Farmers in high-income countries use antibiotics for growth promotion and disease prophylaxis in healthy animals because it is economically rational — antibiotics improve feed conversion, accelerate growth, and reduce the need for expensive biosecurity investments. The cost of the antibiotic is low; the benefit to the farm is immediate.

The cost of resulting human AMR infections is borne by society, not by the farmer. This is a classic externality problem, but it is masked by the fact that the farmer's rational behavior — from the farm's perspective — is globally catastrophic. No individual farmer can solve this by unilateral restriction; the only solution is coordinated regulatory change.