On average, the best time to plant corn in most of Wisconsin is from 20 April to 10 May. The optimum date is around 1 May, but it will vary from year to year. To finish planting by 10 May is a goal most farmers should work toward. In northern Wisconsin, the optimum planting period may be 7 to 10 days later.

The rainy, cold weather and wet, cool soil temperatures we have experienced during April and early May has delayed Wisconsin fieldwork this year. Due to delayed planting, it is likely that some growers will need to modify their hybrid selections and switch corn hybrid maturities from full-season to shorter-season maturities. 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.

Corn producers need to select hybrids that yield well for their maturity. Delayed planting will affect both yield potential and grain moisture at harvest. Full-season hybrids are more affected by delays in planting than shorter-season hybrids. Usually there is some date during the growing season when shorter-season hybrids perform better than full-season hybrids (Fig. 1). The date when the performance of the full-season hybrid equals the shorter-season hybrid would be considered the "switch" date. In the example in Fig. 1 using yield as the performance measurement, the hybrids should be switched on 13 May.

A four year trial conducted at six locations in Wisconsin between 1991 and 1994 investigated the relationship between corn performance and planting date. One objective of this trial was to determine when hybrids should be switched from full- to shorter-season maturity. In this trial, maturity differences between full- and shorter-season hybrids ranged between 5 and 15 days based upon the Minnesota Relative Maturity System depending upon location.

An important aspect of the decision to "switch" hybrids should be the potential use of corn and the "yield versus grain moisture trade-off." Using the data from the 1991 to 1994 study period, I calculated grower return for three corn production systems: livestock (ear corn and high moisture corn), on-farm drying and commercial elevator drying. Grower return is the amount of money left over after treatment costs have been subtracted, which in this example is mostly energy costs for drying. For these scenarios I also subtracted hauling and handling costs of $0.04 and $0.017 per bushel since it costs more to handle higher yields.

Table 1. Date to "switch" corn hybrids from full- to shorter-season
relative maturity in three corn production systems.

The "switch" dates were affected by corn production system. In general, switching between full- and shorter-season corn hybrids grown for livestock and not requiring any energy costs for drying was later than the other production systems where drying costs were necessary (Table 1). Later switch dates were seen with higher corn prices in both on-farm drying and commercial elevator drying corn production systems. Earlier switch dates were seen with increasing energy costs for drying. An exception was at Ashland that might be modified somewhat by the "lake effect." At Arlington, the shorter-season hybrid returned more than the longer season hybrid on every planting date indicating that there are some shorter-season hybrids which approach the performance of full-season hybrids and since they are drier at harvest may have greater grower return.

Table 2 describes some guidelines for switching corn hybrid maturities in various Wisconsin production zones when the end-use is dry grain. The chances of obtaining dry grain when planting in mid- to late-June are low. When using these guidelines, please remember that growing season, site and management influence a particular hybrid's actual days to maturity. Maturities listed are estimates based on the Minnesota Relative Maturity System. This is a widely used system, however, not all companies use this exact day maturity system and there is no industry standard for estimating maturity. Therefore, use the values in Table 2 only as a general guide.

Table 2. Latest acceptable planting date and hybrid maturity for dry corn grain in several Wisconsin production zones.
Hybrid maturity	Southern		South Central		North central		Northern
Hybrid maturity	Latest planting	Relative maturity	Latest planting	Relative maturity	Latest planting	Relative maturity	Latest planting	Relative maturity
	date	days	date	days	date	days	date	days
Full-season	5 May	105-110	8 May	100-105	11 May	95-100	14 May	85-90
Mid-season	15 May	100-105	18 May	95-100	21 May	85-95	24 May	75-85
Short-season	25 May	95-100	28 May	90-95	31 May	75-85	4 June	65-75

Although the penalty for late planting is important, growers also need to be careful to avoid tillage when soil is too wet. Yields may be reduced somewhat this year, but effects of soil compaction can reduce yields for several years to come. Your decision to switch hybrid maturity depends upon:

Desire to accept risk: Longer season hybrids offer the highest yield potentials, but may also increase drying costs and/or delay harvest.
Potential use: For dry grain, relative maturities should be shorter-season within the maturity range for the latest acceptable planting date. For ear corn, high moisture corn, and silage, relative maturities should be longer-season within the maturity range for the latest acceptable planting date.
Field conditions: Shorter-season hybrids within the maturity range for the latest acceptable planting date should be selected when field conditions include heavy crop residue, reduced tillage, and heavy soil textures.
Hybrid dry down and grain quality characteristics: Longer-season hybrids within the latest acceptable planting dates should have fast grain dry-down and high test weight characteristics.
Ease of trading original hybrids for superior shorter-season alternatives.