For the week ending May 15 about 75% of the corn acres were planted in Wisconsin (USDA Statistics). About 1 million acres are still intended to be planted. Traditionally, as May 20 approaches, farmers start switching to shorter-season relative maturities of hybrids.

Factors that need to be considered when making this decision are: 1) knowing the correct full-season maturity that a field can handle, 2) grain price, and 3) fuel costs for grain drying after harvest. Understanding how to pick the correct maturity for a field is difficult. Farmers usually spread risk by selecting a few hybrids of different maturities and planting these hybrids across fields on their farm. The realistic grain price will vary depending upon the producer's ability to market grain. Corn grain used on-farm as livestock feed should be valued at the price it would cost to purchase. Fuel costs will be influenced by market supply and demand, the farmer's purchasing power, and his ability to negotiate and haggle over price.

Many factors influence full-season hybrid relative maturity. Some factors are genetic, others are environmental, and still others are influenced by management. Also, no common method for rating relative maturity of hybrids exists among seed companies. So, any recommendations for switching hybrid maturity during late-May must be made in general terms.

This article evaluates the planting date response of shorter-season hybrids and builds upon an article published a few weeks ago describing planting date response of full-season hybrids between 1997 and 2005 (http://corn.agronomy.wisc.edu/WCM/W183.aspx). In all years, two or more hybrids were grown that differed for RM by 7 or more days. But, for four years (1998-2001), all hybrids grown were greater than 102 days RM, which is considered full-season at the Arlington location. Thus, no shorter-season hybrids were grown during 1998-2001 and no comparisons are made.

Figure 1 shows the relationship between full- and shorter-season hybrids at Arlington where a full-season hybrid is > 102 days RM. In most years, corn grain yield decreases as planting date is delayed (Figure 1). Full-season hybrids provide the best yield potential until ~May 20, when hybrids 7 days RM or more shorter-season begin to produce greater yield. So, by ~May 20, corn hybrid maturity should be switched to 98 days RM or less at Arlington.

In the previous article, full-season hybrids (grown during 1997, 2002-2005) produced a maximum grain yield of 206 bu/A on April 20. Yields were still within 95% of the maximum grain yield until May 20.

Grain yield potential of shorter-season hybrids is lower than full-season hybrids. The maximum yield of shorter-season hybrids was 190 bu/A produced April 23 (Table 1). Yields were still within 95% of the maximum yield until May 26. Thus, the planting date that produces maximum yield is not very different between full- and shorter-season corn hybrids (April 20 v. April 23). Shorter-season hybrids remain within 95% of their maximum yield about 6 days later into May (May 20 v. May 26). In these years (1997, 2002-2005), the best date to switch hybrid relative maturity from full- to shorter-season to maximize grain yield was May 21 at Arlington.

Table 1. Planting date response of shorter-season corn hybrids (<100 d RM) at Arlington, WI .
Year	First treatment	Grain yield
Year	First treatment	Pattern	Maximum	Maximum	Window
	Date		Bu/A	Date	Date
1997	April 18	+L-Q	198	April 25	May 21
1998	All hybrids grown in experiment were full-season for Arlington
1999	All hybrids grown in experiment were full-season for Arlington
2000	All hybrids grown in experiment were full-season for Arlington
2001	All hybrids grown in experiment were full-season for Arlington
2002	April 15	None	177	April 15	June 9
2003	April 15	-Q	195	April 15	May 9
2004	April 12	+L-Q	192	April 30	May 12
2005	April 15	None	190	April 15	June 15
Average	April 15	+L-Q	190	April 23	May 26
Window = Last planting date where grain yield is within 95% of the maximum yield or grower return is within 95% of the economic optimum.