They aren't build to eat it. When Cows are fed Corn they often get infections. This is why Grass fed Beef is better as it has not antibiotics or anti infection agents in them that Humans don't need.
They are ruminants and are supposed to be eating grains and grass... not corn (or corn syrup). Feeding them corn or other unnatural sources of sustenance is bad for them, and also degrades the nutritional benefits of consuming their meat, milk or other edibles derived from them (even before considering the other trash that is injected in them), that is to say, improperly fed and managed livestock is UNHEALTHY to us as well not just to the livestock.
And this is one of many reasons I simply do not consume livestock.
Approximately 85% of corn grown in the U.S. in 2012 is estimated to be genetically modified.
And this is one of many reasons I simply do not consume livestock.