Corn Yield Calculation Example

Example 1: Excellent Iowa Corn Yield

Let's calculate yield for a high-producing field in central Iowa. The farmer takes samples in late August at R5 (dent) stage.

Field measurements:

• Sample row: 17'5" of 30-inch row (1/1000 acre)
• Ears counted: 34 ears
• 5 ear samples — kernel rows: 16, 16, 18, 16, 18 (avg = 16.8, round to 17)
• 5 ear samples — kernels per row: 40, 42, 38, 41, 39 (avg = 40)
• Grain moisture: 23.5%

Step 1: Calculate ears per acre

34 ears × 1,000 = 34,000 ears per acre

Step 2: Calculate total kernels per acre

34,000 × 17 × 40 = 23,120,000 kernels per acre

Step 3: Calculate base yield

23,120,000 ÷ 90,000 = 256.9 bu/acre (raw)

Step 4: Apply moisture adjustment

Factor = 1 + ((15.5 − 23.5) ÷ 100) = 1 + (−0.08) = 0.92

Adjusted yield = 256.9 × 0.92 = 236.3 bu/acre

Final yield estimate: 236 bushels per acre — an excellent Iowa yield.

Example 2: Average Illinois Corn Yield

A farmer in central Illinois samples at black layer (R6) stage.

Field measurements:

• Sample row: 17'5" of 30-inch row
• Ears counted: 32 ears
• Kernel rows (avg of 5 ears): 16
• Kernels per row (avg of 5 ears): 36
• Grain moisture: 18.0%

Step 1: 32 × 1,000 = 32,000 ears per acre

Step 2: 32,000 × 16 × 36 = 18,432,000 kernels per acre

Step 3: 18,432,000 ÷ 90,000 = 204.8 bu/acre (raw)

Step 4: Factor = 1 + ((15.5 − 18.0) ÷ 100) = 1 + (−0.025) = 0.975

Adjusted yield = 204.8 × 0.975 = 199.7 bu/acre

Final yield estimate: 200 bushels per acre — slightly above the US national average.

Example 3: Drought-Stressed Kansas Corn

A western Kansas dryland farmer samples after a stressful growing season with limited rainfall.

Field measurements:

• Sample row: 17'5" of 30-inch row
• Ears counted: 26 ears (some barren stalks)
• Kernel rows (avg of 5 ears): 14
• Kernels per row (avg of 5 ears): 28 (heat stress during pollination)
• Grain moisture: 14.0% (already dried down)

Step 1: 26 × 1,000 = 26,000 ears per acre

Step 2: 26,000 × 14 × 28 = 10,192,000 kernels per acre

Step 3: 10,192,000 ÷ 90,000 = 113.2 bu/acre (raw)

Step 4: Factor = 1 + ((15.5 − 14.0) ÷ 100) = 1 + 0.015 = 1.015

Adjusted yield = 113.2 × 1.015 = 114.9 bu/acre

Final yield estimate: 115 bushels per acre — below-average yield due to drought stress.

Example 4: Irrigated Nebraska Corn

A Nebraska farmer with center pivot irrigation samples in early September.

Field measurements:

• Sample row: 17'5" of 30-inch row
• Ears counted: 36 ears (high population)
• Kernel rows (avg of 5 ears): 18
• Kernels per row (avg of 5 ears): 38
• Grain moisture: 25.0%

Step 1: 36 × 1,000 = 36,000 ears per acre

Step 2: 36,000 × 18 × 38 = 24,624,000 kernels per acre

Step 3: 24,624,000 ÷ 90,000 = 273.6 bu/acre (raw)

Step 4: Factor = 1 + ((15.5 − 25.0) ÷ 100) = 1 + (−0.095) = 0.905

Adjusted yield = 273.6 × 0.905 = 247.6 bu/acre

Final yield estimate: 248 bushels per acre — excellent yield from irrigated Nebraska corn.

Example 5: Adjusting for Small Kernels

Sometimes you need to adjust the 90,000 constant for unusually small or large kernels. Let's revisit Example 3 (drought-stressed Kansas) using 100,000 instead of 90,000 because the kernels are visibly small.

Recalculated with 100,000 constant:

10,192,000 ÷ 100,000 = 101.9 bu/acre (raw)

Adjusted yield = 101.9 × 1.015 = 103.4 bu/acre

The drought-stressed crop had small kernels, so using 100,000 gives a more accurate estimate of about 103 bu/ac — closer to actual harvest yield of 98 bu/ac in this real example.

Rule of thumb: Use 80,000-85,000 for large kernels (excellent grain fill), 90,000 for normal kernels (standard), and 95,000-105,000 for small kernels (stressed conditions).