The impact of hail damage is largely dependent on corn’s stage of development. Hail affects yield primarily by reducing stands and defoliating plants. Most of the hail damage results from defoliation. Generally, the corn plant is little affected by hail prior to the 6-leaf collar stage because the growing point is at or below the soil surface and in the leaf whorl.
However, once the growing point is elevated above the soil surface due to internode elongation, the plant grows rapidly and becomes increasingly vulnerable to hail damage with the tassel stage/pollen shedding stage (VT) being the most critical period.
Severe hail damage prior to the 6 to 7-leaf stage can also result in “twisted” or “tied” leaf whorls as injured plants recover and new leaves try to unroll; however, most plants will grow out of this problem and tied whorls seldom cause major yield loss.
Leaf damage by hail usually looks much worse than it really is, especially during the early stages of vegetative growth. Shredded leaves and plants with broken midribs still have some capacity to contribute to plant growth. Plants not killed outright by hail usually show new growth within 3 to 5 days after injury occurs (i.e. if damage occurs prior to tasseling).
For this reason, estimates of hail damage should be delayed several days to allow for this period of re-growth.
The hail insurance adjustor’s growth staging system counts leaves beyond the last visible collar to the uppermost leaf that is 40-50% exposed whose tip points downward – usually this results in a leaf stage that is numerically 2 leaves greater than the “leaf collar method” (e.g. a V11 plant according to the leaf collar method would probably correspond to a 13-leaf plant according to the hail adjustor’s method).
How do we estimate the potential yield loss from recent hail storms? This year stages of development will vary considerably depending on location, planting date, etc. Within some corn fields, it’s not unusual to see corn differ by three or more growth stages because of differences in soil color, drainage, and previous crop residues.
Most corn has not progressed much beyond the V13 stage in many areas. Based on estimates of the National Crop Insurance Association (see table below), at the 15-leaf stage (or about V13) if 50% of the leaf tissue is destroyed by hail, a corn plant loses 15% of its grain yield potential; if 100% defoliation occurs, a corn plant loses 51% of its yield potential.