Abstract
Antibiotics were discovered over 50 years ago and have since been widely used in the livestock and poultry industries. They represent an extremely important tool in the efficient production of pork, beef, poultry meat, and other animal products. When used at low (subtherapeutic) levels in feeds, antibiotics improve growth rate and efficiency of feed utilization, reduce mortality and morbidity, and improve reproductive performance. Antibiotics are also used at intermediate levels to prevent disease and at high (therapeutic) levels to treat diseases in animals.