Bayer Corp. has asked the Food and Drug Administration to allow it to keep selling its controversial animal antibiotic, Baytril, while it fights an agency ban on the drug in federal court.

Almost five years after the FDA first moved to ban the drug -- which the agency had concluded was contributing to a decrease in the effectiveness of closely related human antibiotics -- FDA Commissioner Lester M. Crawford issued a final rule in July ordering the drug off the market as of Sept. 12.

Bayer, a number of veterinarian organizations and the trade association that represents animal-drug makers filed their petition last week, asking for a permanent delay or a temporary one that would allow them to argue the timing of the ban in court.

The petition argues that the poultry industry needs the drug on the market now because the type of respiratory illness treated by Baytril poses the greatest risk to birds in the autumn.

All antibiotics gradually become less effective as bacteria adapt to them, and the speed of the adaptation is to some extent determined by how much of the medication is used. Because a substantial percentage of the antibiotics used domestically go to treat farm animals or to speed their growth, the FDA and many medical researchers have sought to limit their use on farms to ensure that the antibiotics used to treat humans do not become ineffective.

The agency considers Baytril an especially troublesome animal drug because it is very similar to the important human antibiotic ciprofloxacin.

As the FDA considered proposals to ban Baytril, Bayer sought the help of numerous members of Congress to stop the process. That effort failed, but supporters of the ban worry that the agency will now respond to pressure and allow the drug to stay on the market during a legal appeal, which could take years.

"If the agency decides to leave Baytril on the market, it will add to the growing evidence that FDA's agenda is being driven by politics, not science," said Margaret Mellon of the Union of Concerned Scientists, an advocacy group that supports the ban. "The petition for a stay submitted by Bayer and its allies is based on arguments that the FDA has already considered and soundly rejected."