There is some concern about the cool spring we have been having so far and the impact that later than normal planting dates might have on corn yields. A recent study investigated corn hybrid response to planting date at six locations in Wisconsin over four years between 1991 and 1994. The locations were at Lancaster, Arlington, Hancock, Marshfield, Spooner, and Ashland, WI. The production years of 1991 and 1994 were record production years, while 1992 and 1993 were years characterized by cool wet growing conditions at sometime during the year (Wisconsin Agriculture Statistics).

At every location a "full" season hybrid was grown along with a "shorter" season hybrid. A full season hybrid is defined as a hybrid that uses (or requires) the entire available growing season to reach physiological maturity before killing frost or cool temperatures end the growing season. The maturity difference between the full and shorter season hybrids used in this study averaged 10 to 15 Minnesota Relative Maturity units.

Pooling method	Hybrid maturity	Optimum date	Latest date at 95% of optimum grain yield	Daily yield change between
Pooling method	Hybrid maturity	Optimum date	Latest date at 95% of optimum grain yield	24 April and 8 May	8 May and 22 May	22 Many and 5 June	5 June and 19 June
All locations	All hybrids	3 May	15 May	0	-1	-2	-3
	Full season	3 May	14 May	0	-1	-2	-3
	Shorter season	5 May	17 May	0	-1	-2	-3
Northern locations	All hybrids	10 May	19 May	1	-1	-2	-4
	Full season	8 May	17 May	1	-1	-2	-4
	Shorter season	12 May	21 May	1	0	-2	-4
Southern locations	All hybrids	29 Apr	13 May	0	-1	-2	-2
	Full season	30 Apr	12 May	0	-1	-2	-3
	Shorter season	3 May	16 May	0	-1	-2	-2
1991 and 1994	All hybrids	4 May	16 May	0	-1	-2	-3
	Full season	4 May	15 May	0	-1	-2	-3
	Shorter season	5 May	18 May	0	-1	-1	-2
1992 and 1993	All hybrids	2 May	14 May	0	-1	-2	-3
	Full season	2 May	13 May	0	-1	-2	-4
	Shorter season	5 May	17 May	0	-1	-2	-3

The optimum planting date for full season hybrids was 30 Apr. in southern Wisconsin versus 8 May in northern Wisconsin (Table 1). Lancaster had the earliest optimum planting date for full season hybrids on 24 April and yields were still within 95% of the optimum date yields on 9 May (data not shown). Marshfield and Spooner had the latest optimum planting date for full season hybrids on 9 May, and yields were still within 95% of the optimum on 18 May.

Averaged across all locations, the optimum planting date was 3 May (Table 1). Grain yields were still within 95% of the optimum date yield on 15 May. No daily yield changes were observed for planting dates between 24 Apr. and 8 May. After 8 May, corn yields at all locations decreased at the rate of 1 percent per day. The rate of decrease accelerated to 2 percent per day between 22 May and 5 June, and 3 percent between 5 June and 19 June.

The optimum planting date in southern Wisconsin was about 5 to 11 days earlier than the optimum date in northern Wisconsin (Table 1). In northern Wisconsin, yield increased slightly between 24 Apr. and 8 May. Yield changes for planting dates between 8 May and 5 June were similar for northern and southern locations, but after 5 June both full and shorter season hybrids were more affected by delays in planting in northern locations.

Seasonal differences affected overall grain yield, but within any particular season the optimum date to plant corn was similar regardless of the weather later in the growing season. The optimum planting date during the record production years of 1991 and 1994 for all hybrids was 4 May, while the optimum date during 1992 and 1993 was 2 May (Table 1). Averaged across all hybrids, the daily yield changes between each 2-week period decreased at similar rates.

Often soil temperature is used to determine when to start planting. Soil temperature may be important for early planting, but after 20 Apr. in southern Wisconsin and 30 Apr. in northern Wisconsin, we should discard using soil temperature as a guide and proceed as rapidly as possible with corn planting provided soil conditions are adequate.