By Lori Abendroth and Roger Elmore (Text originially written February 5, 2007)
Planting dates are earlier today than ever before. In general, research has shown a small yield loss with very early planting dates and a larger yield loss with later planting dates. Research conducted during 1998-2000 showed that planting approximately between April 20 and May 20 resulted in 100 percent yield (see Iowa State University Extension's Planting Guide. Based on newer research (2003-2006), we believe that this window is actually earlier; corn can be planted prior to April 20 and reach 100 percent yield.
Again, yields are reduced more when planted too late rather than too early. Because producers will have more acres of corn to plant, the possibility of having corn planted outside the optimum window is increased. We recommend that corn be planted before the optimum planting window rather than after, if seedbed and environmental conditions allow.
The amount of residue will be significantly greater in corn following corn unless it was completely buried beforehand (See Photo 1). Fields covered with corn residue will likely be cooler and wetter in the spring, and these fields should not be the first planted. Corn-soybean acres should be the primary focus when planting before April 20.
Photo 1. The amount of residue is significantly greater in corn following corn fields.
When planting corn early, be aware that your established population will potentially be less than what it would be if planted later due to a prolonged length of time the seed is in the soil. A higher percentage of seeds generally emerge with later plantings. Figure 1 shows Iowa research data across nine planting date trials in 2006.
Figure 1. Final plant population across nine planting date research trials in Iowa, 2006. Percent maximum population (100% is maximum) relative to planting date (three outliers were removed from data set).
Data is graphed relative to the highest established plant population within each research location. Therefore, at each location, the planting date that resulted in the highest plant population was set to 100 percent, and all other data at that site was relative to this. Greater variability in the final population exists with planting dates prior to May 1. If planting early, especially before mid-April, the seeding rate should be taken into consideration and potentially increased by up to 5 percent.