Knee-high by the Fourth of July" is a common benchmark we use to describe the progress of our corn crop. Many benchmarks are used to describe weather effects on corn, but typically calender date and growing degree units (GDU) are most commonly used. This year the temperature pattern during May and June has been cooler than average. A question I have been getting is, "How far behind is the corn crop this year?"

Table 1 describes average benchmarks expected for corn grown using chisel tillage at Arlington, WI. Early in corn development, each leaf collar requires about 50-60 GDUs to emerge. During the first couple of weeks in June we typically accumulate about 14 GDUs per day. Thus, each leaf collar requires about 3.5 to 4.5 days to emerge. On average, corn produces four leaf collars (V4) by June 3 and typically 380 GDUs have accumulated since planting on May 1. In 1995, corn planted on May 1 reached V4 on June 7 after accumulating 397 GDUs. This year a chisel tilled field planted on April 26 was at V3 on June 10; only 340 GDUs have accumulated.

Using the calendar as our benchmark, corn planted early at Arlington is about 10 to 12 days behind in development this year. Corn development is right-on using GDU as a benchmark. We still have a lot of growing season left. Daily GDU accumulation will in all likelihood increase, and corn development can catch up quickly once we return to a more normal temperature pattern.

Table 1. Development benchmarks of corn grown using chisel tillage for an "average" year, during 1995 and 1996 at Arlington, WI
Corn stage	Average year		1995		1996
Corn stage	GDU	Date	GDU	Date	GDU	Date

Planting	0	May 1	0	May 1	0	April 26
VE	125	May 12	138	May 18	130	May 20
V4	380	June 3	397	June 7	340	June 10 @ V3
V8	600	June 15	706	June 23
V12	900	June 30	929	July 5