April 26, 2011

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Early planted crops can have the highest yield potential, but they don't come without some risk and some worry in years like this. With the recent cold, wet weather, some producers who planted early are beginning to wonder whether their corn crop is doomed.

While these concerns are not unwarranted, before dragging the plow out of the shed or filling the sprayer to kill those plants that have come up, give some serious consideration to actual conditions and the scenario you are dealing with.

With the recent rains and cool air temperatures, corn seed that was already planted is clearly going to develop slowly if at all until conditions change.

How a Kernel Develops

Some Nebraska fields have kernels planted 2 ½ inches deep that have germinated with the radical (primary root) just piercing the pericarp (seed coat). Under adequate field conditions, the planted seed first absorbs water (seed imbibition) and then begins to grow. The radical is the first to begin elongation from the swollen kernel, followed by the coleoptile (spike) with the enclosed plumule (embryonic plant), and then three to four lateral seminal roots. This process is significantly slower under cool soil conditions than under warmer conditions.

To get the crop to germinate requires metabolic energy; initially this energy comes from the endosperm (sugary starch) in the kernel. Once emergence begins, the corn plant will slowly shift from relying on the starch and sugars in the endosperm to creating energy through the photosynthetic process.

The growing point of the crop is currently well below the soil surface and will remain below ground until the plant reaches the 5th to 6th true leaf stage, protecting it from cooler air temperatures. While frost damage or slow growth can result from cooler than normal air temperatures, the plant will generally not be killed. Remember that soil temperature is different from air temperature. With recent conditions, air temperatures have fluctuated greatly, but soil temperatures have not moved up and down as rapidly.

For now, the amount of energy stored in that kernel is usually sufficient for three to maybe four weeks after the imbibtion process begins, weather permitting. Knowing this should help ease the stress of whether the young corn crop is going to emerge. Soil temperatures are currently just below the established optimal minimum of 50°F for normal corn development. As temperatures warm up, corn seedlings will start to grow more rapidly.

Until then, cooler than normal soil temperature will retard the normal development, but at this point, it is not yet detrimental to stand establishment. Serious germination and/or establishment problems would be expected only after an extended period of temperatures below 50°F, usually three or four more weeks.

Of course, this can be compounded if we have too much soil moisture and anerobic (without oxygen) conditions develop. This would prevent normal, healthy plant respiration. If this is the case in particular low-lying, swampy field areas, you may need to replant in areas where water has been standing for three or more days. Overall, however, it appears our soils are wet but not too wet for adequate germination. In fact, in the south central part of the state, conditions were considered to be in mild or moderate drought conditions prior to recent rains.

Assessing Your Field

Before rushing out to start over with this year's corn crop, take the following steps to evaluate your situation:

1. Leave the fields that you have already planted until all other fields with high yield potential are planted. Doing this will give you the opportunity to clearly assess the corn stand and yield potential.
    
2. Assess the actual corn population. Use this corn stand as a starting point for whether to replant. Conditions in Nebraska vary widely for ideal corn populations depending on whether the field is irrigated, the average annual precipitation, and the length of growing season.
    
3. Assess the uniformity of germination in the field. Corn fields with nonuniform plant emergence will have lower yield potential than fields with uniform emergence.
    
4. Use the data you have collected on plant population and uniformity of emergence, clearly identify how quickly the field can be replanted and determine if the cost of replanting plus any potential yield increase justifies replanting.
   
   Generally, corn planted after May 1 (depending on your location in the state) will lose about 1 bu/ac/day of yield potential.

In many cases, this assessment will likely indicate that replanting is not justified unless the cold, wet weather conditions persist. However, plant stands may be considerably lower than planting populations because the longer the seedlings sit in the soil before germinating, the more susceptible they are to early season diseases and insects. The best recommendation for addressing this risk is thorough and frequent scouting.

Mark Reed Hinze
Extension Educator, Hall County
Greg Kruger
Extension Cropping Systems Specialist