Each year, federal inspectors find illegal levels of antibiotics in hundreds of older dairy cows bound for the slaughterhouse. Concerned that those antibiotics might also be contaminating the milk Americans drink, the Food and Drug Administration intended to begin tests this month on the milk from farms that had repeatedly sold cows tainted by drug residue.
    But the testing plan met with fierce protest from the dairy industry, which said that it could force farmers to needlessly dump millions of gallons of milk while they waited for test results. Industry officials and state regulators said the testing program was poorly conceived and could lead to costly recalls that could be avoided with a better plan for testing.
    In response, the F.D.A. postponed the testing, and now the two sides are sparring over how much danger the antibiotics pose and the best way to ensure that the drugs do not end up in the milk supply.
    鈥淲hat has been served up, up to this point, by Food and Drug has been potentially very damaging to innocent dairy farmers,鈥 said John J. Wilson, a senior vice president for Dairy Farmers of America, the nation鈥檚 largest dairy cooperative. He said that that the nation鈥檚 milk was safe and that there was little reason to think that the slaughterhouse findings would be replicated in tests of the milk supply.
    But food safety advocates said that the F.D.A.鈥檚 preliminary findings raised issues about the possible overuse of antibiotics in livestock, which many fear could undermine the effectiveness of drugs to combat human illnesses.
    鈥淐onsumers certainly don鈥檛 want to be taking small amounts of drugs every time they drink milk,鈥 said Caroline Smith DeWaal, food safety director of the Center for Science in the Public Interest, an advocacy group. 鈥淭hey want products that are appropriately managed to ensure those drug residues aren鈥檛 there, and the dairy farmer is the one who can control that.鈥
    The F.D.A. said that it would confer with the industry before deciding how to proceed. 鈥淭he agency remains committed to gathering the information necessary to address its concern with respect to this important potential public health issue,鈥 it said in a statement.
    The concerns of federal regulators stem from tests done by the Department of Agriculture on dairy cows sent to be slaughtered at meat plants. For years, those tests have found a small but persistent number of animals with drug residues, mostly antibiotics, that violate legal limits.
The tests found 788 dairy cows with residue violations in 2008, the most recent year for which data was available. That was a tiny fraction of the 2.6 million dairy cows slaughtered that year, but regulators say the violations are warning signs because the problem persists from year to year and some of the drugs detected are not approved for use in dairy cows.
    The question for the F.D.A. is whether cows that are producing milk also have improper levels of such drugs in their bodies and whether traces of those drugs are getting into the milk.
    Regulators and veterinarians say that high levels of drugs can persist in an animal鈥檚 system because of misuse of medicines on the farm.
    That can include exceeding the prescribed dose or injecting a drug into muscle instead of a vein. Problems can also occur if farmers do not follow rules that require them to wait for a specified number of days after administering medication before sending an animal to slaughter or putting it into milk production.
    鈥淔.D.A. is concerned that the same poor management practices which led to the meat residues may also result in drug residues in milk,鈥 the agency said in a document explaining its plan to the industry. In the same document, the F.D.A. said it believed that the nation鈥檚 milk supply was safe.
    Today, every truckload of milk is tested for four to six antibiotics that are commonly used on dairy farms. The list includes drugs like penicillin and ampicillin, which are also prescribed for people. Each year, only a small number of truckloads are found to be 鈥渉ot milk,鈥 containing trace amounts of antibiotics. In those cases, the milk is destroyed.
    But dairy farmers use many more drugs that are not regularly tested for in milk. Regulators are concerned because some of those other drugs have been showing up in the slaughterhouse testing.
    Federal officials have discussed expanded testing for years. But industry executives said that it was not until last month that the F.D.A. told them it was finally going to begin.
    The agency said that it planned to test milk from about 900 dairy farms that had repeatedly been caught sending cows to slaughter with illegal levels of drugs in their systems.
    It said it would test for about two dozen antibiotics beyond the six that are typically tested for. The testing would also look for a painkiller and anti-inflammatory drug popular on dairy farms, called flunixin, which often shows up in the slaughterhouse testing.
    The problem, from the industry鈥檚 point of view, is the lengthy time it takes for test results.
    The tests currently done for antibiotics in milk take just minutes to complete. But the new tests could take a week or more to determine if the drugs were present in the milk.
    Milk moves quickly onto store shelves or to factories where it is made into cheese or other products. The industry worried that, under the F.D.A. plan, by the time a load of milk was found to be contaminated, it could already be in consumers鈥 refrigerators, and that could lead to recalls.
    One Northeast cooperative, Agri-Mark, sent a letter to its members last month instructing them to dump milk if it had been tested by the F.D.A. 鈥淎gri-Mark must ensure that all of our milk sales, cheese, butter and other products are in no danger of recall,鈥 the letter said.
    Other industry executives said that processing plants would refuse to take any milk from a farm that had been tested until the results showed it was drug-free, meaning farmers could end up dumping milk for a week or more while waiting.
    The F.D.A. plan was also criticized by state officials that regulate the dairy industry.
    In a sharply worded Dec. 29 letter, the top agriculture officials of 10 Northeastern states, including New York and Pennsylvania, which are both leading dairy producers, told the F.D.A. that its plan was badly flawed. Among other problems, the letter said, forcing farmers to dump large quantities of milk could create environmental problems.
    The F.D.A. said it would consider the regulators鈥 comments as it reviewed its testing plan.

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