Hail damage always makes corn look bad, and can make for some sleepless nights. But while the physical damage is apparent, the actual effect on yield is not as obvious, says Kraig Roozeboom, Kansas State University agronomist.
The hail damage's effect on corn yields depends on the stage of growth. "Potential corn yield losses from hail gradually increase as the crop gets more mature, up to the silk stage, when peak yield loss occurs," he says. "After silking, yield losses from hail damage normally decline again."
There are several reasons for that, and all are based on the corn plant's growth and development:
Emergence until stem elongation (VE to V5 development stages). Up through the 5-leaf stage of growth, the growing point of corn is below the soil surface. At the worst, hail damage would remove all five leaves, but typically not damage the growing point. A corn plant has 24 to 26 leaves at tasseling. If the plant loses five of those leaves early on, it will still have the potential to have 19 to 21 leaves at tasseling. Yield will be reduced, but by much less than one might expect from the appearance of the plant.
Stem elongation to tassel (V6 to VT development stages). In this stage, the growing point begins extending above ground by the 6-leaf stage, although it is still protected by several layers of leaves and sheaths. The number of rows that will be in the ear is established by the 12-leaf stage. Stress during V8 to V11 can reduce row number. The number of kernels per row is not determined until about V17, just before tasseling. Hail damage and loss of leaf area during these growth stages can cause increasing potential for yield loss. Hail can also cause stalk bruising during these growth stages, but it is hard to determine the amount of damage from stalk bruising until later in the season.
Tassel to maturity (VT to R6 or tassel to silk development stages). At VT to R1, the corn plant is more vulnerable to hail damage than at any other stage. The tassel and all leaves are exposed at that time. No more leaves will be developed, and the corn cannot replace a damaged tassel. Furthermore, the stalk is exposed, with only one layer of leaf sheath protecting it. Unlike wheat, corn cannot fill extensively from the stem if leaves are lost at this growth stage. The six to eight leaves above the ear are the most important, and provide most of the grain fill. If damage occurs later in grain fill, much of the yield is already in the grain. Shredded leaves may prevent or reduce the last few increments of dry matter accumulation, but what has been deposited in the grain already is typically not lost. Losses at this stage occur when large hail stones puncture the husks and allow insects and diseases to access the immature grain.
The four-week period centered around silking is critical for corn, and not only in regard to hail damage, Roozeboom adds.
"Drought stress, excessive moisture, extreme heat, diseases, and even high winds can all stress the plant at this critical stage and reduce yields," he says. "Early in this period, stress can reduce kernel number by limiting potential ear size. Stress right at silking can reduce the number of kernels fertilized. And stress just after silking can cause fertilized kernels to abort."

Source: Kansas State University