🌡️ GDU CALCULATOR

Crop

Relative Maturity (days)

Corn: 80–130 RM. GDU to maturity ≈ RM × 9 + 375 (Rule of thumb)

Planting Date

Today's Date

GDU Entry Method

Avg Daily High (°F)

Avg Daily Low (°F)

GDU/day = ((min(Hi,86) + max(Lo,50)) ÷ 2) − 50. Caps: Hi=86°F, Lo=50°F.

Days Since Planting —

GDUs / Day —

📊 DAILY GDU BY MONTH (WI Avg)

Month	Avg Hi	Avg Lo	GDU/Day	Monthly Total
May	65°F	44°F	7.5	232
June	76°F	55°F	15.5	465
July	82°F	61°F	21.5	667
August	80°F	59°F	19.5	605
September	70°F	50°F	10.0	300

Wisconsin 30-year normals (NOAA). Southern WI runs ~5–8% higher; northern WI lower.

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GDUs accumulated

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VEV6V10VTR3Black Layer

MATURITY ESTIMATES

GDU Target

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to maturity

GDUs Left

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remaining

Days Left

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est. to maturity

Est. Maturity Date —

Progress to Maturity —

CORN GROWTH STAGE MILESTONES

GDU requirements vary by hybrid/variety. Values shown are typical averages.

🌽 CORN STAGE REFERENCE

Stage	GDUs from Planting	Key Events	Management Notes
VE	~100	Emergence	Assess stand; begin pest scouting
V3	~200	Growing point below soil	Side-dress N window opens V3–V6
V5	~350	Ear shoots initiated	Final row count per ear set here
V6	~475	Growing point above soil	Last pass for pre-emerge herbicide
V8	~700	Kernel rows determined	Sulfur and micronutrient uptake peaks
V10	~900	Ear size potential being set	Tassel is fully formed inside stalk
VT	~1135	Tasseling — shedding pollen	Critical drought window begins; fungicide timing
R1	~1400	Silking	Most drought-sensitive stage — protect with water if possible
R2	~1560	Blister	Kernel number locked in
R3	~1750	Milk stage	Grain fill accelerating; leaf diseases reduce yield
R4	~1925	Dough stage	Silage harvest window opens R4–R5
R5	~2190	Dent stage	Stalk integrity issues appear; watch for stalk rot
R6	~2700	Black layer / physiological maturity	Moisture at R6 ≈28–32%; harvest when <25%

🫘 SOYBEAN STAGE REFERENCE

Stage	GDUs from Planting	Key Events	Management Notes
VE	~130	Emergence	Assess stand; 100,000 plants/ac minimum
V1	~200	First trifoliate	Post-emerge herbicide window
V3	~400	Third trifoliate	Last effective post herbicide application
V5	~600	Fifth trifoliate	Branching initiated; canopy closure begins
R1	~800	First flower	Fungicide timing — frogeye & white mold risk begins
R2	~1000	Full bloom	Primary fungicide window; N fixation peak
R3	~1200	Beginning pod set	Drought most damaging here through R6
R4	~1400	Full pod (pods >¾" on 4 nodes)	Irrigation highest priority; pod abortion risk
R5	~1600	Beginning seed fill	Seed size and oil/protein content determined
R6	~1750	Full seed	Harvest timing decisions begin
R7	~1900	Beginning maturity	50% of pods show mature color
R8	~2000	Full maturity	95% of pods tan/brown; harvest when <15% moisture

📐 GDU FORMULA & METHODOLOGY

Corn GDU Formula

GDU/day = ((min(Hi, 86°F) + max(Lo, 50°F)) ÷ 2) − 50

Temperature caps:
• Max daily high capped at 86°F (above this, corn growth doesn't increase)
• Min daily low floored at 50°F (below this, no heat accumulation)

Soybean GDU Formula

Soybeans use the same base formula but are photoperiod-sensitive — flowering is triggered by day length (short-day plant), not GDUs alone.

GDU to R1 varies by maturity group. MG 2.0–4.0 typical in Midwest = ~800–1200 GDUs to first flower.

RM × GDU Rule of Thumb

Corn: GDUs to black layer ≈ (RM × 9) + 375
Example: 108-day hybrid ≈ (108 × 9) + 375 = 1,347 GDUs to black layer

Always verify with the seed company's published GDU requirements for your specific hybrid.

Limitations

• GDUs are a guide, not a guarantee
• Soil temp at planting affects early accumulation
• Cold snaps can reset or delay stage progress
• Hybrid differences of ±100–200 GDUs are common
• Use NOAA or your local extension office for actual daily GDU data