Many more tons of antibiotics are used by farms, even as a growth promoter, and this constant low-level exposure to antibiotics is a very favourable condition for developing resistance. <a href="http://www.wired.com/wiredscience/2013/09/cdc-amr-rpt2/">http://www.wired.com/wiredscience/2013/09/cdc-amr-rpt2/</a> (and many other posts about antibiotic resistance and farming at Superbug)