It was discovered in the 1940s that feeding subtherapeutic levels of antibiotics improved feed efficiency and accelerated animal growth.[27] Following this discovery, American Cyanamid published research establishing the practice of using antibiotic growth promoters.[25] By 2001, this practice had grown so much that a report by the Union of Concerned Scientists found that nearly 90% of the total use of antimicrobials in the United States was for non-therapeutic purposes in agricultural production.[28] Certain antibiotics, when given in low, subtherapeutic doses, are known to improve feed conversion efficiency (more output, such as muscle or milk, for a given amount of feed) and may promote greater growth, most likely by affecting gut flora.[29] The drugs listed below can be used to increase feed conversion ratio and weight gain, but are not legally allowed to be used for such purposes any longer in the United States.