Herbicide, Wind and a Neighbour's Vegetables
A farmer must apply a pre-emergence herbicide to 0.6 ha of cotton. The label states 2.0 L of product per hectare in 200 L of water. His knapsack sprayer holds 15 L. With a flat-fan nozzle he measures an output of 1.2 L/min at his working pressure, and he plans an effective swath of 0.5 m while walking at 4 km/h. A neighbour's vegetable plot lies 30 m downwind. The forecast is 12 km/h wind gusting to 20 km/h, and rain is expected within 24 hours. He has one day available before the rain. He intends to fill the tank, add 2.0 L of product because 'that is the per-hectare rate', and spray the whole field before the rain arrives.
1. Calibrate the sprayer. Calculate the actual application volume, the speed needed for 200 L/ha, and the product dose per 15 L tank at the correct volume.
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Use: L/ha = (600 × output in L/min) ÷ (speed in km/h × swath in m).
Actual volume: (600 × 1.2) ÷ (4 × 0.5) = 720 ÷ 2 = 360 L/ha.
That is 1.8 times the label volume, so his intended plan would over-apply product by 80%.
Speed needed at 0.5 m swath: speed = 720 ÷ (200 × 0.5) = 720 ÷ 100 = 7.2 km/h. That is an unrealistic walking pace for accurate spraying.
Swath needed at 4 km/h: swath = 720 ÷ (200 × 4) = 0.9 m. A single flat-fan nozzle does not give a reliable 0.9 m swath, so this is also impractical.
Practical solution: either fit a smaller nozzle that delivers about 0.67 L/min, or accept 360 L/ha and adjust concentration. At 360 L/ha, product per litre = 2.0 ÷ 360 = 5.56 mL/L, so a 15 L tank needs about 83 mL. At 200 L/ha, a 15 L tank needs 150 mL and covers 0.075 ha.
Tank count at 200 L/ha: 0.6 ha × 200 = 120 L, which is 8 tanks. At 360 L/ha it is 216 L, or about 14.4 tanks. Either way, adding 2.0 L to one tank is a serious overdose.
2. Should he spray today? Give a reasoned decision covering drift, timing and the rain forecast.
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No, not under these conditions.
Drift: 12 km/h with gusts to 20 km/h is too windy for a knapsack sprayer with a flat-fan nozzle, especially with vegetables 30 m downwind. Gusts carry fine droplets much further than the average wind suggests, and herbicide damage to a neighbour's vegetables can be severe, costly and a legal liability. A flat-fan nozzle at normal pressure produces a high proportion of driftable fines.
Timing versus rain: pre-emergence herbicides need to be applied to a clean, moist soil surface before weeds emerge. If heavy rain falls soon after application, the product may run off or be leached below the weed germination zone, wasting the spray. He should check the label's rainfastness interval rather than guessing.
Better plan: wait for a calm period, ideally early morning when wind is below about 5–8 km/h and not gusty. Use a low-drift or air-induction nozzle, keep the boom low, maintain a buffer strip of at least 10–20 m from the neighbour's plot, and spray when the soil surface is moist but rain is not imminent. If the rain arrives before a calm window, delay planting or use a different weed-control approach rather than spraying in unsafe conditions.
3. Describe how he should verify, in the field, that the sprayer is delivering the intended dose, and what records to keep.
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Field check before spraying:
- Measure the actual walking speed over 50 m with a stopwatch while carrying the sprayer, because a loaded knapsack slows walking.
- Measure the swath by spraying water onto dry concrete or a strip of paper and measuring the wetted width.
- Catch nozzle output for one minute in a measuring jug and repeat three times; use the average.
- Recalculate L/ha with the measured values, then calculate product per tank.
- Spray a small test strip and check for streaks or misses before treating the whole field.
During spraying: keep pressure and pace constant, overlap passes consistently, and note tank start and finish times.
Records: date, field, crop stage, product name and batch, rate per hectare, water volume, number of tanks, nozzle type, weather (wind speed and direction, temperature), operator, and the buffer distance left. Also record any incidents such as a gust towards the neighbour's plot.
Why it matters: records allow him to trace poor weed control or crop injury to a cause, demonstrate responsible practice if a neighbour complains, and repeat a successful programme next season. Without them, calibration is a one-off exercise rather than management.