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It's corn planting time

D.R. Hicks, Extension Agronomist

Remember last year? 22% of Minnesota's corn acres were planted in mid April. The seedbed was ideal and the soil temperature was above normal for mid April. But, it turned cold and stayed that way for 30 days. As a result seed laid in the ground for 30 to 40 days. The end result was uneven emerging plants and stands that were substantially lower than the desired plant populations. In addition, stands were not uniform in plant spacing. Yet - we had a record state average corn yield in Minnesota of 156 bushels per acre!

So, when will you plant corn this year?

Fig. 1 Long Term avg.gif
With the poor start to the season last year several were asking if we are planting corn too early in Minnesota. My answer is clearly no! The air temperatures and subsequent soil temperatures last year were not normal. Figure 1 shows the long-term soil temperature at the two-inch depth and the soil temperatures for last year. For the historic average, the soil temperature at the two-inch depth shows an increase in temperature during the month of April and early May. The temperature reaches 50 degrees about April 20, which is the temperature the seed needs to reach to support and promote germination and seedling growth. 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 soil temperature was above the long-term average last year in mid April and then fell below the normal warming trend until late May. There were warming and cooling periods for the next 30 days. And even though stands were lower than expected, one can be surprised that such a high percentage of the seed did continue to survive, germinate, and emerge. With a normal spring, we shouldn't have the long, extended time for emergence that we had last year.

And we can't forget the importance of early planting for maximum yield. I believe that early planting was a major reason for the high yields last year. Of course the rainfall and timing of the rains were important, but early planting sets the stage for high yields and minimizes the first yield-limiting barrier. Production costs are fixed and independent of when corn is planted; planting date is a no cost production practice. So there is greater profitability with early planting and higher yields. The yield relationship with planting date is given in Table 1.

Table 1 Effect of planting date on corn yield.
Planting date Potential yield
percent (%)
April 25100
April 3099
May 597
May 1094
May 1591
May 2588
May 3083

Are uniform stands important and do late emerging plants contribute to yield?

Growers worried about the poor stands and uneven emergence last year and rightly so because cornfields are not as pretty when plants are different heights with gaps between plants and not growing uniformly. I would also prefer to see uniformly emerged plants is every field because each plant has an equal chance at competition for water, nutrients, and light. So how important is this?

Results from a study to delay emergence of various portions of the plants are given in Table 2. When 25% of the stand (full stand was 30000 plants per acre) was missing, the yield was 90% of potential or 10% of the yield was lost. When the stand was full (30000ppa) but 25% emerged 10 days later than the rest of the stand, yield was reduced only 6%, so the late emerging plants contributed 4% to yield. The yield contribution was not equal to the other plants (25% of the plants only produced 4% of the yield), but they did contribute as long as the delay in emergence was not too long. When emergence was delayed 20 days for 25% of the stand, there was no contribution to yield (yield was 90% of potential which was the same yield obtained with 25% of the stand missing).

When stands are extremely non-uniform, late emerging plants contribute more to yield. For example, yield is only 70% of potential with a 50% stand (30% loss). If those 50% of the plants emerge 10 days later the yield loss is only 8% and only 20% if they emerge 20 days later, so the later emerging plants contribute to yield.

And, an important point to remember is that the yield potential of an early planted field with a poor stand is usually better than the yield potential of a later planted field with a full stand of uniformly spaced and similar height plants. That's not what we want to look at in June and July, but has a better profitability potential. And I haven't mentioned that there would be replant costs and later maturity with higher drying costs in the fall! And then there's the question of planting depth with early corn planting.

Certainly the seed zone will be warmer the closer the seeds are to the soil surface because the soil warms from the top down. So one might want to plant shallow with early planting to promote more rapid germination and emergence. I recommend planting corn 2-inches deep regardless of planting date. I think there will be fewer potential problems later with a 2-in depth. With normal weather, the soil is on a warming trend and the extra few days to emerge for corn planted 2-in deep versus 1 ½ - in deep is not worth the risk of poor root development that can occur with shallow planting. Sometimes the seedbed settles, particularly with rain. When this occurs, the seed doesn't move much from where it was placed in the soil, but the planting depth is now shallower because the surface settled. If the surface settles enough, then the crown of the plant is very close to the surface and may result in reduced and poor secondary root development. And these roots are the permanent roots of the corn plant. So, plant corn 2-in deep.

Table 2. Effect of uneven emergence and missing plants on corn yield.
Stand condition
Stand loss Potential yield (%)
25% Missing90
50% Missing70
75% Missing49
Potential yield (%)
Late emergence 10 days later 20 days later
25% late9490
50% late 9280
75% late 9378

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