Farmers are faced with corn replanting decisions every year. Cold temperatures, wet or crusted soils, and/or pesticide or fertilizer injury may reduce seed germination and seedling emergence. After emergence, stands may be further reduced from insects, hail, and/or flooding.

This year Wisconsin farmers have experienced cool temperatures, and corn emergence has been slower than normal. Often farmers become anxious about making the decision to replant before the crop has fully emerged, because each day delay can significantly impact grain yield. Table 1 shows the development progress of corn planted this year at Arlington, WI. Each planting date has taken about 22 days to emerge. GDU accumulation between planting and emergence has ranged from 121 to 139 GDU. Corn planted on May 13 and May 21 has not emerged yet. These planting dates should begin emerging on May 30 and June 3 based on 30-year temperature averages.

Table 1. Number of days and Growing Degree Units (GDU) between planting and emergence of corn grown during 1997 at Arlington, WI (as of May 27).
Growth stage (GDU required)	Planting date	Date growth stage achieved (predicted)	Days between growth stages	GDU between growth stages
VE (125 GDU)	April 18	May 10	22	121
	April 25	May 16	22	126
	May 5	May 27	22	139
	May 13	Not emerged as of May 27 (May 30)	---	87 as of May 27
	May 21	Not emerged as of May 27 (June 3)	---	50 as of May 27
V2 (240 GDU)	April 18	(May28)	---	225 as of May 27

Nearly 50% of Wisconsin's corn acreage was planted between May 4 and May 18. Typically, corn should emerge within 10 to 12 days, but due to below normal temperatures it is taking longer for the germination and emergence process.

How long should we wait before starting the decision process for replanting? The germination and emergence process is driven by temperature. Typically about 125 Growing Degree Units (GDU) are required between planting and emergence. Based on 30-year temperature averages, daily GDU amounts during May range from 10 to 13 GDU per day. However, numerous environment and management factors influence the number of GDU required for emergence (Table 2). For example, in tillage systems where residue is present on the surface, thereby cooling the soil, an extra 30 to 60 GDU are required between planting and emergence. This means an extra 3 to 6 days is required for emergence under high residue situations.

Table 2. Growing Degree Unit (GDU) adjustment for various environment characteristics and management practices.
Environment characteristic or Management practice	GDU Adjustment
Conservation tillage (more than 75% residue)	Add 30-60 GDU
Soil texture: fine coarse	Add 30-60 GDU Subtract 30-60 GDU
Seed-zone soil moisture (below optimum)	Add 30 GDU
Seed-bed condition (soil crusting or massive clods)	Add 30 GDU
Seeding depth (for each inch below 2 inches)	Add 15 GDU
Hybrid differences for emergence	Hybrid specific

Over 80% of the corn acreage planted in Wisconsin was planted after May 4. Much of that acreage has not accumulated 125 GDU yet. We need to be patient, keep checking our fields, and track GDU accumulation. If about 150 to 180 GDU has accumulated since planting, and the corn has not emerged, begin thinking about replanting. Below is a summary of the key steps in the decision-making process for corn replanting in Wisconsin (for more information see Corn replant/late-plant decisions in Wisconsin A3353).

Determine plant population
Evaluate plant health
Assess the unevenness of the stand
Compare the yield potential of a reduced stand to that of a replanted stand
Calculate replanting costs
Factor in risks of replanting