What percent of cow feed is corn?

It depends on the class and breed of animals you are referring to, and the availability of corn to a producer. Some producers will have no corn in the feed for cattle, others will have 85% corn in the ration for their animals (particularly if they're feedlot cattle). Dairy cows, which are lactating, will typically have around 10 to 30 percent corn (or a little more) in their formulated ration.