Long-term studies indicate that corn yields typically start to decline significantly when planting occurs after May 10 to 14, as shown in Figure 2. The exact dates will vary from year to year. About 50 percent of Iowa’s corn is normally planted by this time. One reason for the decline in yield for late-planted corn is that fewer “heat units” are available during the growing season, and this influences the rate of crop development.
How early to start planting requires considerable judgment. Ideal conditions would be a soil temperature of 50°F (10°C) or above at planting depth and a favorable five-day weather forecast. In most of Iowa, if soil conditions and temperatures are favorable, starting to plant the last ten days of April should be advantageous. In May, the major consideration should be the condition of the seedbed.
There is some risk with early planting. Replanting may occasionally be required, but the long-term benefits far outweigh this cost. An added benefit from early-planted corn is lower grain moisture levels at harvest and reduced drying costs.
Most of the same things can be said about planting soybean varieties. The ideal time for planting adapted soybean varieties is between May 1 and May 15. Yields can be expected to decline in most years if planting occurs after May 20, as Figure 3 shows.
Timeliness losses at harvest are due primarily to more dropped ears, field shattering and cracked beans. These losses must be balanced against the cost of artificially drying grain harvested at a moisture level higher than that required for safe storage. Some harvesting losses occur because combining speed is too high or the machine is poorly adjusted.