I'll never forget one of the biggest hailstorms we had when I was growing up at home. It lasted 45 minutes. Afterwards, my mom went straight to her garden with her flashlight. "Oh my tomatoes!" she exclaimed. "They're ruined."
My neighbor, who had joined her by then, said, "Well, what do you think of your corn field?"
Mom hadn't thought of that. Fortunately, the corn was far enough along that yield loss was a lot less than it could have been. The big bushy Williams soybeans of the day took a bigger hit. One day they were too big to cultivate – the next day they weren't. They still made a decent yield for the times.
What you should do is wait three to five days instead of running to your field with a flashlight, says Bob Nielsen, Purdue University Extension corn specialist. Give corn plants a chance to recover and show what they can do. Often they recover more than you would think if you see the field right after the storm.
If hail hits at the knee-high stage, one concern is whether the whorl is damaged. If it becomes knotted up, the next group of leaves may have trouble emerging. You should be able to tell that in three to five days, Nielsen notes. If the leaves emerge from the whorl, then the plant will likely grow normally. Yield loss will likely be minimal.
The National Hail Insurance Board has prepared tables that you can find in the Purdue University Corn & Soybean pocket Field Guide and its companion iPad app. Nielsen says they're fairly accurate as to how much loss to expect based upon how much defoliation occurred.
The other factor you need to include is how many plants didn't survive. Often the table will lead you to a lower percent of yield loss than you might expect.