Cows are supposed to eat grass.

Cows can grow big eating nothing but grass because they have rumens—a gastrointestinal adaptation they share with deer and goats, but one that we humans don’t have. The secret to the success of a rumen lies in the host of bacteria, protozoa and fungi that it contains. These microbes are fantastic fermenters, breaking down the cellulose in grass and converting it into fatty acids that the cow can digest. A cow’s rumen is the first of four stomachs, and by far the largest part of its digestive system—the rumen of a fully grown cow can hold up to 50 gallons of material.

Despite the benefit of their own personal and portable fermentery, cows grow pretty slowly on grass. That wouldn’t normally be a problem except that most of the cows in North America are raised for food, and that means we want them to grow quickly. Since it takes three years or more for a cow to reach slaughter weight on a diet of grass alone, the beef industry has worked out a new system where they switch cows from a diet of grass to a diet of grain and corn, at around 10 to 12 months of age. The cows stay on this diet of corn and grain for the next four to six months until they reach slaughter weight—this is called “finishing.” The new diet shaves more than a year off the time it takes for a cow to reach your dinner plate.

Finishing takes place in a feedlot, not in a pasture. Once they start dining on corn and grain, cows don’t “need” open pasture and the grass it provides, so they are shipped to centralized feedlots that can hold upwards of 100,000 cows at one time. Once there, they are kept in pens, closely monitored and fed specific ratios of corn, grain, and other not-entirely-natural dietary supplements (like fats and proteins). Not only does this approach shorten the amount of time it takes to bring a cow up to slaughter weight, but it leaves the meat much more tender and marbled with fat, and therefore of “higher” quality according to the USDA.

The problem with this approach, however, is that cows are not supposed to eat corn. (Remember, cows eat grass.) The chemical balance of a rumen is very delicate and the system can easily get out of whack. When corn enters the rumen, it overwhelms the fermenting process, and coats everything in a slimy layer. (Picture a coral reef covered in slime because it’s located too close to a sewage outflow.) This slime layer coats the fermenting material and can trap the gases produced during the fermentation process. This can prevent the cow from burping,* which can kill the cow unless a tube is inserted down the throat to release the gases.

The unnatural diet also upsets the pH of the rumen, leading to acidosis and ulcers. The acid eats away at the lining of the rumen, allowing bacteria that are supposed to stay in the rumen to leach into the bloodstream and attack the liver. The process is slowed down by pharmaceutical intervention—including the heavy use of antibiotics—but drugs aren’t foolproof. According to industry studies, almost every cow from a feedlot has significant liver abscesses by the time it is slaughtered.

Cows on a corn diet also have more acidic colons. Some of the starch a cow eats doesn’t get fermented in the rumen, ending up in the large intestine. Once there, microbes start up a second fermentation process which has acidic byproducts. While an acidic colon sounds unpleasant enough on its own, it also promotes the growth of a type of E. coli bacteria that is very dangerous to humans.

The E. coli normally found in cow poop can’t survive our stomach acid. (That helps protect us from contaminated meat, not necessarily from putting cow patties on the evening’s menu, just so ya know.) But E. coli O157:H7 (the type most frequently found in acidic cow colons) are acid-tolerant, so in the event that meat gets contaminated with poop from a cow with an acidic colon, the E. coli will survive in our stomachs  and wreak havoc on our systems.

So why, pray tell, do we feed our beloved cows an unnatural diet of corn and such that leads to unhealthy cows and potentially lethal meat? Economics. Corn is cheaper than hay, and corn-eating cows grow faster than grass-eating cows. Since consumers like their meat cheap and plentiful, finishing a cow on corn instead of hay is the industry’s answer. Of course, some cows still attend finishing school on grass pastures, so grass-fed beef does exist. You just have to look for it.

* The gases produced by burping cows are, in fact, greenhouse gases. But whether the types of gas vary between corn-fed and grass-fed cows, and which is better for the environment is beyond our current discussion. For more on burping cows check out this post.