I was still in medical training when I was called to testify in defense of Oprah Winfrey in the infamous "meat defamation" trial. If you remember, Oprah swore she would never eat another burger again after hearing that cows were being fed the remains of other cattle. After she tried to remind the audience that cows were supposed to be herbivores, the meat industry representative defended the practice by stating, "Now keep in mind, before you view the ruminant animal, the cow, as simply a vegetarian -- remember that they drink milk." The absurdity of the statement aside, it's not even entirely accurate. In modern agribusiness, humans drink the milk. Calves typically get milk "replacer."

Like all mammals, cows can produce milk only after they've had a baby. Most newborn calves in the United States are separated from their mothers within 12 hours -- many immediately after birth -- so the mother's milk can be marketed for human consumption. Though some dairy farmers still wean calves on whole milk, the majority of producers use milk replacer, which too often contains spray-dried cattle blood as a cheap source of protein.

According to the American Protein Corporation, which boasts to be the world's largest spray-dryer of blood, the chief disadvantage of blood-based milk replacer is simply its "different color." Milk replacer containing blood concentrate typically has a "chocolate brown" color, which can leave a dark residue on the bottles, buckets, and utensils used to feed the liquid. "For some producers," a company official remarked, "the difference is difficult to accept at first, since the product does not look 'like milk.'" But the "[c]alves don't care," he was quick to add.

The calves may not care, but Stanley Prusiner does. Prusiner won the Nobel Prize in Medicine for his discovery of prions, the infectious proteins that cause mad cow disease. He was quoted in the New York Times as calling the practice of feeding cattle blood to young calves "a really stupid idea," because it could complete the "cannibalistic" circuit blamed for the spread of the disease.

The European Commission also recommended against the practice of "intraspecies recycling of ruminant blood and blood products" -- the practice of suckling calves on cows' blood protein. Even excluding the fact that brain matter may pass into the trough that collects the blood once an animal's throat is slit, the Commission report concluded a decade ago that "[a]s far as ruminant blood is concerned, it is considered that the best approach to protect public health at present is to assume that it could contain low levels of infectivity." Since then, evidence that blood can be infectious has only grown, yet dairy calves in the United States are still drinking up to three cups of "red blood cell protein" concentrate every day.

Although the U.S. Food and Drug Administration initially proposed to ban the feeding of blood and blood products to livestock, the agency ended up reneging on their much touted promise. Let's hope that the newly reported case of mad cow disease in a California dairy cow will renew interest in closing the loopholes in feed regulations that continue to allow the feeding of slaughterhouse waste, blood and manure to farm animals in the United States.