6.3 Mixing, Loading, and Application-Rate Math

6.3 Mixing, Loading, and Application-Rate Math

Quick Answer: The label gives a rate in product per acre (for example, 1 qt/A). Total product = acres x labeled rate. Total spray solution = acres x GPA. Adjuvant is added as a percent of finished spray. Always add product in the label's mixing order, and never pour water onto concentrated pesticide.

Reading Label Rates

Pesticide labels state the application rate as product per acre. Common units:

Some labels also state the rate as pounds of active ingredient per acre (lb ai/A). To convert between product rate and active ingredient rate, use the percent active ingredient printed on the label.

Calculating Product Needed for a Field

Total product = acres x labeled rate per acre.

Worked Example 1: Liquid Herbicide

• Field: 40 acres
• Label rate: 1.5 qt/A
• Total product = 40 x 1.5 = 60 qt = 15 gallons

Worked Example 2: Dry Flowable

• Field: 25 acres
• Label rate: 8 oz/A (dry)
• Total product = 25 x 8 = 200 oz dry = 12.5 lb

Total Spray Solution and Carrier Volume

The total spray solution the sprayer carries to the field is acres x GPA. The carrier (usually water) is the total spray solution minus the product volume.

Worked Example 3: Full Tank Mix

• Field: 40 acres
• Label rate: 1 qt/A herbicide
• Calibrated GPA: 15
• Total spray solution = 40 x 15 = 600 gallons
• Total herbicide = 40 x 1 qt = 40 qt = 10 gallons
• Carrier water = 600 - 10 = 590 gallons

If the tank holds 300 gallons, you will make two batches of 300 gallons each, each treating 20 acres and each containing 20 qt (5 gallons) of herbicide plus 295 gallons of water.

Adjuvant Rates

Adjuvants (surfactants, oils, drift control agents) are added as a percent of finished spray volume (% v/v) or as a percent of spray volume. Convert percent to a volume per gallon:

fl oz adjuvant per gallon = (% x 128) / 100

Worked Example 4: Surfactant

• Label: add nonionic surfactant at 0.25% v/v
• Per gallon: (0.25 x 128) / 100 = 0.32 fl oz/gal
• For a 300-gallon tank: 300 x 0.32 = 96 fl oz = 0.75 gallon of surfactant

Crop oil concentrate (COC) labels often call for 1% v/v, which is 1.28 fl oz/gal, or 1 gallon per 100 gallons of spray.

Percent Active Ingredient

The percent active ingredient (% ai) printed on the label tells you how much of the formulated product is the active chemical. To compare two formulations or to hit a lb ai/A recommendation:

lb ai/A = (product rate, lb/A) x (% ai / 100)

Worked Example 5: Comparing Formulations

• Product A: 4 lb/gal emulsifiable concentrate, 4 lb ai per gallon
• Product B: 2 lb/gal aqueous solution, 2 lb ai per gallon
• Label calls for 1 lb ai/A
• Product A rate = 1 / 4 = 0.25 gal/A = 1 qt/A
• Product B rate = 1 / 2 = 0.5 gal/A = 2 qt/A

The same lb ai/A needs twice as much Product B because it is half as concentrated.

Dilution Calculations

Dilution problems ask how much of a concentrated product to mix to reach a target concentration. Use the mass-balance relation:

C1 x V1 = C2 x V2

where C is concentration and V is volume, with product 1 the concentrated source and product 2 the finished spray.

Worked Example 6: Dilution

You need 100 gallons of a 1% spray solution from a 50% concentrated product.

• C1 = 50%, C2 = 1%, V2 = 100 gal
• V1 = (C2 x V2) / C1 = (1 x 100) / 50 = 2 gallons of concentrate
• Carrier = 100 - 2 = 98 gallons of water

Tank-Mix Math with Multiple Products

When the label allows a tank mix, sum each product's per-acre amount first, then subtract the total product volume from the spray solution to find the carrier.

Worked Example 7: Two-Product Tank Mix

• Field: 30 acres; GPA: 10
• Herbicide A: 1 qt/A; Herbicide B: 0.5 pt/A; Surfactant: 0.25% v/v
• Total spray solution = 30 x 10 = 300 gallons
• Herbicide A = 30 x 1 qt = 30 qt = 7.5 gallons
• Herbicide B = 30 x 0.5 pt = 15 pt = 1.875 gallons
• Surfactant = 300 x 0.32 fl oz/gal = 96 fl oz = 0.75 gallon
• Total product = 7.5 + 1.875 + 0.75 = 10.125 gallons
• Carrier water = 300 - 10.125 = 289.875 gallons

Mixing Order and Safety

Always follow the label's mixing order. A common sequence is:

1. Start agitation and add half the carrier.
2. Add dry formulations (wettable powders, dry flowables, water-dispersible granules).
3. Add liquids (flowables, emulsifiable concentrates, soluble liquids).
4. Add adjuvants (surfactants, oils).
5. Add the rest of the carrier and keep agitating.

Never add water to concentrated pesticide; always add pesticide to water. Keep agitation running during spraying so suspensions do not settle. Mix and load on a pad or away from wells and surface water to prevent contamination of Iowa groundwater.