In the early years of antibiotic usage, there was concern that the popularly used antibiotics, such as chlortetracy-cline, oxytetracycline, penicillin, tylosin, and others would eventually lose their effectiveness because of resistance development in harmful organisms or by selecting for harmful organisms that were naturally resistant. Certain bacteria do develop resistance and this should be considered in antibiotics use programs. However, the problem is not as great as some suggest. Appropriate management of therapeutic and subtherapeu-tic use in combination with sound housing, management, and nutrition programs has resulted in profitable benefits from these antibacterial agents for more than half a century. Comprehensive statistical evaluations of experiments conducted over a period of more than 25 years show that those antibiotics first introduced are still effective. This report also included the results of an experiment that demonstrated positive responses to tetracycline in a facility in which tetracycline had been used continuously in the feed for three years prior to the experiment. The pigs used in the experiment had been fed diets containing tetracycline prior to being allocated to diets with or without the tetracycline. A positive response to the antibiotic continued. Over the years, numerous antibacterial agents have been tested singly or in combination with others. Some combinations provide greater antibacterial activity and greater improvements in rates of gain or efficiency of feed conversion. Some have been effective, but never approved for use, either because they showed no unique advantage or because they were uncompeti-tive cost-wise.
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