this is what happens with corn:
Cattle fed on a heavy diet of corn will eventually become sick and die. Via a transcript from Fresh Air with Terry Gross (emphasis added):
GROSS: Let’s get back to the cow’s stomach.
Mr. POLLAN: Yeah.
GROSS: So the cow now is eating corn instead of eating grass. Its stomach is made for digesting grass and turning it into protein. How does the cow’s digestive system handle corn?
Mr. POLLAN: Well, very poorly. It’ll go kablooey if it’s not done very gradually. And I talked to people who said that most cows, most beef cattle getting a heavy diet of corn–and again, they can tolerate some of it, but when you crank it up to 70, 80, 90 percent grain, their stomachs go haywire. They suffer from a range of different phenomenon, one of which is bloat.
You know, the rumen, this organ, is always producing copious amounts of gas, and these are expelled during rumination, you know, when the animal kind of chews its cud. It regurgitates this bolus of grass and in the process releases all this greenhouse gas, essentially methane and things because when you’re digesting grass much gas is produced. But when they’re eating corn, this layer of slime forms over the mass in the rumen, and it doesn’t allow the gas to escape. So what happens is the rumen begins to expand like a balloon until it’s pressing up against the lungs of the animal. And if nothing is done to release the pressure of that gas, the animal suffocates. It can’t breathe anymore. So what do they do? Well, if it gets to that point, they force a hose down the esophagus of the animal, and that releases the gas, and they very quickly put them back on hay for a little while.
So that’s one of the things that can go wrong. Well, perhaps the most dramatic. But a whole other range of problems are created because the corn acidifies the rumen. The rumen has basically a neutral pH when it’s healthy and getting grass, and that’s very significant for a lot of reasons. But you feed it corn and it gets a lot more acidic. And the rumen can’t deal with acids, and what happens is the acids gradually eat away at the wall of the rumen, creating little lesions or ulcers through which bacteria can pass. And the bacteria get into the bloodstream and travel down to the liver, which collects all such impurities, and infects the liver. And that is why more than 13 percent of the animals slaughtered in this country are found to have abscessed livers that have to be thrown away and is a sign of disease.
But this low-level sickness, acidosis or even subacute acidosis, as they call it, afflicts many, many–probably the majority–of feedlot calves, and it leaves them vulnerable to all sorts of other diseases. Their immune systems are compromised. So they get this, you know, horrifying list of feedlot diseases. You know, we have these diseases of civilization, you know, heart disease and such things. Well, they have their own diseases of civilization: feedlot polio, abscessed livers, rumenitis, all these kinds of things that cows in nature simply don’t get.
Then the cows are given loads of antibiotics... which condition bacteria to become resistant.
http://conservationreport.com/2009/11/30/industrial-farm-animal-production-the-cost-of-feeding-cattle-corn/
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