How to calculate percent stand loss from a gap map

Percent stand loss is just planted population minus emerged population, divided by planted population, times 100. The math is simple. The hard part is getting an emerged population number you trust before the replant window closes.

The formula, and where it breaks down

Say you dropped seed at 34,000 per acre and your scout comes back with a stand count of 27,500. That's:

(34,000 - 27,500) / 34,000 × 100 = 19.1% stand loss

Clean enough on paper. Where it falls apart is the input. A hoop count at three or four spots per field gives you a stand estimate built off a handful of samples. It says nothing about where the thin spots sit.

A field can average 19% loss and still have one 4-acre corner at 45% because of standing water or a planter skip, while the rest of the field is fine. Average loss tells you whether to worry. It tells you nothing about where to replant, or whether a strip-till pass makes more sense than reseeding the whole 80.

A gap map fixes the "where" problem because it's built from a count across the field instead of a few transects extrapolated up. Once you have a verified emerged-plants-per-hectare figure for each zone of the field, you plug that into the same formula, zone by zone, instead of one number for the whole boundary. That's the version of the calculation that tells you where to send the drill.

Building the replant worksheet

Once you've got per-zone emergence, the worksheet is three columns and one decision rule:

1. Planted population by zone (seed drop rate, adjusted for any variable-rate planting)
2. Emerged population by zone (from your count, or from a gap map pulled off a drone pass)
3. Percent stand loss by zone (the formula above, run per zone instead of per field)

Most agronomists run a threshold against row 3, something like 15 to 20% loss in a contiguous zone of a certain size, though the number shifts by hybrid, planting date, and what the replant seed costs relative to what the thin stand would have yielded anyway. Below threshold, you leave it. Above threshold, you decide between spot-replanting the zone or working the whole field, and that decision has a shelf life: once the canopy closes over the gaps, replant plants that do come up get shaded out and the whole exercise is wasted seed.

Getting a reliable emerged-population number across the whole field, fast enough to still act on it, is what eats the clock. Walking transects with a stand-count board works, but it's slow, and by the time you've covered a few hundred acres the earliest fields you checked have already pushed another leaf stage. A single drone pass over the field gets you the same per-hectare emergence figure, plus a gap layer showing exactly which zones are below threshold, while there's still time to send a drill.

A quick sanity check on your number

Before you commit a zone to replant, it's worth checking the stand loss figure against two things: planting date and current growth stage. A 20% loss in a field planted ten days late and still at V2 is a different decision than the same 20% in a field pushing V5 that planted on time. The formula doesn't know your calendar. You do.

If the gap map shows loss concentrated in a pattern, a straight line down a planter pass, a low spot that holds water, a headland where the planter lifted, that pattern tells you something the average doesn't. Planter error gets fixed in the shed. Standing water takes longer, usually a tile project for next year. Either way, it's worth noting on the worksheet next to the number, not just the percentage itself.

Run the numbers early enough and the worksheet does its job either way: replant, or leave it alone and bank the acres.

If you'd rather start from a verified count than a transect estimate, Plant Counting turns one flight into the per-hectare figure and gap map that worksheet needs.