Storms packing tremendous wind rolled through central Iowa and into east central Iowa in the early morning hours of July 11, causing a lot of destruction. Damage includes quite a few corn fields flattened, a number of empty grain bins and machine sheds blown into nearby fields, grain elevator legs knocked down, electric utility poles toppled, power lines down, roofs of houses ripped off and windows blown out.

"With that tremendous wind comes damage to a very good looking 2011 corn crop in these areas," observes Mark Licht, an Iowa State University Extension field agronomist who covers central and west central Iowa.

The wind damage to corn started north of Woodward and moved east through Slater, Huxley, State Center, Marshalltown and on eastward. "I've heard reports the damage was 8 miles wide in several areas. From what I've seen at this early stage is mostly lodged corn, and some of it is blown over so badly that it's either laying on the ground or is within a foot of the ground. I've also seen fields with some slight green snap."

Jim Fawcett, another ISU Extension crop specialist, covers east central Iowa and he says a number of corn fields have been damaged there, too. Soybeans are growing lower to the ground so they didn't suffer much damage if any. It's mostly corn that we're concerned about, says Fawcett.

When did winds hit? How close was corn to tasseling?

Will corn that's been blown down that bad grow upright again? From here on it is mostly a waiting game to see if the corn does straighten itself up, at least somewhat, as it continues growing.

"Most of this corn is between 3 and 7 days from tasseling and in some fields the first tassels became visible this past weekend," says Licht. "The rule of thumb I follow is if the corn is more than 10 days from tasseling, lodged corn will 'goose neck' and grow upright at least enough to form reasonable rows. But 10 days prior to tasseling the amount of corn that will gain vertical orientation again decreases. And if it's after tasseling when the wind hits hard and the corn is flattened, then very little of the lodged corn will regain vertical orientation."

How much yield loss is associated with lodged corn?

One question Licht always gets is how much yield loss can be associated with lodging? There is very little data available but he has no doubt that some yield loss will occur compared to unaffected fields. The straighter the corn stand, the better the odds of having better yields.

If there was green snap, the rule of thumb is 1% yield loss for each 1% of stand loss. This is not completely true, he says, since neighboring plants can compensate with increased grain fill, but 1% yield loss per 1% stand loss is an easy rule to follow.

"True crop damage from a ferocious storm like this one will take several weeks to assess the amount of yield loss," he notes. "But some beginning assessments can be made in about three to five days."

Several weeks before you can assess amount of yield loss

Fawcett says the straight-line winds that struck around 3 a.m. on Monday July 11 laid corn down from Story County in central Iowa along U.S. Highway 30 to Cedar Rapids. Strong winds of near 100 mph hit many fields in Marshall County, Tama County and western Benton County. He looked at corn damage on Monday after the storm in eastern Jones County.

"So this storm stretched from northern Polk County, towards Cedar Rapids and then on toward Dubuque," notes Fawcett. Some corn fields suffered more wind damage than others. "The damage is spotty, it's worse in some fields than others depending on row direction, wind direction and wind speed. Also, soil moisture, stage of corn development and hybrid make a difference."

What should a farmer look for when evaluating wind damage to corn? First, check to see if it's just root-lodging or if it is green snap, says Fawcett. Green snap is worse, of course. At this stage of the game once the corn stalk has snapped off, it's done. However, if the corn is leaning over badly with root lodging, the corn plants will grow to upright themselves, at least up-righting the upper half of the plant. That will greatly improve the ability to pollinate the crop.

Contact your crop insurance agent if you've had damage

Should you contact your crop insurance agent? "The main thing is whether or not you have greensnap or whether the corn is just leaning over," says Fawcett. "Particularly if you've got a lot of greensnap, get hold of your insurance agent. But if the majority of the corn is just leaning, that's much better news. It should grow and straighten out in another week or two and look a lot better."

Here is an article from ISU's ICM newsletter July 2006 on green snap and yield reductions: http://www.ipm.iastate.edu/ipm/icm/2006/7-10/greensnap.html. Yield reductions from root lodging relative to stage of development is explained in the following ICM article from 2002 where a similar situation occurred: http://www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/yieldeffects.html