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Want To Know How Old That Cow Is?First, look at its teeth.

By Daniel Engber
Posted Thursday, Feb. 10, 2005, at 7:20 PM ET

The secretary of agriculture announced yesterday that part of a ban on importing cattle from Canada will be lifted next month: Starting March 7, live cows less than 30 months old will be permitted into the United States while older cattle, alive or dead, will remain off-limits (in an effort to prevent the spread of Bovine Spongiform Encephalopathy). How can inspectors tell how old a cow is?

By the cow's teeth while it's alive, and by its carcass when it's dead. The process would be much simpler if ranchers kept careful calving records. Individual animals often pass from owner to owner before they're slaughtered, and information about their dates of birth rarely goes with them. Inspectors must learn what they can from the cow itself.

To determine the age of a cow by "dentition," a rancher looks into its mouth and checks the incisors at the very front. A calf, like a human child, has baby teeth (or "milk teeth") that loosen and fall out. These are whiter and narrower than the adult versions, which only begin to emerge after 15 months. Between 24 and 30 months, a second pair of incisors erupts from the gums. Once these teeth are fully in, the cow is deemed to be at least 30 months old.

Looking at dead animals can provide more specific information. In the process of grading beef for its quality (prime, choice, or select), officials from the Department of Agriculture examine the bones and flesh of an animal to determine its age. As a cow gets older, its lean tissue becomes rougher-grained and darker in color, its ribs flatten and narrow, and some of its soft cartilage hardens into bone. Because an older animal will have tougher meat, an expert grader (one with at least 4,000 hours of training) considers these factors alongside the degree of marbling in the meat. The meat is assigned an age that ranges from A0 to A100, with A40 corresponding to about 20 months.

The A40 has special significance. Since our domestic mad cow scare in 2003, Japan has refused to import beef from American cattle over 20 months old. Without a good dental marker for that age, American ranchers must use the grading method, with A40 as a cut-off. But the grading method was designed to evaluate meat quality, not protect public health, so Japanese officials wanted proof that it worked. A recent government study of around 8,000 animals confirmed its accuracy, and as of yesterday Japan agreed to import A40-grade meat.

There are other ways to determine the age of cattle. As part of a national health program dating back to 1934, some calves are vaccinated for brucellosis, sometimes called "contagious abortion" or "Bang's disease." Vaccinated cattle get either a tag or a tattoo on their right ears. Numbers in the tattoos give the approximate date of their vaccination and, therefore, their birth.

Next question?

Explainer thanks Gary Weber of the National Cattlemen's Beef Association.

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Daniel Engber is an associate editor at Slate. He can be reached at .
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