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We’ve had a significant portion of Minnesota corn acreage planted prior to May 1 for the past two years (2002 and 2003). In both years, plant stands have not been ideal - spacing between plants has not been uniform and final stands have been lower than expected. Yet both years have been excellent corn production years for Minnesota growers. The state average yield was a record 157 bushels per acre in 2002 and an impressive 146 bushels per acre in 2003.

Early planted corn seed will lay in the ground longer before emergence because of soil temperature. About 100 heat units are necessary to cause seed to germinate and emerge. Soil temperature where the seed lays in the soil needs to be 50 degrees or higher to promote germination and seedling growth. In late April and early May, the long-term average daily soil temperature at the two-inch depth (about where the seed is) is below 50 degrees (Figure 1). The soil temperature fluctuates around this average temperature with part of the day when the temperature is above average and part of the day when the temperature is below average.

As the warming trend continues, there is a greater portion of the day when the seed zone soil temperature is above 50 degrees. That's why early-planted corn requires more time to emerge than does later-planted corn. The number of days from planting to emergence is given in Figure 2 for planting dates beginning April 15. With average soil temperatures, corn seed put in the ground mid-April will lay there for about 25 days before emergence. This is usually not a problem because the temperature where the seed is laying is too low for rotting organisms to flourish, so seeds germinate and emerge after some time.

When corn is planted later, there are fewer days that the seed lays in the ground because soil temperatures in the seed zone are higher. Plants will emerge in 5 to 7 days when planted in mid to late May.

Because soil temperature is so marginal in late April with respect to the temperature necessary to promote germination and emergence, small differences in soil temperature can cause uneven emerging plants with slow growth rates in mid to late May. Soil temperature differences in the seed zone occur because of differing water conditions and residue pieces on the surface, both of which cause the soil to warm more slowly. Uneven emerging fields may not be as pretty to look at as are fields planted later that emerge more uniformly and appear to be growing faster.

Uniform stands (both spacing between plants and time of emergence) are important to give all plants equal competition to water and nutrients. However, non-uniform stands are productive and profitable stands because late emerging plants do contribute to yield. Yields from corn stands with late emerging plants are given in the Corn Planting Newsletter (4/15/03) at http://www.corn.umn.edu.

There will be years again like the past two where stands will not be as good as growers intend. But these early-planted stands (lower than desired populations or uneven emerging plants or both) have a higher yield potential than do the later-planted stands that emerge more uniformly and appear to grow faster. Early planting sets the stage for high yields and the greatest profitability (see the Corn Planting Newsletter cited above for the relationship between planting date and corn yield in Minnesota).

There are two planting windows in Minnesota. The first is between April 15 and May 5. Then there is a higher probability of rainfall that stops field work. The second window for planting begins about May 15. Times and durations of these planting windows vary every year, but the pattern is the same. For maximum profitability, don’t miss the first planting window!