Iowa Delayed Corn Planting Recommendations


As planting time approaches and wet soils remain, many growers may consider switching to earlier maturity corn products to offset late planting. Yield potential can decrease with delayed planting due to a number of factors including a shorter growing season, insect and disease pressure, and moisture stress during pollination. For delayed planting, the risks and rewards of switching to earlier-maturity corn products vary with the specifics of each farming operation; it is not a decision to base only on calendar date.

Corn Maturity

Recommendations from your local agronomist regarding full-season corn relative maturity (RM) groups and RM switch dates for Iowa are listed in Table 1. Careful consideration should be given prior to switching to an earlier corn product. Full-season corn products for a given area typically have the highest yield potential, which can help offset an increase in drying costs. As planting is delayed, corn product maturities will come closer together. Growing degree unit (GDU) accumulation increases as the growing season progresses. As a result, corn generally requires 6.8 GDUs less each day to reach physiological maturity (black layer) as planting is delayed beyond about May 1.1 This means that late-planted products mature in fewer than expected GDUs. Therefore, corn planted in late May compared to an optimum date may actually take 125 to 200 GDUs less to reach black layer.


When to Switch Corn Maturity

The yield for late-planted corn will vary greatly depending on the rest of the growing season. The decision to switch maturity with delayed corn planting is difficult because of variations in growing seasons relative to available GDUs, first frost date, and fall drying conditions.

Table 2 (page 2) lists accumulated GDUs, at several locations over several weeks, based on a May 1 planting date. It can help with the decision of when to switch to an earlier maturity by determining the potential GDUs remaining from a given planting date to typical maturity or killing frost in a given area. Table 3 (page 2) shows 30-year average first frost dates for those same locations across Iowa.

For example, consider if planting was delayed until the week of May 15 in the Osceola area. In that time, 147 GDUs would have been lost from May 1. If the first killing frost date is October 18, the maximum potential GDUs remaining for Osceola is approximately 2816 (2963 - 147). Therefore, a product with a GDU to black layer rating of 2800 GDUs could still be planted because its rating is below the 2816 estimated GDU potential that may occur before the first killing frost. If the reduced GDU requirement after May 1st is also taken into consideration, the product is even less likely to encounter a killing frost before physiological maturity:

2800 GDU requirement - (6.8 less GDUs/day X 15 days) = 2698 GDUs

The numbers given are based on averages and should only be used as a reference. Growers must decide what is best for their operation. Remember that the main reason for switching corn product maturity is not so much for yield, but to reduce the risk of immature and wet grain in the fall.

Product Considerations

Insect protection and crop safety become even more important with later planting. Corn products with traits that offer insect protection and herbicide tolerance, such as products with SmartStax® technology should be considered. Additionally, even with delayed planting, it is still important to try to minimize the risk of adverse weather during critical growth stages by planting a package of products that range in GDU requirements to flowering as well as maturity. Several new products flower early, which can help to lower the risk of an early frost.


Sources: 1Nielsen, R.L. 2017. Hybrid Maturities for Delayed Planting. Purdue University. Nielsen, R. L. 2009. Late planting & relative hybrid maturity decisions. Purdue University Extension. Web sources verified 4/19/18.