The spring of 2012 brought agronomists across the state thousands of questions about cornfields that initially emerged relatively evenly but soon began looking worse. Fields looked decent from VE to around V2 to V3 growth stage, but then uneven spots started to show up. When the corn reached about V5 to V7, the better plants really took off, and the bad areas looked like they were “stalling out.”
Often the changeover from the seminal root system to the nodal root system intensifies these differences in plant height; the larger plants typically switch over to the nodal root system quicker and take off.
Growers and agronomists did a ton of digging around in those spots in fields. We found that most of the areas of shorter corn had a common theme: inhibited nodal root systems. We found a lot of reasons for the limitations of nodal root growth and subsequently uneven corn in fields.
When corn hit the later vegetative growth stages in mid- to late June, some differences in plant size were masked by the rapid increase in growth, as many fields took off after some much-needed rainfall. Now that we are in the heart of tasseling time, much of the uneven corn has reared its head again.
Why is corn so uneven?
Growers are wondering why fields are tasseling so unevenly; many of the issues can be traced back to the uneven corn we witnessed in May and June. Agronomists and growers have discussed this in great depth; here are some thoughts on why the corn has been so uneven this year.
• Cold, wet soils early in the season limited early root formation. This was followed by dry soils that weren’t as conducive to root growth, as plants struggled to change from the seminal to the nodal root system. Small differences in planting depth, soils and seed environments led to big differences in plant-to-plant variation.
• Heavy and variable crop residue areas in fields are another factor. Agronomists and growers noted there was much more residue this year due to dry weather since last fall, with subsequent slower decomposition of crop residue. Uneven distribution of some of this residue led to variability in soil temperature and moisture, directly impacting germination and early growth.
• Compacted areas in the seed placement zone — both in the sidewall and right under the seminal root system — about 3 to 4 inches down are another cause of uneven corn stands. Some agronomists theorize that the warm winter and lack of a good freeze-and-thaw cycle contributed to this. We even found this compaction in long-term no-till fields, which may tell us this isn’t strictly something we did, but there was also an additive issue of Mother Nature.
• This seed zone soil compaction seemed to be more prevalent in corn on corn. Agronomists are asking if the rotation to a different type of root system, like tap-rooted soybeans, helped to offset this somewhat. Field observations indicated in some areas rotating corn with beans did help with this. The flip side is that rotation decisions are tough to base on one really odd — and nearly absent — winter freeze-and-thaw cycle.
Will deep tillage help?
Growers are also asking if they should do some sort of mid-level or deep tillage to correct this later in the fall or next spring. These soil compaction issues have been so variable within fields that such tillage may not address the problem as well as a “real” Iowa winter.
It is a tough call to recommend that farmers run through a lot of acres burning diesel fuel, using up time and risking erosion with deep tillage for spotty and shallow “tight soil” issues.
As you pull the crop off this fall, work with your agronomy team and make an area-by-area evaluation of your soil structure when you have a chance.
Factors leading to uneven stands
Keep in mind uneven stands can be caused by anhydrous ammonia burn to corn roots, or by insect injury to the corn and/or mesocotyl of a corn plant. Early-season seedling diseases of the mesocotyl and/or crown of the corn plant also can cause unevenness.
Tillage patterns, especially anything done midspring after we started getting some moisture, are another factor. Also, loose, dry and “fluffy” soil (not my term, by the way) in the seed zone that settled with some of the early rains added to the problem. In effect this caused shallower planting depth by a half inch to an inch or more in some areas. As weather turned hot and dry, this set the corn up for trying to put nodal roots into hot, dry soil.
In some fields where heavy rains washed soil out of the row, the result was very shallow-planted corn (1 inch or less). And there was quite a bit of sidewall smearing and compaction with some of our middle planting windows this year. Even in areas where corn was planted in relatively good conditions, the incredible rapid drying of the seed zone we saw this spring sometimes led to these sidewall issues — something we would normally avoid with a few timely showers.
Combination of causes
Combinations of the above problems were common; rarely does one single factor create uneven corn. More often there is a cascade of factors like those mentioned above that combine to limit growth. The most common question that comes up after “Why is my corn uneven?” is “What can we do about it for next season?”
When making management decisions to try to prevent this problem from occurring in the future, remember it varies by field. But in general there aren’t great answers to contend with the root cause (pun intended) of the uneven corn this season — the extreme weather conditions that started last winter and are extending into this summer so far.
Extended periods of stress magnify the small differences within fields and improve the odds of seedling disease, insect injury, chemical injury and other crop challenges. This will generally make the plant more susceptible to slow root and top growth from any of the above factors.
There are dozens of seemingly small management changes that can help with some of these issues, such as creating a true “V” seed trench, using floating trash whippers, slowing planting speeds, and adjusting straw choppers and spreaders on combines to distribute residue evenly. It’s always a good idea to look over the field as a whole for any patterns to the uneven corn that may help determine causes.
Since each field is different, the best idea is to work with your local dealers and ISU agronomists to see what management factors can help in the future.
The bottom line is that many of the stunted and uneven areas can improve to a degree, especially with some moisture. But it is unlikely they’ll improve as much as we would like and yield to the potential of the “good” areas of the fields. As you probably guessed, with the limited root systems, stunted and late corn plants will continue to suffer more than the good parts of the field if we have extended dry weather.
McGrath is an ISU Extension field agronomist, co-leader of the crops team and author of this Corn-Soybean Insight column. Contact him at cmcgrath@