If corn is planted prior to April 20 in southern Wisconsin or April 30 in northern Wisconsin, the decision should be based on field conditions and soil temperature (55 1F).
After April 20 and April 30, corn planting should proceed as rapidly as possible and be completed by May 5 for southern Wisconsin and May 12 for northern Wisconsin.
Corn grain yield decreases 0.3 to 0.5 percent per day when planted during early May and 1.5 to 2.3 percent per day when planted during late May.
Full season corn hybrids yield 5 to 15 percent more than shorter season hybrids early in the planting period which results in increased grower return at $10 to $20 per acre.
Producers should consider switching maturities from full to shorter season corn hybrids between May 5 and 10 for southern Wisconsin, and between May 20 and 25 for northern Wisconsin.

Many questions are asked by producers about planting date response of corn. Those who plant early are concerned with: slow emergence and stunted early growth, purple or yellow color of plants, and frost. Those who plant late because of wet weather or equipment breakdowns are concerned with: when to switch to earlier hybrid maturities, which hybrids should be used to ensure maturity before fall frost, and the effect late planting will have on yield?

Typically, a starting point for corn planting is soil temperature of 55 1F. Little information is available on corn planting date response for regions where full-season corn is 90-95 day relative maturity or earlier. Also many growers are planting corn under reduced tillage conditions, in which soils are cold and wet early in the season.

Experiments were conducted between 1991 and 1994 by staff at the University of Wisconsin Agricultural Research Stations at Arlington, Ashland, Hancock, Lancaster, Marshfield, and Spooner to evaluate the influence of planting date and hybrid maturity on corn grain yield and harvest moisture. Two or three corn hybrids ranging from "full" to "mid/short" season maturity for each location were planted every 10 to 14 days from 20 April to 10 June. At all sites grain yield and moisture were measured.

Grower return was calculated using a corn grain price of $2.25 per bushel. Hauling costs were $0.04 per bushel, handling costs were $0.017 per bushel, and drying costs were $0.015 per point per bushel (Duffy and Judd, 1992). Grower return equals gross payment minus treatment costs which were hauling, handling, and drying costs. Data were grouped according to corn production zones in Wisconsin and whether or not it was a "full" or "shorter" season hybrid for that production zone.

Grain yield increased slightly during early planting (approximately 0.4 percent per day), achieved an "optimum", and then decreased; at a slow rate initially and then faster during late May and June (Figs. 1a and 1c). There was no obvious yield penalty for very early planting, even though early-planted corn froze twice at same locations in 1992. In all years, corn planted in late May yielded at least 30% less than corn planted just one to two weeks earlier.

In the south and south central corn production zones of Wisconsin, the optimum planting date for grain yield of full season hybrids was April 30 and for shorter season hybrids was May 3 (Fig. 1a). In 3 of 12 tests, the highest yielding treatment was in April. Yield decreased approximately 0.3 percent per day in early May and 1.5 percent per day during late May. Full season hybrids yielded more than shorter season hybrids during early May. Shorter season hybrids yielded more than full season hybrids by May 15. However, grower return of shorter season hybrids was greater than full season hybrids by April 30 due to the higher drying cost associated with the full season hybrids (Fig. 1b).

In the north and north central production zones of Wisconsin, the optimum planting date for grain yield of full season hybrids was May 11 (Fig. 1c). Corn planted earlier than these dates usually had slightly lower grain yields. The highest yielding treatments were never observed during April. Yield decreased approximately 0.5 percent per day during early May and 2.5 percent per day during late May. Even though full season hybrids yielded 10 to 15 percent more than shorter season hybrids early in the planting period, it only resulted in a $15 to $20 per acre increase due to the higher drying costs associated with the full season hybrids (Fig. 1d). The shorter season hybrids yielded more than full season hybrids by June 1 (Fig. 1c), but grower return was greater for shorter season hybrids after May 22 (Fig. 1d). Thus, growers should consider switching to shorter season hybrids between May 20 and 25.

If corn is planted prior to April 20 in the south and south central Wisconsin corn production zones and April 30 in the north and north central zones, the decision to begin planting should be based on field conditions and when soil temperature reaches 55oF at any time during the day. After April 20 and 30, corn planting should proceed as rapidly as possible and be completed by May 5 for the south and south central corn production zones and May 12 for north and north central zones in Wisconsin.

REFERENCES

Duffy, M., and D. Judd. 1992. Estimated costs of crop production in Iowa 1993. Iowa State University Extension FM1712.