Boom Calibration, a Bad Nozzle, and a Resistance Problem
An applicator is calibrating a boom sprayer with eight nozzles on 20-inch spacing and a 300-gallon tank, targeting a post-emergence broadleaf herbicide at 1.5 pints per acre. He drives 200 feet in 30 seconds. Catching each nozzle for 30 seconds, he records 27, 28, 28, 27, 22, 28, 27, and 28 ounces. The field has been sprayed with the same Group 4 product at the same rate three times since April, and a few weeds have survived each time. The label requires a nonionic surfactant at 0.25% v/v and prohibits application above 85°F or when wind is below 2 mph or above 10 mph. The forecast is 88°F with 3 mph wind and a temperature inversion expected to hold until 9 a.m.
1. Calculate gallons per acre and the tank mix for a full 300-gallon load, then interpret the 22-ounce nozzle.
Read the worked answer
Speed. 200 ft ÷ 30 s = 6.67 ft/s = 4.55 mph.
Average output. The eight readings total 215 oz over 30 s = 26.9 oz per nozzle per 30 s = 53.8 oz/min = 0.42 gpm.
GPA. GPA = (gpm × 5940) ÷ (mph × nozzle spacing in inches) = (0.42 × 5940) ÷ (4.55 × 20) = 2495 ÷ 91 = about 27.5 GPA
Tank mix (300 gal).
- Acres per tank = 300 ÷ 27.5 = 10.9 acres
- Product = 1.5 pt/ac × 10.9 ac = 16.4 pt ≈ 2 gallons plus 0.4 pint
- Surfactant = 0.25% of 300 gal = 0.75 gallon (96 fl oz)
The 22-ounce nozzle. The mean is 26.9 oz; 22 oz is about 18% below it. A tip that flows less than its neighbors is partially plugged, damaged, or the wrong size - wear normally increases flow, so this is not ordinary wear. At 18% low, both rate and pattern are off: the strip under that nozzle is under-dosed and the pattern is distorted, which is a likely contributor to the weed escapes. Replace the tip, re-check the whole boom against a new tip, and recalibrate. Do this before mixing any product.
2. The forecast is 88°F with 3 mph wind and an inversion until 9 a.m. Do you spray? Reason it through.
Read the worked answer
No - not on that forecast. Two independent problems, either one sufficient:
- Temperature. 88°F exceeds the label's 85°F ceiling. That is a flat prohibition, not a guideline. High heat also increases volatility of ester formulations and increases the chance of vapor drift after the spray has landed.
- Wind and inversion. Three mph is technically inside the label window, but combined with a temperature inversion it is the worst case: the air is stable, fine droplets do not disperse or mix upward, and they hang and travel laterally until the inversion breaks. When it breaks, that concentrated cloud comes down somewhere - often on a garden, a tomato planting, or a nursery crop, all of which are highly sensitive to a Group 4 herbicide.
What to do instead: wait until the inversion has broken and the temperature has dropped below 85°F - typically late afternoon or evening, once mixing has begun and wind is steady between roughly 3 and 10 mph. Re-check the wind at the boom height, not at the truck. If it is still 88°F at dusk, spray another day. Open water, houses, and sensitive broadleaf plantings within the drift window should be scouted before the decision, not after.
3. A few weeds survived each of the last three Group 4 applications. What is the resistance-management response, and why is raising the rate the wrong answer?
Read the worked answer
First, separate escapes from resistance. The 22-ounce nozzle has been under-applying a strip, and calibration was never verified. Survivors in a pattern that follows a nozzle, a wheel track, or a field edge are application failures, not resistant biotypes. Fix the equipment and re-evaluate before blaming biology.
If survivors are scattered at random and survived full-rate applications:
- Three sequential applications of the same Group 4 mode of action in one season is textbook selection pressure. Any naturally tolerant individuals are the ones setting seed, and the population shifts.
- Do not raise the rate. A higher rate of the same site of action kills the same susceptible plants and simply selects harder against the intermediate ones; it also moves further from the label and adds cost and carryover risk. Resistance to synthetic auxins is not reversed by rate.
- Rotate to a different mode-of-action group with activity on the same weed, and use a tank-mix partner from another group so both mechanisms hit the same plant.
- Layer non-chemical tactics: crop or turf competition, mowing timing before seed set, fertility, and mechanical removal of the few survivors before they seed. Hand-rogueing a handful of escapes is cheap compared with losing the chemistry.
- Clean the equipment afterward - residue in the boom can injure the next sensitive crop.
Document the field history and the mode-of-action groups used; that log is what an inspector or agronomist will ask for.