Part of the indiscriminate use was out of necessity, but it is secondary to the fact that many antibiotics seems to promote growth and improve conversion of fee

Part of the indiscriminate use was out of necessity, but it is secondary to the fact that many antibiotics seems to promote growth and improve conversion of feed into weight gains. The exact mechanism is not well understood. A good example is nitarsone which is an organoarsenic additive commonly found in turkey feed until it was banned earlier this year. AFAIK it has not been approved for human use ever.