VRA Prescription

VRA — Variable Rate Application — is the practice of applying inputs (spray, seed, fertilizer) at different rates across a field instead of a single uniform rate. The portal's Map Prescription module produces VRA prescriptions; the field controller drives the actual variable-rate application during the flight.

The simple version#

A traditional sprayer applies the same rate everywhere. A VRA sprayer applies more in some zones, less in others.

The decision of how much per zone is the prescription. The portal generates it. The controller follows it.

How a prescription is built#

The portal's workflow:

1. NDVI raster as input (from satellite or drone). Each pixel has an NDVI value.
2. Clustering groups similar-NDVI pixels into zones. You pick how many zones (Low / Medium / High = 2 / 3 / 5) and which method (K-Means / Jenks / Quantile / Equal Interval).
3. Block grid overlays the plot. Each block (3–50 m on a side) inherits the zone of the majority pixel inside it.
4. Rate per zone is computed from the prescription preset:
   • Recovery: more product where NDVI is low.
   • Maintenance: more product where NDVI is high.
   • Neutral: uniform.
5. Exclusion zones override the prescription with rate = 0 wherever drawn.
6. The result is exported as a shapefile with a per-polygon rate attribute.

What the controller does#

When the drone enters a polygon during the flight:

1. The controller looks up the polygon's rate and unit in the shapefile.
2. It dispatches the appropriate command to the spray pump (or seeder mechanism).
3. As the drone crosses into the next polygon, the rate changes.

The rate changes continuously, not in discrete steps — the controller interpolates over the polygon edge to avoid abrupt pressure changes.

Spraying vs. Spreading#

The Zone Editor's VRA Prescription card has a toggle:

• Spraying — for liquid products. Units: gal/ac or L/ha.
• Spreading — for granular products (seed, fertilizer prill). Units: lb/ac or kg/ha.

Choose based on what you're going to load into the drone. The toggle affects the output shapefile's unit field.

Recovery vs. Maintenance — picking a strategy#

The two main strategies trade off rescue vs. protect:

Recovery — invest in the stressed areas#

The hypothesis: low-NDVI areas are reachable. Bump them up.

• More product where NDVI is low.
• Common for foliar nitrogen, micronutrient rescue.
• Risk: spending on areas that won't respond.

Maintenance — invest in the high-yielding areas#

The hypothesis: high-NDVI areas are your moneymakers. Protect them.

• More product where NDVI is high.
• Common for late-season fungicide, sulfur maintenance.
• Risk: low-NDVI areas continue to underperform.

For a season's first VRA pass, Recovery is the default. For maintenance passes later, switch to Maintenance.

Reading the Application Rate by Zone table#

The table shows one row per zone with:

• Zone label (e.g., "Zone 1: Low Vigor").
• Application rate in your chosen unit (gal/ac, L/ha, lb/ac, kg/ha).
• Area covered by that zone.

You can edit each rate directly. The Avg. rate updates as you edit; the Required Product total on the Estimate step updates accordingly.

Sanity checks#

Before clicking Generate VRA Prescription:

• The min rate is above the controller's minimum mechanical rate (~0.5 gal/ac for typical sprayers).
• The max rate is below the controller's maximum (~5 gal/ac for typical drones).
• The rate spread isn't absurd (a 10× spread between zones is unusual).
• Exclusion zones are drawn where you need them.

If any of these are off, edit the rates or change the preset before generating.

What you can't do (yet)#

• Multi-product prescriptions — one zone gets product A, another gets product B. On the roadmap.
• Time-of-day modulation — different rates morning vs. afternoon. Not planned.
• Multi-pass strategies — automatic split between two flights. Workaround: generate two prescriptions, fly each separately.

Related#

• Management Zones.
• NDVI.
• Map Prescription reference.
• VRA and zones guide.