Figure 1. Corn field damaged by June 14, 2009, hail storm. Photo on June 18 near Bloomfield, Iowa. R. Elmore photo.
Hail decreases yields by reducing stands as well as destroying leaves. The severity depends on the crop’s growth stage. Corn has an advantage over soybean early in the season when storms roll through since corn’s growing point remains below ground until about the sixth-leaf stage. Young plants like this are not killed if only leaf or stem tissue is lost. Before we go on though, remember that the staging method used by most agronomists differs from that employed by hail adjusters The sixth-leaf stage of the ISU leaf-collar system correlates to the seventh-leaf stage used by hail adjusters. From V8 and following (ISU system), the hail adjuster method is about two leaves ahead. We will use the ISU leaf-collar system in this discussion. By destroying leaves and reducing stands, hail can wreak havoc on a corn crop, especially this late in the season.
Figure 2. Corn plant damaged by hail on June 14, 2009, with base of stem cut lengthwise. Brown discoloration above growing point will likely kill the plant's growing point. Weather following the hail storm was not conducive for vigorous plant growth and recovery. Bloomfield, Iowa, June 18, 2009. R. Elmore photo.
4) Estimate yield losses from defoliation. As just mentioned, leaf loss or defoliation will rarely affect yield before the sixth leaf stage. Plants with six leaves or greater will experience yield losses depending on the extent of the defoliation - see Table 1. Table 1. Estimated percent yield reduction from leaf loss caused by hail damage. Corn can withstand a substantial loss of leaf area without major yield losses. A reduction in leaf area less than 50 percent does not reduce yield if it occurs before V13. For example, at V13 - thirteen collared leaves - a 50 percent reduction in leaf area reduces yield by only 10 percent. However, when the crop is tasseled, VT, yield is reduced by 31 percent. 5) Estimate yield losses from stand reductions. Stand loss may occur following significant hail storms. Small reductions in plant survival do not impact yields much; for example a one-third reduction in stand will only reduce yield by 1o percent if it occurs before V8. Neighboring plants compensate to some extent for the lost plant. However, after V8, yields are reduced by the same amount that the stand is reduced. A one-third reduction in stand will reduce the yield potential by one-third. We are conducting research in conjunction with the crop insurance industry to determine if this is still valid with modern hybrids and management. Twisted whorl plants – a.k.a. tied or buggy whipped - may result from hail injury. A study on twisted-whorl plants by Thomison and Mangen at the Ohio State University found that in fields with major hail damage exhibited 36 to 61 percent twisted whorls. One month later that number was reduced to 0 to 9 percent; most plants grew out of it. The site that had the largest yield losses experienced major stand losses. Once plant survival is established, use Table 2 to determine yield potential of the current stand based on the original planting date and plant population. Compare this to the yield potential of a replant. Replanting corn now is difficult to justify based on normal planting date responses. Table 2. Relative yield potential of corn by planting date and population. Note: Values based on preliminary Iowa research and modeling; 100% yield potential is estimated to occur with 35,000 plant population and early planting. From: Iowa State University Extension, Corn Field Guide, CSI 001. 2009. Overall, remember the key is to assess plant viability thoroughly once plants have had a good chance to recover. Contact your crop insurance company before destroying the crop or replanting. Roger W. Elmore and Lori Abendroth