The reason for this improvement in average daily gain and feed efficiency was not known but the beneficial effects resulting from feeding antibiotics were consistently reproduced by researchers around the world. Over the last 50 years, many new antimicrobial agents have been developed that produce the same positive effects on feed efficiency and average daily gain as the original antibiotics did 50 years ago.
Today there are many public concerns about the routine use of antibiotics in livestock production.
The chief concern is the development of resistance by bacteria to these antibiotics. Resistant bacteria can be passed from farm animals to humans and cause serious, sometimes fatal illnesses. The best known bacteria in this category are Salmonella. Another concern associated with the use of antibiotics in livestock production is the occurrence of drug residues in meat. Finally there are public relations issues arising from the perception that farmers need to use drugs throughout the production cycle to successfully raise livestock.