ost corn growers will tell you that the maximum corn
yield potential of a hybrid exists while the seed is still in the bag. Once the
seed is in the ground, the challenge is to protect that yield potential from
the many potential stresses that await the developing crop. This growing season
is doing its best to reinforce that belief.
Around the state, folks are lamenting the fact that fields of corn that had emerged uniformly and initially developed uniformly are now fields that contain plants of uneven color and size. Other fields did not emerge uniformly to begin with. The recent spate of unusually cool temperatures and (in some places) excessive rainfall has contributed to the further development of fields that can only be labeled as ugly.
Why worry about uneven stands of corn? Simply put, it leads to unfair competition among adjacent plants and, ultimately, to a lower yield potential for the field. Research from Illinois and Wisconsin documented potential yield losses ranging from 8 to 20% due to uneven emergence, depending on the degree of delayed emergence timing and the extent to which the field was affected. Basically, if delayed emergers are two leaves or greater behind the original emergers, the delayed emergers will likely be barren at the end of the year. Yield losses to competition among plants of similar age but varying degrees of health or vigor is more difficult to document, but likely mimics that due to uneven emergence.
What causes uneven emergence? The three common causes are a) uneven soil moisture in the seed furrow, b) uneven seed to soil contact, and c) uneven soil temperature in the seed furrow. The order in which I listed these causes is probably the order of frequency in which they were responsible for uneven emergence in 2001.
I’ve seen more instances of delayed emergence due to soil moisture problems this year than in many recent years. In some cases, the problem was related to a) uneven seeding depth or b) uneven soil moisture at the selected seeding depth. In other cases, preplant tillage left a cloddy seedbed, especially in the tillage tire tracks, and those areas of the planted field had to wait for the recent soaking rains before germination occurred. It is not uncommon to find fields with plant variability for growth stage as great as four leaves, meaning that if the original emergers are at the V6 stage of development, the delayed emergers are at growth stage V2.
What causes fields that emerged uniformly to later turn ugly? Almost invariably the cause lies below ground. Several weeks ago, I briefly reviewed how roots are supposed to develop in corn. In that article, I emphasized the importance of maintaining the health of the seed and the mesocotyl until the nodal root system was successfully established. As the nodal roots develop, the importance of the seed reserves and the mesocotyl (the 'pipeline' to those reserves) declines.
For all practical purposes, the time from planting to about V4 to V6 can be labeled the 'critical stand establishment' period for corn. If a field of corn successfully develops to the V4 to V6 (four to six visible leaf collars) stages with no damage to mesocotyls or seeds, chances are that field will look very uniform. If, however, mesocotyl or seed damage occurs prior to substantial nodal root development, seedlings will either die or be severely stunted. Consequently, a field that may have emerged uniformly and initially looked quite uniform may become very uneven in appearance if initial seedling development has been affected by one or more stresses.
What stresses are we talking about? The list of potential stresses that can injure the seed, seed roots or mesocotyl is not particularly long, but deciding which ones to blame can be difficult since more than one is usually present in any given field. The list of possible offenders includes:
| Seed rot fungi | Wireworms |
| Seedling blight fungi | Seedcorn maggots |
| Excessively wet soils (death by drowning) | Grubs of various types |
| Excessively dry soils (death by desiccation) | Prying agronomists |
| Anhydrous ammonia fertilizer 'burn' | Starter fertilizer ‘burn’ |
Other factors that influence the effects of these early stresses include cool soils in general, sandblasting injury, herbicide injury, excessively dry conditions and ponding. All of these weather-related stresses slow or hinder the early growth of corn seedlings and make them more vulnerable to the above list of stresses. Similarly, while most herbicides that can injure corn do not do so to such young seedlings, later injury will subsequently hamper the recovery of an already struggling crop.
Seed quality and the hybrid's inherent seedling vigor also play an important role in determining the consequence of injury during stand establishment. Otherwise minor stresses during stand establishment can have major effects on overall plant health if seed quality is less than acceptable or if seedling vigor is simply average.
So, what’s a guy to do? The bad news is that if stand establishment this year is crappy (an agronomic term meaning uneven), there is little you can do about it now. As you think about next year, there are a few things you can keep in mind to minimize the future risk of crappy stands.
A Final Comment. Any time that germination, emergence AND/OR stand establishment are delayed significantly, the developing crop is simply exposed to a lengthier period of stresses from the list above. The consequence of such stresses on a slowly developing crop is exactly the uneven stands of corn that some corn growers are lamenting about now.
For other
information about corn, take a look at the Corn Growers Guidebook on the World
Wide Web at http://www.kingcorn.org