The Catch Test That Read 72 Ounces, With Rain Coming
A pull-type sprayer with a 60-foot boom (36 nozzles on 20-inch spacing) and a 500-gallon tank is set up for 15 GPA at 10 mph. After the first tank, the applicator runs a one-minute catch test from one nozzle at operating pressure and collects 72 ounces. The product is a preemergence herbicide labeled at a maximum of 2.0 pints/A with a 15 GPA carrier. Wind is 12 mph from the west, temperature is 88°F, the field’s east edge is a cattail slough, and there is a shelterbelt gap on that side. Thunderstorms are forecast for late afternoon and he wants to finish before they arrive.
1. What should the catch test have read, and what does 72 ounces tell you about the sprayer?
Read the worked answer
Compute the expected output, then convert the error into acres and product.
Expected flow per nozzle: GPM = (GPA × mph × spacing in inches) ÷ 5,940 = (15 × 10 × 20) ÷ 5,940 = 0.505 gpm = 64.6 oz per minute.
- 72 oz ÷ 64.6 oz = 1.115, about 11.5% high.
- Actual output: 15 × 11.5% ≈ 16.7 GPA.
- Tank coverage: 500 gal ÷ 16.7 = 29.9 acres, not the 33.3 acres the plan assumed.
- Product at 2.0 pt/A: 29.9 × 2.0 = 59.8 pt in the tank. If he filled for 33.3 acres (66.6 pt), he would be applying 2.23 pt/A - over the labeled maximum.
Diagnosis before blame: worn nozzle tips (most likely, and it is only one nozzle checked), pressure above the gauge setting, or ground speed lower than 10 mph. Check pressure and speed first, then catch-test several nozzles across the boom. If nozzles vary more than about 10% from the average or from new, replace the set - not just the one that read high, because the rest are the same age.
Rule: recalibrate, then refigure the tank mix to the true GPA. Never “top up” product to force the original tank acres.
2. Given the wind, temperature, slough, and forecast, should he spray this afternoon? Reason it through.
Read the worked answer
No - the forecast, not the calendar, decides this one.
- Wind: 12 mph already exceeds the wind limit on many labels for this kind of product and is moving from the west, straight toward the slough and the shelterbelt gap. Read the label’s wind restriction - it is a hard number, not a suggestion.
- Temperature: 88°F raises volatility and evaporation of fine droplets, and hot afternoons are when inversions begin to set up. A surface inversion would hold droplets and vapor in the field and move them in an unpredictable direction at dusk - exactly when the storm arrives.
- Approaching storm: gust fronts ahead of a thunderstorm shift wind direction and speed abruptly, often within minutes. Spraying into an oncoming front is how drift complaints are made.
- Sensitive area: a cattail slough is surface water and a wildlife habitat; the label buffer applies. The shelterbelt gap is a direct path for drift.
What he should do instead: wait for a morning with steady wind below the label limit, moving away from the slough, moderate temperature, and no inversion. Use a coarse, drift-reducing nozzle at a labeled height, keep the buffer on the slough, and do not treat “just the east passes” as a shortcut. A preemergence that goes on two days late is a scheduling problem; one that goes on ahead of a storm is a complaint, a crop injury claim, and possibly a label violation.
3. Suppose he had already sprayed 40 acres with the worn setup before catching the error. What now?
Read the worked answer
Contain the consequences, document everything, and stop compounding the error.
- Stop and recalibrate before any more product is loaded. The remaining acres should go on at the true, labeled rate.
- Quantify the overage. At 16.7 GPA he applied about 2.23 pt/A on those 40 acres - roughly 11.5% over the labeled maximum. Note the actual acres, product, rate applied, date, and time.
- Check the label for a maximum annual rate per acre. If the season total is now near or over the cap, no further application of that product on those acres - that decision is made on paper before the sprayer moves.
- Assess crop risk with the dealer or manufacturer agronomist; a preemergence overage may mean injury, stand loss, or a replant restriction. Get the recommendation in writing.
- Do not blend the over-applied ground into a field-average record. Map the sprayed blocks so the error area can be identified later if injury appears.
- Fix the machine: replace the nozzle set, verify pressure and speed, and keep the calibration record with the sprayer so the next operator starts from a known baseline.
The applicator’s own record - what happened, what was done, who was told - is what protects him. An undocumented 11.5% overage looks far worse later than it does today.