Calibrating Insecticides in Row Crops

Every year I get a number of calls about calibration of sprayers. Proper calibration can mean the difference in control or failure of a product against the targeted pests and potentially thousands of dollars of savings to the grower. I know that many growers are now using rate controls, but there is a need to check these occasionally to make sure they are delivering what they say they are delivering. I am going to address a number of common questions and do my best to make this as simple as possible. Some will be very elementary but I think there will be many that may pick up on something useful they did not understand.

What is “A gallon to X acres”?  I get this a lot from students just entering the industry. Most are not taught this in school but it is the most commonly used expression of rates among growers and consultants. It is very common in the agricultural world to hear 1 to 40 or 1 to 90 or 1 to whatever.  This simply means that one gallon of the product will cover X number of acres. This likely came about by the ease of knowing how much chemical to purchase when expressed this way. If you want turn a grower off quickly, start quoting rates in pounds of active ingredient per acre to him.

Example 1:  A farmer wants a use rate of 1 to 32 (1:32). Again, this simply means one gallon of the product will cover 32 acres. If you want to know what the actual use rate of the product is in ounces per acre for a 1:32 rate, just divide 128 (ounces in 1 gallon) by 32= 4. So the grower is applying 4 oz. of product per acre. Conversely, if you know the use rate in ounces of product per acre, you can simple divide 128 by the use rate in ounces (in our example it is 4) = 32.  Now you’re back to 1:32.

Example 2: You can also use the active ingredient rates to get the same answers. For example:  If you have a 4EC (product contains 4 lbs. active ingredient per gallon of formulated product) product and the use rate is 0.04 lbs. a.i per acre.  You can simply divide the 4EC by the use rate of 0.04 a.i per acre and get 100.  So in this example, the rate would be 1:100. If now I want to know how many ounces that is, again divide 128 by 100 and you get 1.28 oz. of actual product per acre. So in this example 1:100 = 0.04 a.i lbs. acre = 1.28 oz. of product.

Calibrating broadcast sprayers: There are many ways to calibrate sprayers. I am going to share one that I think is the simplest. Here is a formula that you need to know.

xxxxxxxxMPH x NSI x GPA

GPM = —————————-

xxxxxxxxxxxxx5940

GPM= Gallons per minute

MPH= Miles per Hour

NSI= Nozzle Spacing in Inches

5940= this is a constant

What this formula tells you is how many gallons you would have to catch in one minute (GPM) going X miles per hour (MPH) with X nozzle spacing (NSI) to deliver X gallons per acre (GPA).

Here is an example: A farmer wants to drive his sprayer 5 MPH.  His broadcast boom has each of his nozzles on 20 inch spacing and he wants to put out 10 gallons per acre.

xxxxxxxx5 x 20 x 10                              1000

GPM = ———————     =                    ——-      =         0.16835 (Gallons Per Minute)

xxxxxxxxxxx5940                                  5940

So you would have to catch 0.16835 gallons in 1 minute to deliver a rate of 10 gallons per acre driving 5 miles per hour with your boom set up on 20 inch nozzle spacing. Since measuring 0.16835 gallons is not really feasible, we should convert that number into a smaller unit like ounces. Since there are 128 oz. in one gallon, we can simply multiply 0.16835 by 128 and we get 21.5 ounces we need to catch in one minute. Most people will also now take the 21.5 oz. and divide it by 2 so they only have to catch for 30 seconds rather than 1 minute (11 oz.).

Now we have a rig calibrated to deliver 10 gallons per acre and we got a rate of 1:32 we want to put out. How much product do I pour in the tank?  Think about it like this, if we are delivering 10 gallons per acre and we know 1 gallon will cover 32 acres, we need 320 gallons of water to spray the 32 acres.  So every time you put 320 gallons in the tank you need to add 1 gallon of the insecticide. If you have a 500 gallon tank just divide 500 by 320 and this tells you that if you filled the tank up you would need to put 1.6 gallons in the 500 gallon tank.

Another way to think about this is using the ounces of product per acre rate. In our example of 1:32, this = 4 oz. of product per acre. Since we are calibrated at 10 gallons per acre, for every 10 gallons of water you put in the tank you need to add 4 oz. of insecticide. In our 500 gallon tank example you would have to add 200 oz. of insecticide (500/10 = 50 and 50 x 4oz. = 200 oz.). To get this number back to gallons you would divide by 128 = 1.6 gallons again.

Calibrating banded applications:  The calibration of banded insecticide applications can sometimes be very confusing. To calibrate your rig use the same formula we used before except substitute your band width in inches (BWI) you desire to use for the nozzle spacing in inches (NSI). Everything else is the same.

xxxxxxxMPH x BWI x GPA

GPM = —————————-

xxxxxxxxxxxxx5940

Let’s assume you are on a 38” row and you want to deliver a 50% band behind the planter for cutworm control and you want to apply 5 GPA. Since 50% of 38 is 19 (.5 x 38 = 19), this goes in the BWI part of the formula.  Also, you will be planting at 4 MPH so this would be your speed since you are spraying off the back of the planter. The formula would look like this.

xxxxxxxxx4 x 19 x 5                             380

GPM=  ———————-     =                ——-       =          0.06397 (Gallons Per Minute)

xxxxxxxxxxx5940                                 5940

Again to get 0.06397 in ounces multiply by 128 and you get 8.2 ounces you need to catch in one minute (or 4.1 oz. in 30 seconds) from one nozzle to deliver 5 gallons per acre on your 19 inch band at 4 MPH.

Here is where some of the confusion comes in. Let’s go back to our 1:32 or 4 oz. of product per acre example. You will still mix everything exactly the same as in the broadcasts example.  Since we are delivering 5 gallons per acre this time, for every 5 gallons of water in the tank, you add 4 oz. of product. Or for every 160 gallons of water in the tank you will add one gallon (5 x 32 = 160). Here is the difference. On the 19 inches (the treated acre) you are spraying, you are delivering 5 gallons per acre and 4 oz. of product but on a land acre you are only putting out 2 oz. of product and 2.5 gallons of water because it is a 50% band. Remember, you are applying the exact same rate of product and gallons per acre to the area you are treating as you would be if you were treating a broadcast acre but you will go 50% farther before your tank runs out. For instance, since it is a 50% band your tractor will cover 2 land acres for every 5 gallons of water. If someone asks you your rate it is 4 oz. per acre not 2 oz.

Multiple nozzles per row: It is not uncommon to have more than one nozzle contribute to a band. We see this often times early in the season making spider mite or thrips sprays on small cotton.  This is useful if you want to get better coverage. If you have multiple nozzles per row it has to be accounted for in your calibration. For example, if you have 2 nozzles contributing to a 10 inch band, you can catch from one nozzle to calibrate, but you have to multiply the answer by 2 since each nozzle is assumed to deliver equal amounts. All other calculations are the exact same as the banded example. Calibrations get more complex with some of the multi-nozzle hoods.

In-furrow calibrations: In furrow calibrations are not difficult but are calculated a little differently. One of the main differences is with in-furrow spray calibration is you use the row width in inches in the NSI part of the formula. In-furrow sprays are common for fertilizers and insecticides. Many insecticides used in-furrow are recommended at use rates of ounces of product per 1000 row feet rather than ounces of product per acre. This is because when you use ounces of product per 1000 row feet, you still deliver the exact same rate of product across a variety of row spacing in the furrow. However, as row spacing narrows it takes more product per acre to keep the rate per 1000 row feet the same.

Here are some examples with Capture LFR we run into this season. The table below shows how many ounces of product per acre you need to apply at several row spaces to keep the rate constant at 0.25 oz. of product per 1000 row feet.

You can see from the example above, to keep the rate per 1000 row ft. equivalent; you would have to actually mix for different rates per acre depending on your row spacing. If you are on a 38” twin row, this would be the same as 19” row spacing in the table since each row is delivered independently from each other. So it would cost you twice as much to apply the material as it would somebody on a single 38” row keeping the rate the same at 0.25 oz. per 1000 row ft.

To figure this out by hand you must first know how to figure out how many row feet are in an acre on a given row spacing. Example: 38” rows = 3.167 ft. (38″ divide by 12″ to put in feet). There are 43560 sq. feet in one acre so if you divide 43560 by 3.167 you get 13756 linear feet in one acre on 38” rows. Since the rate is 0.25 oz. per 1000 row feet, I need to know how many 1000 row feet are in my row spacing. Simply divide 13756 by 1000 and you get 13.756.  Now simply multiply that by the 0.25 and you get 3.4 oz. of product per acre has to be mixed for to deliver 0.25 oz. of product per 1000 row feet on a 38” row.