When it comes to corn yields, conformity counts. “Rapid, uniform germination and emergence of corn help set the stage for maximum grain yield,” says Purdue University Agronomist Bob Nielsen. That's why it pays to get out of the tractor during planting to check seed placement. It's your best defense against yield-robbing height differences.

Uneven seedling emergence is common. “It's almost like hail,” Nielsen says. “Every year — somewhere, someplace — we'll see areas of uneven emergence.”

The problem was widespread in 2007. In Indiana, for example, a wet spring followed by an unexpectedly rapid dry-out in early May caused cloddy seedbeds and poor contact between seed and soil in many corn fields, Nielsen says.

“That contributed to several different germination flushes of the corn crop and a lot of delayed emergence.” On two Purdue research farms, “we had corn that emerged over a one-month period, leading to big differences” in plant height. In one field, the earliest emerging plants were head high, while “segments of rows were only 3-4 in. tall. That's as bad as it gets.”

It was the same story in Iowa and much of the Corn Belt, says Agronomist Lori Abendroth of Iowa State University (ISU). “Fields tended to be worked or planted wet, leading to more sidewall compaction, poor seed-to-soil contact and poor emergence.”

UNEVEN CORN STANDS produce lower yields than stands where every plant looks alike, she says. That's because younger plants are at a disadvantage when they compete with larger plants for sunlight, moisture and nutrients, so they produce less grain. And early emerging corn plants don't fully compensate for losses from neighboring runts, she says.

“It's tempting to tell yourself that the crop will grow out of it — make up for the effects of uneven emergence later,” Nielsen says. But “uneven emergence and variable plant spacing are problems that will haunt you the whole season.”

What's the penalty for uneven seedling emergence?

It depends on how many plants are delayed and how far behind they are, Abendroth says. If one-fourth of the crop emerges about one week late, expect a yield loss of about 6%, she says. If half the plants come up two weeks late, losses can reach 17%, and a three-week delay could push the yield penalty to 20% or more. (Abendroth and ISU Corn Specialist Roger Elmore have put together an online tool to estimate yield loss from uneven corn heights at: www.agronext.iastate.edu/corn/production/management/early/height.html.)

Uneven emergence is caused by a variety of factors. Among the main culprits:

  1. Variable soil moisture in the seed depth zone due to different soil characteristics and topography or secondary tillage. “Uneven soil moisture throughout the seed zone is the primary cause of uneven emergence,” Nielsen says, “the results of which can easily be yield losses of 8-10%.”

  2. Uneven seed-to-soil contact due to cloddy seedbeds, old root balls near the surface of continuous corn fields, inability of no-till coulters to slice cleanly through residue, worn disc openers or misadjusted closing wheels.

  3. Variable soil temperatures in the seed zone due to uneven residue distribution, uneven seeding depth or differences in soil color, soil type or topography. “Temperature variability during germination and emergence is most critical when average soil temperatures are hovering around 50° F,” the minimum germination threshold, Nielsen says.

  4. Variable soil crusting due to excessive tillage, heavy rainfall before emergence or excessive down-pressure on the planter closing wheels.

    What can you do to avoid these problems?

    Often, environmental conditions beyond your control are to blame, Abendroth says. Still, good management can improve your chances of uniform germination, she and Nielsen say. Their advice: Prepare the best possible seedbed and monitor seed placement field-by-field during planting. And, don't neglect planter maintenance and adjustment. They offer these tips:

  5. Avoid tilling soils that are too wet. This often creates cloddy seedbeds and shallow compaction. Reduce tillage trips in wet conditions, Abendroth says. “Although it is hard to wait, tillage at the wrong time can cause season-long problems that will never disappear because of poor root development.”

  6. Choose a seeding depth that ensures adequate, uniform moisture. The correct seeding depth is based on seedbed conditions and weather outlook, Nielsen says. That means evaluating every new field as you pull in. If seed-zone moisture is marginal, you might need to increase seeding depth to 2.5-3 in., he says. But don't plant above 1.5 in., which raises the risk of uneven germination and places the main root system too shallow, Abendroth says.

  7. Distribute surface residue evenly. Seed-depth soil temperatures can vary significantly if crop residues in reduced-tillage systems aren't distributed evenly, Nielsen says. Soil under heavy residue will be much colder than under barer spots. Use row-cleaning attachments to move aside surface residue and expose the seedbed to warming sunlight.

    “Whippers, wipers, movers, fingers and other similar trash management gadgets for the planter are most beneficial when you are challenged with rocky, cloddy or trashy surface-soil conditions,” he says. Nielsen also recommends row cleaners for planting corn after corn to move old root balls out of the way.

  8. Check seed depth and seed-to-soil contact in several areas of the field. If placement is inconsistent, adjust seed openers and closing-wheel pressure. “It takes a lot of discipline to check seed-zone moisture and depth and make a lot of adjustments when you're trying to go full speed to finish planting,” Nielsen adds.