Corn seeding rate: how to set population and seed drop

The formula

Three steps, in order.

1. Seed drop from target stand.

Seed drop (seeds/ac) = target final stand (plants/ac) ÷ (germination × expected survival)

Germination comes from the seed tag as a decimal (0.95 for 95%). Expected survival is the share of germinable seed that becomes a harvestable plant; Purdue's planting-rate trials averaged 95% stand establishment overall, with a range of 79 to 100%, and Purdue's replant worksheet defaults survival to 95% when you have no better number. If you only track one factor, "percent stand" (final plants ÷ seeds dropped) from your own stand counts at V6 is the most useful.

2. Row-feet per acre from row spacing.

Row-feet per acre = 43,560 sq ft ÷ row spacing in feet

• 30-inch rows: 43,560 ÷ 2.5 = 17,424 ft
• 20-inch rows: 43,560 ÷ 1.667 = 26,136 ft
• 15-inch rows: 43,560 ÷ 1.25 = 34,848 ft

3. Seeds per foot and in-row spacing.

Seeds per foot = seed drop ÷ row-feet per acre; spacing (inches) = 12 ÷ seeds per foot.

For metric planters, a hectare has 10,000 m² ÷ 0.76 m = 13,158 m of row at 76 cm. Seeds per metre = seeds per hectare ÷ 13,158, and seeds per hectare = seeds per acre × 2.471.

Inputs and where to get them

Target final stand. This is a plants-per-acre number, not seeds. The published ranges:

Purdue's economic optimum sits "several thousand lower" than the agronomic optimum because the top of the response curve is flat. Iowa State puts the economic rate typically 3,000 to 5,000 seeds/ac below the rate that maximises yield.

Hybrid rating. Pioneer ranks hybrid second only to field productivity in setting seeding rate, with optimum populations differing considerably between hybrids of the same maturity. Purdue's paired-hybrid trials found significant differences in only 11 of 27 comparisons, so treat the hybrid rating as a nudge of a few thousand, not a different world. Get the number from the seed company's population guidance for your hybrid and yield level.

Germination. The tag figure is a warm test at 77 °F (25 °C). Iowa State recommends a cold or cold-saturated germination test for early planting, and a higher seeding rate when germination is below 90%.

Expected survival. Use your own V6 stand counts from past years. Ontario's guide uses a simple rule: seed 10% above the desired final stand when planting early into cold soil, 5% when soils are warm. Lower survival is likely in no-till or high-residue seedbeds, cloddy or compacted soils, and fields with seedling disease or insect history.

Worked example

A grower in central Illinois wants a final stand of 32,000 plants/ac (79,100 plants/ha) on productive ground with a hybrid rated for medium-to-high populations. Tag germination is 95%. Past V6 counts show about 95% of germinable seed surviving when planted into warm soil.

Seed drop = 32,000 ÷ (0.95 × 0.95) = 35,457, so set the planter at 35,500 seeds/ac (87,700 seeds/ha).

• 30-inch rows: 35,500 ÷ 17,424 = 2.04 seeds/ft, one seed every 5.9 inches.
• 20-inch rows: 35,500 ÷ 26,136 = 1.36 seeds/ft, one seed every 8.8 inches.
• 76 cm rows: 87,700 ÷ 13,158 = 6.67 seeds/m, one seed every 15.0 cm.

Seed need: 35,500 ÷ 80,000 = 0.44 bags/ac, so 44.4 bags for 100 acres (40.5 ha).

Seed cost sensitivity. Suppose a bag costs $300, which is $3.75 per 1,000 seeds. The 3,500 seeds/ac added to cover germination and mortality cost about $13/ac. If the grower assumed 90% survival instead of 95%, seed drop would rise to 37,400 seeds/ac and cost a further $7/ac; Purdue's estimate is that improving stand establishment from 90% to 95% saves about $6/ac at that bag price. Each additional 1,000 seeds/ac has to return $3.75 ÷ corn price in extra yield to break even, which is why the economic optimum sits below the agronomic one and why a flat response curve argues for the lower end of a hybrid's range when margins are tight.

Caveats and adjustments

• Count, then correct. Purdue recommends calibrating the planter and counting plants no earlier than V6 (1/1,000 acre is 17 ft 5 in of row at 30-inch spacing; multiply by 1,000). If your percent stand is consistently 90%, use 0.90, not the default 0.95.
• Variable-rate seeding. Purdue's 2018 analysis found little relationship between within-field yield level (140 to 245 bu/ac) and optimum population, and suggests there may be minimal value in variable-rate seeding for many fields. Pioneer's data, by contrast, show optimum population rising with yield level (around 30,000 seeds/ac at the low end of the yield range to over 44,000 at the high end for one 111 CRM hybrid). The honest reconciliation is that water-limited zones (sands, pivot corners, shallow soils) do justify lower rates, while differences between good and very good zones are small. On-farm strips settle it for your fields.
• Late planting. Keep the same target stand when replanting; adjust only the survival factor, which improves in warm seedbeds.
• Silage. Ontario cites Cornell data showing no benefit above about 86,500 plants/ha (35,000 plants/ac) for silage, with digestibility declining as populations rise.
• Row spacing. The formula changes seeds per foot, not the target stand. The Purdue trials cited here were all in 30-inch rows; whether a narrow-row hybrid wants a different population is a question for the seed company's data.
• Canadian readers. Ontario's optimum population rises by about 1,112 plants/ha (450 plants/ac) for each 0.94 t/ha (15 bu/ac) increase in yield potential, a useful rule when you have a field-by-field yield history. The Ontario Agronomy Guide tables give spacing by population and row width directly.

Do it for your field

Enter your target stand, tag germination, past percent stand and row spacing, and the result is the seed drop to set on the monitor and the bags to order.