If you could pull an umbrella over every corn pant during a hailstorm, maybe you could protect your investment. Since you can't, you're often left to hope, pray and wait to assess the damage. Many times, it's not as bad as it first looks. Sometimes it is. The difference often boils down to how many leaves are removed and the stage of growth at which the damage occurs.
Two years ago hail pummeled one field in northern Indiana. Assessing it four days afterwards, the best the seedsman could come up with was 60 bushels per acre. Others in the party looking at the damage thought he was far too optimistic. The problem was it was too late to replant to corn, being the last of June, and too risky to come back in with soybeans, based upon what had been sprayed in the field for weed control.
The problem was that besides the stalks that were straight out destroyed, many were bruised where hailstones hit. Bob Nielsen, Purdue university agronomist, would later say that bruises at or below the growing point were a definite concern. Remaining population in this field, counting some bruised plants, was 14,000-16,000, at least in the worst spots.
The farmer left it. If you had bought it for 70 bushels per acre, which he would have gladly accepted that day, you would have harvested a bargain. It wound up making just over 100 bushels per acre. Perhaps some spots weren't hit as bad as either, even though he had walked the entire field. And obviously, the weather cooperated from that point forward for the rest of the season.
If the timing of the hail is wrong and defoliation is too great, you won't be that lucky. Nielsen demonstrates this every year to participants at the Purdue University Crop Diagnostic Training Center. The crew artificially strips leaves off plants at various stages of growth using a weed eater to simulate hail damage. Then Nielsen strips back ears during the clinic days to show what pollinated and what didn't make it. He uses the shake test, holding ears outright and shaking them, to determine how many kernels pollinated. If the kernel pollinated, the silk falls. If not, it remains attached.
Here are examples from the chart used by hail adjustors to determine percent loss to be expected from hail damage on corn. It is published in the 2008 Purdue Corn & Soybean Field Guide. Suppose your corn suffers 50% leaf loss due to hail at the 10-leaf stage. You're only looking at 6% yield loss. That assumes you don't get a lot of buggy whipping and inability of the plant to continue growing/
Suppose that same 50% leaf loss occurs much later, at the tassel stage. Now you're up to 31% yield loss, on average, or nearly a third of the original yield potential. That's the maximum sensitive period for loss, with it gradually declining again as the plant goes through its later stages of development. At late milk stage, a 50% leaf loss should translate into about a 12% yield loss. At higher percentages of leaf loss, the penalties are more severe, especially if the plant is in the early reproductive stages, including tasseling.