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Recommendations from your local agronomist regarding full-season corn relative maturity (RM) groups and RM switch dates for Wisconsin 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 1.6 GDUs less each day to reach flowering and 6.8 GDUs less each day to reach physiological maturity (black layer) as planting is delayed beyond about May 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.
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 an April 28th 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 median first frost dates for some cities in Wisconsin.
For example, consider if planting was delayed until the week of May 15 in the Eau Claire area. In that time, 155 GDUs would have been lost from April 28. If the first killing frost date is October 7, the maximum potential GDUs remaining for Eau Claire is 2597 (2752 - 155). Therefore, a product with a GDU to black layer rating of 2500 GDUs can still be planted because its rating is below the 2597 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:
2500 GDU requirement - (6.8 less GDUs/day X 15 days)= 2398 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.
Insect protection and crop safety become even more important with later planting. Corn with Genuity® traits that offer insect protection and herbicide tolerance, such as Genuity® SmartStax® corn and Genuity® SmartStax® RIB Complete® corn blend products, 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.
In choosing the best corn product for your farm, maturity is only one of many criteria. The reason for switching corn products is to reduce the risk of immature and wet grain in the fall. Keep in mind, conditions during the growing season and harvest are other factors in corn maturation and harvest kernel moisture.
Sources: 1 Wisconsin State Climatology Office. Online: http://www.aos.wisc.edu (verified 4/30/14); North Dakota Agricultural Weather Network. Online: http://www.ndawn.ndsu.nodak.edu (verified 4/30/14); Lauer, J. 2011. Corn hybrid relative maturity switch dates for 2011. Integrated pest and crop management. University of Wisconsin. Online: http://ipcm.wisc.edu (verified 4/30/14); Brouder, S. et al. 2008. Corn & Soybean Field Guide. ID-179. Purdue University. Online: http://www.agry.purdue.edu (verified 4/30/14); Nielsen, R. L. 2009. Late planting & relative hybrid maturity decisions. Purdue University Extension. Online: http://www.agry.purdue.ede. (verified 4/30/14).