From Union of Concerned Scientists

on the web:
http://www.ucsusa.org/food_and_environment/sustainable_food/they-eat-what.html

When many Americans think of farm animals, they picture cattle munching grass on rolling pastures, chickens pecking on the ground outside of picturesque red barns, and pigs gobbling down food at the trough.

Over the last 50 years, the way food animals are raised and fed has changed dramatically—to the detriment of both animals and humans. Many people are surprised to find that most of the food animals in the United States are no longer raised on farms at all. Instead they come from crowded animal factories, also known as large confined animal feeding operations (CAFOs).

Just like other factories, animal factories are constantly searching for ways to shave their costs. To save money, they've redefined what constitutes animal feed, with little consideration of what is best for the animals or for human health. As a result, many of the ingredients used in feed these days are not the kind of food the animals are designed by nature to eat.

Just take a look at what's being fed to the animals you eat.

Same Species Meat
Diseased Animals
Feathers, Hair, Skin, Hooves, and Blood
Manure and Other Animal Waste
Plastics
Drugs and Chemicals
Unhealthy Amounts of Grains

Are these ingredients legal? Unfortunately, yes. Nevertheless, some raise human health concerns. Others just indicate the low standards for animal feeds. But all are symptoms of a system that has lost sight of the appropriate way to raise food animals.

Same Species Meat, Diseased Animals, and Feathers, Hair, Skin, and Blood

The advent of "mad cow" disease (also known as bovine spongiform encephalopathy or BSE) raised international concern about the safety of feeding rendered[1] cattle to cattle. Since the discovery of mad cow disease in the United States, the federal government has taken some action to restrict the parts of cattle that can be fed back to cattle.

However, most animals are still allowed to eat meat from their own species. Pig carcasses can be rendered and fed back to pigs, chicken carcasses can be rendered and fed back to chickens, and turkey carcasses can be rendered and fed back to turkeys. Even cattle can still be fed cow blood and some other cow parts.

Under current law, pigs, chickens, and turkeys that have been fed rendered cattle can be rendered and fed back to cattle—a loophole that may allow mad cow agents to infect healthy cattle.

Animal feed legally can contain rendered road kill, dead horses, and euthanized cats and dogs.

Rendered feathers, hair, skin, hooves, blood, and intestines can also be found in feed, often under catch-all categories like "animal protein products."

Manure and Other Animal Waste

Feed for any food animal can contain cattle manure, swine waste, and poultry litter. This waste may contain drugs such as antibiotics and hormones that have passed unchanged through the animals' bodies.

The poultry litter that is fed to cattle contains rendered cattle parts in the form of digested poultry feed and spilled poultry feed. This is another loophole that may allow mad cow agents to infect healthy cattle.

Animal waste used for feed is also allowed to contain dirt, rocks, sand, wood, and other such contaminants.

Plastics

Many animals need roughage to move food through their digestive systems. But instead of using plant-based roughage, animal factories often turn to pellets made from plastics to compensate for the lack of natural fiber in the factory feed.

Drugs and Chemicals

Animals raised in humane conditions with appropriate space and food rarely require medical treatment. But animals at animal factories often receive antibiotics to promote faster growth and to compensate for crowded, stressful, and unsanitary living conditions. An estimated 13.5 million pounds of antibiotics—the same classes of antibiotics used in human medicine—are routinely added to animal feed or water. This routine, nontherapeutic use of antibiotics speeds the development of antibiotic-resistant bacteria, which can infect humans as well as animals. Antibiotic resistance is a pressing public health problem that costs the U.S. economy billions of dollars each year.

Some of the antimicrobials used to control parasites and promote growth in poultry contain arsenic, a known human carcinogen. Arsenic can be found in meat or can contaminate human water supplies through runoff from factory farms.

Unhealthy Amounts of Grains

One last surprise. While grain may sound like a healthful food, the excessive quantities fed to some animals are not. This is especially true for cattle, which are natural grass eaters. Their digestive systems are not designed to handle the large amounts of corn they receive at feedlots. As a result of this corn-rich diet, feedlot cattle can suffer significant health problems, including excessively acidic digestive systems and liver abscesses. Grain-induced health problems, in turn, ramp up the need for drugs.