It is not just humans that began to eat more corn. Corn has replaced grass as the primary food source for cows. Cows, unlike humans have a second stomach, called a rumen. This rumen allows them to eat and digest grass. Were humans to eat grass, the stomach would be unable to digest it and would expunge it without digesting it. People can eat corn with little consequence, but cows may only eat corn with human intervention.
Pollan wrote an article for the New York Times (which he adapted for The Omnivore's Dilemma) in 2002 describing the consequences of cows eating corn. He writes:
"Perhaps the most serious thing that can go wrong with a ruminant on corn is feedlot bloat. The rumen is always producing copious amounts of gas, which is normally expelled by belching during rumination. But when the diet contains too much starch and too little roughage, rumination all but stops, and a layer of foamy slime that can trap gas forms in the rumen. The rumen inflates like a balloon, pressing against the animal's lungs. Unless action is promptly taken to relieve the pressure (usually by forcing a hose down the animal's esophagus), the cow suffocates.
A corn diet can also give a cow acidosis. Unlike that in our own highly acidic stomachs, the normal pH of a rumen is neutral. Corn makes it unnaturally acidic, however, causing a kind of bovine heartburn, which in some cases can kill the animal but usually just makes it sick. Acidotic animals go off their feed, pant and salivate excessively, paw at their bellies and eat dirt. The condition can lead to diarrhea, ulcers, bloat, liver disease and a general weakening of the immune system that leaves the animal vulnerable to everything from pneumonia to feedlot polio."
These illnesses require intervention and medication. There are reasons doctors tell nursing mothers to not use certain drugs--because the drugs are passed to the nursing child. Yet the USDA not only allows, but encourages feedlot managers to give drugs to cows, which will subsequently be eaten, and passed along to the eater. This is an amazing contradiction in thought.
When corn became the primary diet for cows, they no longer required pasture, as they had always done before. The cows were gathered together in Concentrated Animal Feeding Operations, putting hundreds of cows together in close proximity, typically on concrete. Naturally these cows are left to wade and lay down in their own feces for the duration of their stay at a feedlot. Meat processors are unable to ensure that fecal matter residue does not get onto the processed meat, so they then use irradiation to kill the bacteria. The result is that we may be eating fecal matter on our beef. It is no wonder that many are worried about E Coli, nor is it a surprise that E Coli breakouts are more common now.
This industrial system is being used for pork, chicken, and turkey as well. The results are poor quality meat which is often tainted with bacteria because of cramped confined living quarters in which animals are demanded to grow faster than nature alone would allow. These enterprises are an affront to traditional animal husbandry in which the farmer cares for the animal, treats it humanely, and allows it to live in a manner consistent with its created nature. Of course there are consequences to treating animals without respect to the way God created them. This should be no surprise.
But corn is not the only thing that cows eat on feedlots. The cows are given liquid vitamins, antibiotics, beef tallow (a fat byproduct from cattle slaughterhouses), feather meal from chickens, pig and fish proteins, and as Joel Salatin points out, they often eat chicken manure. Sounds like a healthy, well-rounded diet, no?