Effect of Speed on Application Rate

This is the relation people get wrong in the field, in the moment, with the sprayer already moving. Nozzle output does not know how fast the tractor is going. If the pressure is unchanged, each nozzle keeps delivering the same litres per minute while the ground passing beneath it doubles, so the rate applied per hectare halves. Rate and speed are exactly inversely proportional.

The consequences are larger than they look. A five percent speed increase — the difference between 8.0 and 8.4 km/h, which no one would notice — moves the applied rate by five percent. Most pesticide labels permit something like ±5% on rate, so a driver who speeds up slightly to finish before the wind gets up has quietly taken the application outside its label. Speeding up by a fifth, from 8 to 10 km/h, applies 20% less than calibrated, and an underdosed herbicide does not merely fail: it applies selection pressure at a sublethal dose, which is one of the recognised routes to resistance.

Rate controllers exist precisely to break this relation. They monitor real ground speed and modulate pressure to hold the rate constant, which works — within the limits of the previous formula. Because flow follows the square root of pressure, holding rate constant across a 2:1 speed range demands a 4:1 pressure range, and no nozzle sprays acceptably across that span. This is why pulse-width modulation systems appeared: they hold pressure and spray quality fixed and vary the duty cycle of a solenoid at each nozzle instead, which is the only honest way to get a wide rate range from one tip.