The old adage “if a little is good, more is better” doesn't apply to crop protection chemicals.

When it comes to weed-control products, you need to be fairly precise in measuring, mixing and applying. Too little active ingredient applied runs the risk of weed escapes. Too much wastes money and may cause crop injury.

“You've heard it before: Read the label,” says Bill Casady, University of Missouri (MU) ag engineer. “The chemical manufacturer knows how his product performs in different conditions, and the label gives the recommended range of application rates for this specific formulation.

“When you're mixing chemicals, keep in mind your allowable error and measure accordingly,” Casady adds. “If you have to respray, that error costs 100%, whether the error was due to faulty calculations or poor sprayer calibration.”

Calculations for mixing liquids are different from calculations for dry materials, Casady adds. He suggests these steps:

Liquid pesticides

Usually, the label lists application rates per acre in pints or quarts, which simplifies mixing calculations. However, some may show the application rate in pounds of active ingredient, usually per gallon. In this case, it's necessary to calculate the volume of chemical to apply per acre.

For example, say you want to apply 1.5 lbs/acre of 2,4-D and the label says the chemical contains 4 lbs of active ingredient per gallon:

Divide 1.5 lbs/acre by 4 lbs/gallon to equal 0.375 gallons/acre, or 3 pints/acre.

If you want to apply 20 gallons of spray per acre, and your sprayer tank holds 500 gallons, a full tank will spray 25 acres. Using the above example, you'll need to mix 75 pints or 9.37 gallons of 2,4-D in the tank. Note: If your sprayer uses two or more tanks, remember to divide the chemical proportionally among the tanks.

Dry materials

Some chemicals are sold as dry materials (powders or granules) to be mixed with water. The label usually recommends units of weight (ounces or pounds) per acre, with the amount of active ingredient shown as a percent.

Recommended rates are shown as either pounds of active ingredient or pounds of total product. If the rate is shown as pounds of product, you simply calculate the number of acres sprayed by each full sprayer tank. If the rate is listed as pounds of active ingredient, you may need to do some intermediate math to determine the concentration of active ingredient in the formulation.

Say you want to apply 1.5 lbs/acre of atrazine and the label shows the active ingredient at 80% concentration. The weight of atrazine to apply per acre is the application rate (lbs/acre) divided by the percent concentration:

Multiple 1.5 lbs/acre times 100, divide by 80 to equal 1.875 lbs/acre.

To calculate the number of acres sprayed by a full tank, follow the procedure outlined for liquid pesticides.

“Always partially fill the sprayer tank with water before adding pesticides,” says Casady. “And keep in mind that some elements will require sprayer re-calibration every now and then.”

In what order should materials go into the tank?

“As a rule of thumb, put the hard-to-mix stuff in first, and the easiest-to-mix elements in last,” suggests Andy Kendig, MU extension weed specialist, who recommends this order: water, fertilizers (when the recipe calls for urea ammonium nitrate or ammonium sulfate), wettable powders, dry flowables, emulsifiable concentrates (they turn white in water), water soluble products, and, finally, any surfactants and crop oil concentrates.