The general shape of the planting date response at a location has been described previously. The last time a statewide corn planting date experiment was conducted at numerous locations was during 1991-1994 (Lauer et al., 1999). Full- and shorter-season corn hybrids were planted on five to eight planting dates between April 19 to June 22.
For the northern sites of Ashland, Spooner and Marshfield, the date when maximum corn yield occurred averaged May 12, while the southern sites of Hancock, Arlington and Lancaster were one week earlier averaging May 5 (Table 1). Corn yields were still at 95% of the maximum yield on May 17 in the north and May 12 in the south. By June 1, corn yield was decreasing at an average rate of 2.3 bu/A per day in the north and at a slower rate moving south to Lancaster.
Table 1. Corn grain yield response to planting date of full-season hybrids at various locations in Wisconsin during 1991 to 1994 (derived from Lauer et al., 1999).
Optimum corn planting dates likely vary with farm, hybrid, field, tillage system and other management factors. In northern Wisconsin, these factors along with latitude result in lower yield potential and when combined with grain moisture and drying costs described previously often makes corn production marginal as we move north.