By Roger Elmore, Department of Agronomy
Straight line — derecho — winds sometimes exceeding 100 miles per hour ravaged central, eastern and north-eastern Iowa corn fields early Monday morning, July 11. The storm followed its destructive path into Wisconsin and northern Illinois. Winds in Iowa leveled corn in the worst affected areas — to less than knee high. This follows glowing reports of how well the crop looked, before the wind came along. The lodging was not likely related to corn rootworm feeding.
Crop and soil conditions
Monday's USDA-NASS Crops and Weather report listed 82 percent of Iowa's corn in the 'Good' to 'Excellent' category; only four percent of our crop had tassels and 1 percent silked. This differs from the average of 25 percent tasseled and 13 percent silked, and dramatically contrasts with that of last year when 47 percent was tasseled and 24 percent silked. By the end of this week, tassels and silks will emerge on a good share of our acres.
According to the current USDA- NASS report, top- and sub-soils across most of Iowa had 'Adequate' to 'Surplus' soil moisture. In the areas with the worst wind damage, central and east central Iowa, 83 to 84 percent of the top soils were listed as having either 'Adequate' or 'Surplus' and 83 percent soil moisture and 92 to 93 percent of the sub-soils ranked in those highest categories too. Last year though, 99 to 100 percent of central and east central top- and sub- soils ranked in that category for the same week.
Types of damage
Corn is susceptible to root lodging, pinching and greensnap prior to silking. All three likely happened to varying degrees in the damaged area although reports are still coming in at this time. Let's talk briefly about each one:
"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 root lodged by strong winds. Fortunately, plants root lodged before R1/R2 [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." (Taken from an 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 percent. My colleague, Joe Lauer, University of Wisconsin corn agronomist, summarized this data as well as the greensnap data in an Agronomy Advice article yesterday.
Corn root lodging in eastern Iowa field.
"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 percent of plants are broken this will result in a 10 percent 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." (Taken from an 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.
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.
A thought on seed production fields
The research summarized above relates to grain production fields. Unfortunately, the area affected by the winds also contains large areas of seed production fields. 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.
A few things to be thankful for….
Belief it or not, yield reductions from the derecho winds could have been even more severe. Let me explain:
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 crop was tasseling or silking at the time the derecho hit, our losses would have been much greater.
Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at email@example.com or (515) 294-6655.