Application Equipment, Calibration, and Application Techniques
Equipment basics
Nozzle choice sets droplet size and pattern. Flat-fan nozzles suit broadcast herbicide and preemergence work; even flat-fan gives a uniform pattern for bands; hollow-cone produces fine droplets for foliage and insecticide work; drift-reduction nozzles produce coarser droplets. Higher pressure means smaller droplets and more drift. Worn nozzles (usually more than about 10% above rated flow) cause uneven patterns and over-application; replace them as a set.
Calibration
Calibration matches output to the labeled rate. The standard boom-sprayer equation is:
GPA = (GPM × 5940) ÷ (MPH × nozzle spacing in inches)
Measure nozzle output in gallons per minute (average several nozzles), measure ground speed, and read the spacing off the boom. The 1/128-acre method is a field shortcut: 1/128 acre is 340 ft², so for 20-inch spacing you treat 204 linear feet.
Worked example
A sprayer with 20-inch nozzle spacing, a nozzle averaging 0.4 GPM, traveling 4 MPH: GPA = (0.4 × 5940) ÷ (4 × 20) = 2376 ÷ 80 = 29.7 GPA (about 30 GPA). If the label calls for 20 GPA, the applicator must increase speed, reduce pressure, or switch to a smaller nozzle - and re-check the pattern.
Application technique and traps
Maintain correct boom height for overlap; check the pattern with water on a flat surface. Time applications to labeled growth stages and weather (wind speed, temperature, humidity, inversions). Avoid inversions - fine droplets can hang and move far. Clean equipment between crops to prevent carryover injury, and remember the label's grazing/harvest restrictions for treated forage.
Trap: mixing units (GPM vs GPA, feet vs inches, MPH vs ft/min) is the most common exam error. Write the units down.