Ears of corn normally remain erect until some time after physiological maturity has occurred (black layer development), after which the ear shanks eventually collapse and the ears decline or 'droop' down. In recent weeks, corn field connoisseurs have reported droopy ears in fields that have not yet reached physiological maturity.
Droopy ears are cute on certain breeds of dogs, but droopy ears on corn plants prior to physiological maturity are a signal that grain fill has slowed or halted. Premature ear declination (the fancy term for this problem) results in premature black layer formation, lightweight grain, and ultimately lower grain yields per acre.
What Causes Droopy Ears? I believe the primary cause of premature ear declination this year is the fact that cumulative effects of heat stress, drought stress, and, in some cases, disease stress has simply caused premature plant death. Similar instances of premature ear declination occurred during the drought of 1991. In a few cases, collapsed ear shanks I've inspected have also had 1 or more European corn borer larvae tunneling in them. Such tunneling weakens the ear shank, allowing it to collapse, and can ultimately also cause ear droppage from the plant.
Impact on Yield? Remember that the ear shank is the final "pipeline" for the flow of photosynthates into the developing ear. An ear shank that collapses prior to physiological maturity will greatly restrict, if not totally prevent, the completion of grain fill for that ear and will likely cause premature black layer development in the grain. If the droopy ears you've looked have not black layered yet, they will soon.
The timing of the onset of droopy ears determines the magnitude of the expected yield loss. If grain fill were totally shut down at the full dent stage of grain development (milk line barely visible), the yield loss would be as much as 40 percent. If grain fill were totally shut down at the late dent stage of grain development (milk line halfway between dent and tip), yield losses for the affected ears would equal about 12 percent.
Multiplying the percentage of affected ears in a field by the estimated yield loss per ear will give you an estimate of whole field loss. For example, if ten percent of the field contained plants whose ears drooped prematurely at the late dent stage, whole field loss would be estimated at 1.2 percent (10 percent of the ears multiplied by 12 percent yield loss per ear).