This past weekend many parts of Wisconsin had hail on corn. Those who will be advising growers faced with the likelihood of hail damage should get ready by consulting the National Corn Handbook NCH-1 "Assessing Hail Damage to Corn". This publication does a good job of describing factors to consider, and has charts used by the National Crop Insurance Association for assessing yield loss due to 1) stand reduction through tenth-leaf stage only, and 2) defoliation.

Hail affects yields primarily by reducing stands and defoliating the plant. Defoliation causes most of the loses.Knowing how to recognize hail damage and assess probable loss is important for decision making.

Because it is difficult to distinguish living from dead tissue immediately after a storm, the assessment should be delayed 7 to 10 days. By that time regrowth of living plants will have begun and discolored dead tissue will be apparent.

Some corn in Wisconsin is past the V-10 stage of development. As the season progresses, hail injury and losses could become more significant. Some comments on concerns not covered by NCH-1:

After the tenth leaf stage, yield and stand reductions are on about a one-to-one ratio (eg. 80% stand = 80% potential) and are in addition to losses shown in the defoliation chart.
Plants with bruised, but not severed stalks or ears will usually produce a near normal, harvestable ear.
Growers should monitor stalk rot of severely defoliated plants which have a good-sized ear. Photosynthate will be mobilized towards the ear rather than the stalk. This could weaken the stalk and encourage stalk rot development. These fields may need to be harvested early to avoid standability problems.
Nitrate levels in corn may become elevated. Animal performance could be reduced. Growers with complete defoliation and high soil nitrogen levels (due to fertilizer, manure, or legume plowdown) should test nitrate levels and probably ensile the corn before feeding.
Late season leaf loss will allow more light to penetrate to the soil and late-season weed growth may flourish.

After touring some of the affected areas in Rock, Columbia, Dane, Dodge, Jefferson, and Walworth counties, I would not expect significant yield losses from this particular hail storm. Hail was widespread, but not extremely intensive and usually only upper leaves of the canopy were tattered.