After more than half a century of extensive usage worldwide, the use of low levels of antibacterial agents in the feed or water for livestock and poultry continues to be effective in increasing growth rate, improving feed conversion, and reducing morbidity and mortality. Those first introduced, such as tetracyclines, penicillin, streptomycin, and certain compounds containing arsenic continue to be effective. As would be expected, those agents with a wide antibacterial spectrum are, on average, more effective as routine additives.

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