Knapsack calibration, a worn nozzle and 2,4-D at the wrong stage
Gurmeet Kaur has 1.6 ha of wheat sown on 12 November. At 50 days after sowing, with the crop already jointing, she sprays 2,4-D sodium salt (80% WP) to control bathua and hirankhuri that have grown above the canopy. The label dose is 1.0 kg product per hectare in 320 L of water. Her knapsack holds 16 L; she walks at about 3 km/h with an effective swath of 0.75 m, refilling whenever the tank empties and mixing each tank at label concentration. The nozzle is two seasons old and now delivers 1.8 L/min where a new one delivers 1.2 L/min. She sprays between 1 pm and 3 pm at 33 °C with a light breeze blowing towards a neighbouring berseem field. Five days later the wheat shows twisted, fused and rolled leaves in broad overlapping streaks, worst along the headlands and where she stopped to refill; the crop is late and uneven, and the berseem next door has curled leaf tips.
1. Calculate the spray volume and product dose she actually applied, and explain the streaked injury.
Read the worked answer
Area covered per minute = speed × swath = 50 m/min × 0.75 m = 37.5 m²/min.
With a new nozzle: 1.2 L/min ÷ 37.5 m²/min = 0.032 L/m² = 320 L/ha - correct.
With the worn nozzle: 1.8 ÷ 37.5 = 0.048 L/m² = 480 L/ha, i.e. 1.5 times the intended volume.
Dose: she mixed each tank at label concentration (1.0 kg per 320 L ≈ 3.1 g/L). Delivered dose = 480 × 3.1 g ≈ 1.5 kg product/ha - a 50% overdose, applied to a crop at jointing, the most sensitive stage.
Why streaks: with a single nozzle and a 0.75 m swath she must overlap passes. Where passes overlapped, where she slowed at headlands and refill stops, and where the spray was held over already-treated strips, the local dose reached 2–3 times normal - hence broad bands of twisted, fused leaves separated by comparatively normal wheat. The injury pattern is a calibration and overlap record, not a random disease.
2. Identify the agronomic and operational errors and give the correct practice for each.
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- Wrong stage. 2,4-D belongs at 30–35 days after sowing, before jointing. At 50 days the growing point is above the soil and floral initiation has begun, so the hormone disrupts the ear and causes sterility and distortion. Correct practice: spray on stage, not on convenience; if jointing has started, use a herbicide cleared for that stage instead.
- Worn nozzle, no calibration. A 50% over-application came from a ₹100 part. Correct practice: replace nozzles, then calibrate on 50 m using L/ha = (L/min × 600) ÷ (km/h × swath in m) and fix the area sprayed per tank.
- Weather and drift. Spraying at 33 °C in the afternoon with a cross-breeze carried fine droplets onto the berseem, causing the curled tips. Correct practice: early morning or evening, wind below about 8 km/h, low pressure and coarse droplets, a buffer strip and a drift guard.
- Weeds above the canopy. Large weeds shield the crop and catch most of the spray, so both weed kill and crop injury are uneven. Correct practice: control weeds when small, at 2–4 leaves.
3. What should she do now, and what must change before the next spray season?
Read the worked answer
Now:
- Apply no further herbicide, and above all no second hormone spray - a repeat at this stage would multiply the sterility.
- Give a light irrigation to help the crop recover, and complete any nitrogen still due.
- Do not try to compensate with extra nitrogen or growth regulators; badly affected patches will simply yield less. Record the damaged patches so the loss is documented.
- Warn the neighbour, and do not feed the affected berseem strip immediately; take a later cut.
Before next season:
- Buy new nozzles and a spare, plus a pressure gauge; keep a dedicated sprayer or a clearly labelled nozzle set for hormone herbicides.
- Calibrate before every season: measure output for one minute, walk a measured 50 m, confirm L/ha against the label.
- Keep a spray diary - date, crop stage, product, dose, water volume, temperature, wind - and a marked area per tank so overlaps are avoidable.
- Spray at the right stage: for wheat, 30–35 days after sowing, not when the calendar and canal rotation finally allow.