'It's not that they can't adjust,' he continues, 'and now we're breeding cattle to do well in a feedyard.' One way to look at the breeding work going on at ranches like the Blairs' is that the contemporary beef cow is being selected for the ability to eat large quantities of corn and efficiently convert it to protein without getting too sick. (These, after all, are precisely the genes prized in the steer's father). The species is evolving, in other words, to help absorb the excess biomass coming off America's cornfields. But the cow's not there quite yet, and a great many feedlot cattle- virtually all of them to one degree or another, according to several animal scientists I talked to- are simply sick.
Bloat is perhaps the most serious thing that can go wrong with a ruminant on corn. The fermentation in the rumen produces copious amounts of gas, which is normally expelled by belching during rumination. but when the diet contains too much starch and too little roughage, a rumination all but stops, and a layer of foamy slime forms in the rumen that can trap gas. The rumen inflates like a balloon until it presses against the animal's lungs. Unless action is taken promptly to relieve the pressure (usually by forcing a hose down the animal's esophagus), the animal suffocates.
A concentrated diet of corn can also give a cow acidosis. Unlike our own highly acidic stomachs, the normal pH of a rumen is neutral. Corn renders it acidic, causing a kind of bovine heartburn that in some cases can kill the animal, but usually just makes him sick. Acidiotic animals go off the feed, pant and salivate excessively, paw and scratch their bellies, and eat dirt. The condition can lead to diarrhea, ulcers, bloat, rumentitis, liver disease, and a general weakening of the immune system that leaves the animal vulnerable to the full panoply of feedlot diseases- pneumonia, cocidiosis, enterotoxemia, feedlot polio. Much like modern humans, modern cattle are susceptible to a set of relatively new diseases of civilization- assuming, that is, we're willing to put the modern feedlot under the rubric of civilization.
Cattle rarely live on feedlot diets for more than 150 days, which might be about as much as their systems can tolerate. 'I don't know how long you could feed them this ration before you'd see problems,' Dr. Metzin said; another vet told me the diet would eventually 'blow out their lives' and kill them. Over time the acids eat away at the rumen wall, allowing bacteria to enter the animal's bloodstream. These microbes wind up in the liver, where they form abscesses and impair the liver's function. Between 15 percent and 30 percent of feedlot cows are found at slaughter to have abscessed livers; Dr. Mel told me that in some pens the figure runs as high as 70 percent.
What keeps a feedlot animal healthy- or healthy enough- are antibiotics. Rumensin buffers acidity in the rumen, helping to prevent bloat and acidiosis, and Tylosin, a form of erythromycin, lowers the incidence of liver infection. Most of the antibiotics sold in America today end up in animal feed, a practice that, it is now generally acknowledged (except in agriculture), is leading directly to the evolution of new antibiotic-resistant superbugs. In the debate over the use of antibiotics agriculture, a distinction is usually made between their clinical and nonclinical uses. Public health advocates don't object to treating sick animals with antibiotics; they just don't want to see the drugs lose their effectiveness because factory farms are feeding them to healthy animals to promote growth. But the use of antibiotics in feedlot cattle confounds this distinction. Here the drugs are plainly being used to treat sick animals, yet the animals probably wouldn't be sick if not for the diet of grain we feed them.
I asked Dr. Mel what would happen if drugs like Rumensin and Tylosin were banned from cattle feed, as some public health experts advocate. 'We'd have a high death rate [it's currently about 3 percent matching the industrial average] and poorer performing cattle. We just couldn't feed them as hard.' The whole system would have to change and slow down."
The Omnivore's Dilemma, pp. 77-9