when
the recommendation is given only as a surface acre application to answer environmental
questions.
Herbicide concentration in water is usually referred to in parts per million
(ppm) on a
weight:weight basis. Before determining concentration or amount of herbicide to use for a given
concentration we must first measure surface area as described above and water depth so that we
can
calculate water volume. If water depth is not uniform it is important that average depth is
measured.
In small ponds, depth should be measured across the pond in at least two directions. The number
of
different directions that will be needed will depend on the shape and bottom uniformity of the
pond
and will have to be determined on site. Greater number of depth measurements will result in
greater
accuracy. Average depth is calculated as follows:
.
AVERAGE DEPTH =
number of measurements
.
A convenient unit for measuring water volume for determining herbicide
concentration is the
acre-foot which is determined by multiplying the average depth times the surface area in acres as
follows:
.
ACRE-FEET = average depth x surface area (acres)
.
Amount of herbicide needed for a desired concentration can now be calculated by using the
following
equation:
.
HERBICIDE NEEDED (POUNDS A.I.) =
.
The number 2.7 is a constant because one acre-foot of water weighs 2,700,000
pounds.
In other words, every 2.7 pounds of a substance in one acre-foot of water is equivalent to 1
ppm.
One more calculation must be made. Because the amount of herbicide needed has been
calculated for the active ingredient, the amount of formulation needed must be determined.
For
dry
formulations (wettable powders, dry flowables, pellets and granules) the amount of active
ingredient
is divided by the percent active ingredient expressed as a decimal, i.e. if the formulation is an 80
WP
divide by 0.80. For liquid formulations divide by the amount of herbicide per gallon to
determine
gallons of formulation to apply (i.e., for a 4-pound per gallon EC divide by
4).
Example 8. A farm pond is found to be 1.6 acres in surface area and the
following depths are
measured: direction one 2, 3, 4, 4, 4, 6, 6, 4, 2 feet; direction two 2, 4, 6, 6, 6, 6, 6, 4, 2 feet.
How
much copper sulfate is needed for a copper (as elemental) concentration of 0.5 ppm, for algae
control? Hint - copper sulfate is sold as cupric sulfate pentahydrate, which is 25% elemental
copper.
.
AVERAGE DEPTH =
number of measurements
.
AVERAGE DEPTH =
18
.
AVERAGE DEPTH =
.
ACRE-FEET =
.
ACRE-FEET =
.
ACRE-FEET =
.
HERBICIDE A.I. =
.
HERBICIDE A.I. =
.
HERBICIDE A.I. =
.
Since copper sulfate is 25% elemental copper:
Pounds copper sulfate =
.25
.
Pounds of copper sulfate =
.
The concentration of herbicide in water can be calculated for a given application rate by
re-arranging the last equation so that:
.
PPM =
acre-feet x 2.7
.
Example 9 (Optional). Granular 2,4-D (20% butoxy ethanol ester) is applied
at a rate of 150
pounds
per acre to 5 acres of water 6 feet deep in a large lake to control spatterdock. A homeowner asks
if this herbicide application will be toxic to fish. Although you are confident that there is no
potential
problem because there was no caution on the label and from past experience, you can give the
homeowner concrete facts to support your answer.
A call to the County Cooperative Extension Service office reveals that the experimental toxic
concentration of this 2,4-D formulation is about 3.3 ppm to fathead minnow (a sensitive test
organism).
The maximum potential concentration of 2,4-D in the water must be calculated so that it can
be compared to the toxic level. First calculate the amount of 2,4-D active ingredient applied by
multiplying 150 pounds, the amount of formulation applied per acre, by 0.20, the percent active
ingredient expressed as a decimal. Therefore, 30 pounds (150 pounds x 0.20 = 30 pounds) a.i. of
2,4-D per acre were applied.
Before calculating the concentration of herbicide, acre feet must be calculated as follows:
.
ACRE-FEET =
.
ACRE-FEET =
.
ACRE-FEET =
.
The concentration of 2,4-D can now be calculated as follows:
.
PPM =
acre-feet x 2.7
.
PPM =
30 x 2.7
.
PPM =
.
By comparing the toxic concentration of 3.3 ppm to the maximum potential concentration of
0.37 ppm the homeowner can be told that even if the maximum concentration did occur there
would
be almost a tenfold safety factor. It can be further explained to the homeowner that this
concentration would probably not occur because the herbicide is quickly taken up by aquatic
plants,
diluted in the surrounding water and adsorbed to bottom sediments where it degrades over
time.
Some herbicide labels recommend a proportion of herbicide to final spray tank mix. For
example a label may recommend using one gallon of herbicide formulation to 20 gallons of spray
mix.
Calculate the amount of herbicide to use for spray tank volumes other than that given on the label
as
follows:
.
HERBICIDE PER TANK=
spray mix amount given
.
This equation can be used for any units as long as the same units are used on both sides of the
equation.
Example 10. A herbicide label recommends mixing 1 gallon of herbicide
formulation to 20
gallons
final spray mix and spraying vegetation to wet. How much herbicide would be used with 500
gallons
of spray mix?
.
HERBICIDE PER TANK =
spray mix amount given
.
HERBICIDE PER TANK=
20
.
HERBICIDE PER TANK =
.
Spraytank concentrations are often recommended as a specified percent solution
(volume:volume). Percent actually means parts per one-hundred. Therefore, for each 100 parts
(volume) of spray tank mix one part (volume) of herbicide would be added for each one percent
recommended. The amount of herbicide to be added to a spray tank for a recommended percent
volume:volume solution is calculated as follows:
.
HERBICIDE PER TANK =
100
.
The solution to this equation will be in what ever units the spray tank volume is in. Note: The
concentration of herbicide active ingredient in the tank will depend on the amount of active
ingredient
per gallon of formulation. Therefore, percent solutions of different formulations of the same
herbicide
may not be the same.
Example 11. Four gallons of 1.5% spray mix are to be prepared for
application to emergent
vegetation on a pond bank using a back pack sprayer. How much herbicide should be mixed in
the
spray tank before bringing up to the final volume?
.
HERBICIDE PER TANK =
.
HERBICIDE PER TANK =
.
HERBICIDE PER TANK =
.
Since containers graduated in hundredths of a gallon are rare, to say the least, the answer can be
converted to fluid ounces by multiplying by 128, the number of fluid ounces in one gallon as
follows:
.
FLUID OUNCES =
.
FLUID OUNCES =
.
FLUID OUNCES =
.
Alternatively, liquid ounces could have been used initially in the equation to obtain the same
answer
as follows:
.
HERBICIDE PER TANK =
.
HERBICIDE PER TANK =
.
HERBICIDE PER TANK =
Adjusting application equipment so it delivers the correct amount of pesticide is called
calibration. Calibration of equipment for application of herbicides to agricultural crops must be
precise because there is often a thin line between the rate at which a herbicide will damage the
crop
or carry over to a following crop. With the tractor, or special equipment mounted sprayers used
in
agriculture, precise calibration, and constant application rate is possible. However, maintaining
constant application rate is difficult when applying aquatic herbicides because the equipment is
mounted in a boat and it is difficult to maintain a constant speed and perfect course due to
environmental factors such as wind velocity and speed, water flow and vegetation density.
Although
our initial calibration of equipment for application of aquatic herbicides should be precise, the
applicators constant attention is necessary to make the application rate as constant possible.
When
applying herbicides to water, the herbicides will mix within the treatment area so that precision
becomes a little less important. Still, without any question, proper calibration and constant
attention
by the applicator is the most important component of any application.
The following materials will be needed to calibrate aquatic herbicide application
equipment:
(2) calculator (unless you are good at doing math in your head)
(3) tape measure - or known length of your steps
(4) 100-ft floating rope with a float tied to the end
(5) buoys or poles
(6) scale that will weigh up to 20 pounds
(7) blank herbicide granules
(8) enough 5-gallon buckets to measure water from each nozzle of your spray
boom, one of
these
marked or graduated in pints or quarts
(9) quart container calibrated in ounces (only if calibrating suction/induction
system)
There are various ways to calibrate equipment and any is acceptable as long as the correct end
is achieved. In any of the methods we must determine the gallons per acre
(GPA) of spray
solution
for tank-mix equipment, actual GPA of herbicide for suction systems, or
pounds per acre (PPA)
of
granular or pellet herbicide formulations.
The equations for determining GPA and PPA are:
.
GPA=
.
and
.
PPA=
.
GPM or POUNDS PER MINUTE are measured directly from the
equipment. Note: GPM will
change with viscosity of the spray solution. When using spray thickeners such as polymers GPM
should be determined with the polymer for greatest accuracy.
There are several ways to determine GPM for boom applied tank mix
equipment:
If you measure the amount from each nozzle you can determine nozzle uniformity as follows:
add the quantities from each nozzle up and divide by the number of nozzles to determine the
average
nozzle output. Then, determine the percent deviation of each nozzle by dividing the actual
nozzle
output into the difference between the average and actual output and multiplying by 100. If a
nozzle
deviates by 10-15% (depends on accuracy needed for particular type of application) check it for
clogging or size (wrong size or wear). (Alternatively, a quick check for nozzle uniformity can be
made visually.) The equation for nozzle uniformity is:
.
%DEVIATION=
.
(2) If nozzle uniformity is not a concern and hoses are long enough all hoses
can be placed into
one
bucket and the output measured and the pump run for 1-minute. Using this method, if the pump
output is greater than 5 GPM it will be necessary to run the test for less time.
For example, run
the
pump for 30 seconds and multiply the output by two for the GPM.
(3) If nozzle uniformity is not a concern the tank refill method can be used as an alternative. Put some water in the tank and run it until all air is out of the lines. Fill the tank completely and run the pump for 1 minute. Measure the amount of water that is necessary to refill the tank to determine GPM. For greater accuracy, the pump can be run for a longer period of time and the amount required to fill the tank divided by the number of minutes that the pump was run to obtain GPM.
GPM for handgun applications can be determined either by the tank refill
method or by
catching the output from the handgun in a bucket for 1 minute (or known period of time).
GPM for direct metering equipment is determined by measuring the
amount of liquid that is
drawn out of a container graduated in ounces in a minute or greater period of time. Ounces are
converted to gallons by dividing by 128 (because there are 128 ounces in a gallon).
Pounds per minute can be measured by two methods.
(2) If a container is not available to catch the output (be careful with plastic bags because they tend to get caught and tangle in the spreader's paddle) weigh a known quantity, 10 pounds is good, of blank herbicide and measure the time in seconds that it takes for all of the material to go out. To obtain pounds per minute divide the pounds that were put in the hopper by the seconds that it took to run out and multiply by 60 (number of seconds in a minute).
Now ACRES PER MINUTE must be determined. The following equation
can be used to
determine ACRES PER MINUTE:
.
ACRES PER MINUTE =
43,560
.
For ease of calculation this equation can be simplified by dividing both the top and bottom of the
equation by 88 (the number of feet traveled per minute at 1 mph). The new equation is:
.
ACRES PER MINUTE =
495
.
Since it is easy to multiply by 2 and divide by 1000 (move the decimal point three places to the
left)
this equation can be simplified further (by multiplying both top and bottom of the equation by 2
and
rounding 990 in the bottom to 1000, with the introduction of only 1% error) to:
.
ACRES PER MINUTE =
.
Swath width is measured directly from the equipment to be used. For agricultural boom
applications swath width is defined as nozzle spacing times the number of nozzles. Swath width
for
boom applications to submersed aquatic weeds can be considered the boom length or an
approximation. Often, only every other strip is treated in the treatment area and the swath width
becomes twice the boom length. It is assumed that the herbicide, with the help of turbulence
caused
by the sprayboat and environmental factors, will mix throughout the water column. Swath width
for
a handgun is measured by actually spraying water as the boat comes in toward shore and having
someone on shore mark the distance covered. Swath width of a granular spreader is measured in
a
similar fashion.
Another measurement must be made - SPEED.
To determine boat speed measure a distance at least 100 ft long in the lake using the 100-ft
floating rope and mark the distance with the buoys or poles. Operate the boat, with the spray
tank
half full and the number of persons and other gear that will be carried in the boat when actually
applying, at a comfortable speed (usually a fast idle or equivalent to a fast walking pace) and
approach the front marker. As you pass the first marker start timing, and as you pass the end
marker
note the time in seconds that it required to cover the distance. Do this in the opposite direction
and
average the two numbers to compensate for going with and against the wind. Determine MPH
by
the following equation:
.
MPH =
.
or this equation can be simplified to:
.
MPH =
.
or if you have a stopwatch that reads in tenths of a minute you can measure your time in minutes
and
use the equation:
MPH =
The drawback of using the previous equations for determining ACRES PER
MINUTE is that
speed
must be calculated in mph. An alternative "one shot equation" is:
.
ACRES PER MINUTE=
43,560 X time to cover distance (sec)
.
If a stop watch that measures in tenths of a minute is used 60 in the top part of the equation can
be
eliminated if time to cover the distance is used in minutes.
.
Rate of application can be regulated by varying equipment output, GPM or
POUNDS PER
MINUTE, or by changing the rate at which the equipment covers area,
ACRES PER MINUTE.
GPM can be varied by changing pressure or changing the size or number
of nozzles.
Changing the size or number of nozzles is used for larger changes and changing the pressure
regulator
for small changes (output will only double when pressure is increased four times).
Herbicide withdrawn by direct metering systems can only be varied by changing the metering
orifice in the suction line. There is a small difference in the output between consecutive sizes,
making
accurate calibration possible. Changes in output of centrifugal granular spreaders are made by
changing settings on the spreader.
Swath width and speed determine the rate of coverage. Changes in swath width or speed will
inversely change rate of coverage. For example if swath width or speed is doubled, rate of
coverage
or ACRES PER MINUTE will be cut in half. When spraying with a handgun,
the swath can be
varied considerably. Although actual boom length is fixed, effective swath width of a boom for
submersed weed control applications can be changed by applying in bands. Forward speed can
be
varied within a relatively narrow range. Normally the speed is 3 to 4 mph when treating by boat
at
idle speed. Greater or lesser boat speeds can be used under certain circumstances.
All of the equations, along with some alternatives, that are needed to calibrate equipment have
now been explained. The following examples should help further clarify the process of
equipment
calibration.
Example 12. Assume that a 100 gal tankmix sprayer mounted in a john-boat
is being calibrated.
The
sprayer has an 8-foot boom with four drop hoses. A herbicide is to be applied at the rate of 2
gallons
per acre for hydrilla control. A 200 foot distance is marked off and it is found that the boat
traveled the 200 ft one direction in 54 sec and the opposite direction in 56 sec. The numbers are
averaged:
2
MPH is calculated as follows:
MPH =
5,280 X time to cover distance (sec)
.
MPH =
5,280 X 54
.
MPH =
Calculate acres per minute as follows:
Acres per minute =
495
.
Acres per minute =
495
.
Acres per minute =
.
Output was measured from each drop hose for 1 minute to determine GPM and the following
values were obtained:
hose 1 - 60 fluid ounces
hose 2 - 100 fluid ounces
hose 3 - 110 fluid ounces
hose 4 - 105 fluid ounces
Obviously there was a problem with hose #1 because it is substantially lower than the others.
Deviations from the average output are calculated as follows:
.
AVERAGE OUTPUT =
4
(93.75 can be rounded off to 94 for ease of calculation)
% DEVIATION =
average nozzle output
.
% DEVIATION (hose #1) =
94
.
% DEVIATION (hose #2) =
94
.
% DEVIATION (hose #3) =
94
.
% DEVIATION (hose #4) =
94
.
Since one nozzle was very low in this example it made the average very low and several
deviations are high. However, hoses 2,3 and 4 are consistent with each other so they are
probably
okay. Removal of the nozzle from hose #1 reveals that it is clogged with pieces of a disc that
was
replaced in the pressure regulator last week. Since foreign matter is found in the system, all
nozzles
are removed and the system is flushed in order to avoid another problem. Remeasuring output
for
one minute, the following values are found:
Check these to see if they fall within 15% of the average output.
.
GPM can now be calculated by adding up the output from all four nozzles as
follows:
GPM = 115+120+110+118
.
GPM = 463 fluid ounces
.
Convert fluid ounces to gallons by dividing by 128, the number of fluid ounces per gallon, as
follows:
128 fluid ounces per gal
.
Now calculate GPA as follows:
GPA =
ACRES PER MINUTE
.
GPA =
.04
.
GPA =
.
Two gallons of herbicide are to be applied per acre and the spray volume is 90
GPA, 2 gallons of
herbicide plus adjuvants will be mixed with enough water to make a final volume of 90 gallons
and
this will treat 1 acre. But REMEMBER that this is an
approximation because environmental
conditions can change while applying and the applicator must pay constant attention to his output
and
the area being treated (i.e., when 1/4 acre has been treated 1/4 of the 90 gallon tank should have
been
applied; if not, the boat must be slowed or speeded up accordingly).
Remember that herbicide can be applied in strips and assume diffusion between the strips. In
this example every other 8 feet (swath width) could be treated to give an effective swath width of
16
feet. This will be advantageous because it will cut our spray time in half and cut our spray
volume
in half so that we will save additional time in tank mixing. It might be obvious that this change
will
double ACRES PER MINUTE and cut GPM in half but the
calculations go as follows:
.
ACRES PER MINUTE =
495
.
ACRES PER MINUTE =
495
.
ACRES PER MINUTE =
.
GPA =
ACRES PER MINUTE
.
GPA =
0.08
.
GPA =
.
Four gallons of herbicide, sufficient adjuvants and sufficient water are placed in the tank for a
total
volume of 90 gallons and this will treat 2 acres.
Example 13. A direct metering system is to be calibrated to apply herbicide
by handgun to
control
Waterhyacinth. The herbicide label requires an application rate of 2-4 quarts of herbicide per
acre
in a spray volume of 100 - 200 gallons of water. Since the label requires a specified spray
volume
two measurements must be made, output from the handgun and rate of herbicide metered into the
system.
Because this is a foliar application to waterhyacinth, it is desirable to go as slowly as possible
to insure good coverage. Idling the airboat down as low as possible, it takes an average of 114
seconds to cover the distance of 200 feet (MPH will not be used in this solution but the student
should use the equation previously given to calculate MPH for practice - it should work out to
1.2
MPH). Swath width is measured to be 15 feet and output from the handgun is
measured to be 6
GPM.
Calculate ACRES PER MINUTE as follows (using the "one shot
equation"):
.
ACRES PER MINUTE =
43,560 X time to cover distance (seconds)
.
ACRES PER MINUTE =
43,560 X 114
.
ACRES PER MINUTE =
.
Now calculate GPA as follows:
GPA =
ACRES PER MINUTE
.
GPA =
.036
.
GPA =
.
The equipment is delivering the desired spray volume of 100-200 gallon per acre.
Now measure GPM of herbicide being metered so that this can be used to
calculate GPA of
herbicide being applied. Since the rate of herbicide withdrawal is low, the pump is run for 5
minutes
while measuring herbicide withdrawal and it is found that 17 fluid ounces are withdrawn in the 5
minutes. Therefore, 17 divided by 5 or 3.4 fluid ounces are withdrawn per minute. This is
converted
to GPM by dividing by 128, the number of fluid ounces in a gallon, therefore
GPM is .027. Now
calculate GPA as follows:
.
GPA =
ACRES PER MINUTE
.
GPA =
0.036
.
GPA =
.
Because there are 4 quarts in a gallon, 0.75 gal is equivalent to 3 quarts. Three quarts per 167
gallon
will be perfect to allow for small amounts of underspray and overspray and still be within the
label
requirement of 2-4 qt in 100-200 gal of spray mix per acre.
Note: If herbicide withdrawal had been too low a larger orifice would have to be used and
conversely, if it had been too high a smaller orifice would have to be installed.
Example 14. In Example 12 herbicide withdrawal was measured and it turned
out to be the
desired
amount. It would be better to calculate beforehand the desired GPM (in fluid
ounces) that are
required for a given boat speed, swath width and application rate. Using the information from
Example 12 the fluid ounces that should be withdrawn can be calculated as follows:
Re-arrange the equation,
.
GPA =
Acres per Minute,
.
to
.
GPM = Acres per Minute X GPA.
.
Then
.
GPM =
.
GPM =
.
or
.
0.027 GPM X 128 fluid ounces per gallon = 3.5 fluid ounces per minute.
.
If small waterhyacinths were being treated and we wanted to use the low end of the label rate
or 2 quarts (0.50 gallon) it would be calculated as follows:
.
GPM =
.
GPM =
.
GPM =
.
or
.
0.018 GPM X 128 fluid ounces per gallon = 2.3 fluid ounces per minute.
Example 15. A centrifugal spreader must be calibrated to apply 40 pounds of
herbicide
formulation
per acre. Swath width was measured to be 40 feet and speed was calculated to be 2.5 MPH as in
Example 11 (it may be helpful for the student to go back and recalculate this now for practice in
working a problem completely through).
Calculate ACRES PER MINUTE using the simplified equation as
follows:
.
ACRES PER MINUTE =
1000
.
ACRES PER MINUTE =
1000
.
ACRES PER MINUTE =
.
Calculate how many POUNDS PER MINUTE are required for the
necessary 40 PPA at the
given 40 foot swath and 2.5 MPH (similar to the method that was used in
example 13 for
calculating
ounces to be withdrawn) as follows:
Re-arrange the equation,
.
PPA =
Acres per Minute
.
to
.
POUNDS PER MINUTE =
.
then
.
POUNDS PER MINUTE =
.
POUNDS PER MINUTE =
.
To calibrate the spreader a setting must be found that will deliver 8 POUNDS PER
MINUTE.
However, it is found that the lowest setting on this spreader, where clogging does not occur
delivers
more pellets than 8 POUNDS PER MINUTE. By increasing our boat speed,
which will in turn
increase ACRES PER MINUTE, a greater POUNDS PER MINUTE
can be used so that the
spreader can be calibrated. The boat is operated through the 200-foot course, at increased engine
RPM, and is timed at an average of 34 seconds (down and back).
Calculate ACRES PER MINUTE using the "one shot equation" (the
student should also
calculate MPH and use alternative equations for practice and to gain
confidence) as follows:
.
ACRES PER MINUTE =
43560 X time to cover distance (sec)
.
ACRES PER MINUTE =
4350 X 34
.
ACRES PER MINUTE =
.
Calculate the POUNDS PER MINUTE that will result in 40 pounds per
acre and 40 foot
swath at the new speed as follows:
.
POUNDS PER MINUTE =
.
POUNDS PER MINUTE =
.
POUNDS PER MINUTE =
.
To calibrate the spreader at the new speed a spreader setting that will deliver 12
POUNDS PER
MINUTE must be found. It is helpful to determine spreader output (in
POUNDS PER
MINUTE)
through a range of setting on the spreader to refer to for future applications at various rates and
speeds. The following outputs are determined for the spreader in this example:
.
| Spreader setting | Pounds per Minute |
|---|---|
| 1 | clogs |
| 2 | clogs |
| 3 | 10 |
| 4 | 15 |
| 5 | 20 |
.
It is not necessary to move the spreader setting small increments because variables such as boat
speed
that will vary during the application will have a greater effect on PPA than
small changes that
can be
affected with the spreader setting. In this example a spreader setting between 3 and 4 will give
the
desired 12 POUNDS PER MINUTE, which will in turn result in the required
application rate of
40
PPA at the measured speed.
REMEMBER that calibration is the first approximation of
delivering the correct application
rate. The applicator must pay constant attention to maintain constant speed and continually be
aware
of area covered and amount of herbicide applied.
Note: Spreader output will be different at given settings for different sized particles. Separate
calibration tables must be made for different sized pellets and granules.
Acres per tank =
GPA
.
Herbicide formulation per tank =
.
ACRE-FEET OF CANAL =
43,560 ft2/acre
.
ACRE-FEET OF CANAL =
43,560 ft2/acre
.
Volume of a rectangular tank (ft3) =
.
Volume of a rectangular tank (gal) =
.
Volume of a cylindrical tank (gal) =
.
Speed (mph) =
88
.
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