The Sprayer That Was 20% Too Wet

Actual output (per-nozzle formula, spacing 0.5 m):

L/ha = (1.2 L/min × 600) ÷ (8 km/h × 0.5 m) = 720 ÷ 4 = 180 L/ha

Cross-check by area: 133.3 m/min × 12 m = 1,600 m²/min = 0.16 ha/min; 28.8 L/min ÷ 0.16 ha/min = 180 L/ha. ✔

Consequence. One tank covers 1,200 ÷ 180 = 6.67 ha, not 8 ha. If Dev adds 2.0 L/ha × 8 ha = 16 L of product and then covers only 6.67 ha, he delivers 16 ÷ 6.67 = 2.4 L/ha - a 20% overdose of active ingredient, plus 20% more carrier than the label volume. Overdosing a Group 2 herbicide on a field with suspected resistance is the worst possible combination: more selection pressure, more crop injury risk, and more cost.

Correction options (pick one and re-verify):

1. Recalibrate to 150 L/ha. Flow varies with the square root of pressure: 300 × (150/180)² = 208 kPa, or keep 300 kPa and raise speed to 8 × (180/150) = 9.6 km/h, or fit smaller nozzles.
2. Keep 180 L/ha only if the label permits that carrier volume, then add 2.0 × 6.67 = 13.3 L per tank.

Either way, Dev must recalculate the tank rather than reuse last year's number.

Interpretation. Uniform failure confined to distinct patches, while the rest of the field is controlled, points to selected (evolved) resistance, not to a bad tank mix or an application error - those would show a pattern following the sprayer (streaks, headlands, boom-width strips), not stable patches. Group 2 (ALS-inhibitor) resistance in common ragweed is well documented in Ontario, and Group 2 herbicides share one site of action, so cross-resistance among ALS products is expected.

Plan:

• Stop using the same active/Group on that field. A different Group 2 product is not a rotation.
• Manage the patches separately: map them, harvest them last or not at all, and hand-rogue or spot-treat before seed set. Seed movement on the combine is how patches become fields.
• Layer effective modes of action: use tank-mix or sequential partners from different groups (e.g., a Group 4 synthetic auxin, a Group 14 PPO inhibitor, a Group 9 EPSP synthase inhibitor, a Group 5/6 pre-emergent) where each partner has activity on ragweed and the label allows the mix.
• Include non-chemical tactics: crop rotation, narrower rows, competitive cover, delayed seeding windows, and a stale-seedbed burn-down timed to the weed's emergence.
• Verify: keep a untreated check strip so next year's result can be judged, and consider a resistance test before committing to a program.
• Clean out: clean the combine and sprayer between fields to avoid moving resistant seed.

Do not spray under these conditions.

• Gusts to 22 km/h exceed the wind speeds most labels allow and make droplet travel unpredictable; a steady 14 km/h understates what the boom will actually see.
• 29 °C with low humidity promotes evaporation, fines, and inversion behaviour later in the day, and volatilization risk for some formulations.
• A raspberry u-pick 60 m downwind is a sensitive, marketed crop. Group 4 (synthetic auxin) chemistry in particular causes visible distortion at minute doses, and the liability is not just agronomic - it is a customer-facing business.

A defensible decision:

1. Check the label's wind speed statement, buffer zone, and any temperature restrictions - the label governs, not the forecast.
2. Delay to a calmer window (early morning after dew lifts, or evening) with wind steady and below the label limit, and record the actual measured wind at the boom.
3. Reduce off-target movement regardless: lowest workable boom height, coarser droplets/drift-reduction nozzles, a buffer strip, and a spotter on the downwind edge.
4. Talk to the neighbour before the next window; a two-minute conversation prevents a complaint file.
5. Record the weather and the decision to wait - that record is what protects Dev if someone claims damage later.