Substances that are produced by living organisms having a bacteriostatic or bactericidal activity As growth stimulator - at sub-therapeutic levels in diets, antibiotics improve growth rate and efficiency of feed utilization. Antibiotics have been widely used in the livestock and poultry industries since their discovery more than 50 years ago. They represent an extremely important tool in the efficient production of animal products such as milk, meat and eggs. Reduce mortality and morbidity and improve reproductive performance. At high levels, antibiotics help to prevent disease in exposed animals and to treat diseases.