About 100 farmers and others gathered on a farmstead south of Vinton in east central Iowa on Thursday July 14 to find out information about what they should be doing with wind-damaged corn fields and what they can expect in terms of yield potential as a result of a tremendous July 11 windstorm. Iowa State University Extension specialists conducted the meeting.

"Damaging winds have set these fields of corn back, as the winds flattened many of the plants," noted Jim Fawcett, ISU Extension field agronomist. "But these lodged corn fields are recovering and are looking a lot better now than just four days ago after the windstorm struck with gusts of up to 100 mph or more."

However, in the fields where stalks were snapped off, it's a different story, notes Fawcett. Where "greensnap" occurred, those plants are dead and yield losses will be a lot greater than where corn is just lodged. Lodged corn, although it was laying flat on the ground on Monday, was up and re-gaining its normal stature again by Thursday afternoon. Horizontal stalks are becoming vertical once more, said farmers attending the field day.

Corn is susceptible to root lodging, pinching and greensnap prior to silking, explained Fawcett.  All three likely happened to varying degrees in parts of central, eastern and northeast Iowa where the storm hit July 11. Moist soil kept a lot of this corn from breaking off in the ferocious wind. 

Straight line winds sometimes exceeding 100 miles per hour ravaged central, eastern and north-eastern Iowa corn fields early Monday morning, July 11, 2011. The storm followed its destructive path into Wisconsin and northern Illinois.

"Winds in Iowa leveled corn in the worst affected areas," says Roger Elmore, ISU Extension corn agronomist. "On Monday after the storm the corn plants in many fields were knocked down to less than knee high. This follows glowing reports of how well the 2011 Iowa corn crop looked, which was before the wind came along. The lodging was not likely related to corn rootworm feeding."

Wet soils helped keep stalks from breaking off in tremendous wind

Things could have been worse, Elmore points out. USDA's regular weekly weather and crop conditions report comes out on Monday each week but it is released the afternoon of July 11. So the Monday July 11 report was based on crop conditions and information gathered statewide on Sunday July 10, prior to the publishing of the Monday report.

Monday's USDA-NASS Crops and Weather report listed 82% of Iowa's corn in the 'Good' to 'Excellent' category; only 4% of the state's 2011 corn crop had tassels and 1% was silked. This differs from the average of 25% tasseled and 13% silked in a normal year. And it dramatically contrasts with that of last year when 47% was tasseled and 24% silked. By the weekend of July 16, tassels and silks will be emerging on a good share of Iowa acres.

According to the USDA-NASS report, top- and sub-soils across most of Iowa had 'Adequate' to 'Surplus' soil moisture. In areas with the worst wind damage, (central and east central Iowa) 83% to 84% of the top soils had either 'Adequate' or  'Surplus' and 83% soil moisture and 92% to 93% of the sub-soils ranked in those highest categories too. Last year though, 99% to 100% of central and east central top-soil and sub-soils ranked in that category for the same week.

Evaluate the type and extent of damage you have in wind-hit fields

"Corn is susceptible to root lodging, pinching and greensnap prior to silking, says Elmore. "All three likely happened to varying degrees in the damaged area. Offering the following explanation, Elmore talks briefly about each one:

* Root lodging:  Roots act as guy ropes and props that anchor corn plants against lodging. Initially both windward and leeward roots play a role with slow wind speeds, however, as wind speeds increase, the role of the windward and leeward roots change. During high wind events, windward roots are pulled from the soil while leeward roots are pushed into the soil. Although it might make sense that lodging comes from windward roots that fail to hold fast to the soil, the fragile link in rooting structures is the weakness in compression of the leeward corn roots from bearing large downward loads. A rotation of 10 degrees is enough to cause the leeward roots to buckle and the plant to lodge.

Root mass reaches its maximum at silking (R1). Brace roots provide support to the stalk and are of considerable importance in "resurrecting" plants that are root lodged by strong winds. Fortunately, plants root lodged before the R1/R2 stages of corn growth [silking and blister stages] are somewhat able to compensate for the canopy disruption caused by the lodging. After a couple of days, the upper portions of these plants resume a vertical growth pattern, 'goosenecking.'

Although this rearrangement of the crop canopy may limit potential yield losses, it does make harvesting slower and increases the potential for ear loss during harvest. For more information see the ISU Agronomy Extension Corn Production web article.

Corn in a University of Wisconsin study with moist soil was lodged by hand at different growth stages. Lodged corn in the V17 to R1 (silking) stages reduced yields by 12% to 31%. "My colleague, Joe Lauer who is a corn agronomist at the University of Wisconsin, summarized this data as well as the greensnap data in an Agronomy Advice article," says Elmore.

Corn root lodging in eastern Iowa field.

* Greensnap: Corn is most susceptible to greensnap prior to tasseling, when it is rapidly growing….We've learned from previous greensnap events in Iowa and Nebraska that yield loss is directly related to the amount of stalk breakage that occurred. In other words, yield loss from broken plants is directly related to stand loss. If 10% of plants are broken this will result in a 10% yield reduction. This is due to a lack of compensation ability to this reduced plant competition by the corn plant this time of year. The maximum of what that ear is capable of has already been largely determined.

Hybrids vary dramatically in their tolerance to greensnap. Several companies provide growers with greensnap ratings that may prove useful in selecting less susceptible hybrids. Stage of growth affects breakage too as mentioned above. Factors that increase early season growth tend to increase breakage susceptibility, such as high N, P and K rates; spring-applied N; tillage; and high organic matter." This is described in an ISU Agronomy Extension Corn Production web article.

Most of this previous research was with breakage below the primary ear node.

However, new data indicates that the impact may not be as severe as a 1:1 loss.  Melanie Knaak - in her creative component for her Distance Masters Degree in Agronomy at Iowa State University, 2011 - found that yields resulting from breakage below the ear were not as severe as we previously thought. In the two years of work in Minnesota, the percent broken:percent yield reduction ranged from about 1:0.50  to 1:0.73, considerably better than the older reports of a 1:1 loss. Breakage above the ear had even less negative yield consequences. This new information suggests that standing plants compensate to some degree for the loss of their neighboring broken plants.

In the worst case situation, yield reduction from greensnap may range up to a 1:1 percent broken : yield loss. It is possible that these losses will be as low as 1:0.73 or even 1:0.50 losses.

* Pinching: Little data exists to my knowledge on the outcome of pinched plants. They will certainly remain alive and attempt to gooseneck so that the top of the plant will be upright. I would expect yield losses somewhere between that of root-lodged plants and those of plants broken by the winds - greensnap.

What about seed production fields that sustained lodging?

The research summarized above relates to grain production fields, notes Elmore. Unfortunately, the area of Iowa that was affected by these July 11 winds also contains large areas of seed production fields, grown to produce seed for various seed corn companies. In addition, to the yield reductions mentioned above, rogueing, detasseling, field inspection and other operations will be dramatically affected because of the disruption in the canopy including position of the female rows  and their orientation.

Keep in mind there are a few things to be thankful for…

Belief it or not, yield reductions from these derecho or straight-line winds could have been even more severe. Elmore explains:

1) Soils at the time of the derecho were wet, as discussed above. This is why root lodging occurred. If soils were drier, we would have experienced more greensnap and thus higher yield losses.

2) Also as mentioned above, crop development is behind normal this year because of the wet spring and delayed planting. If more of our 2011 Iowa corn crop was tasseling or silking at the time the derecho hit, our losses would have been much greater.