Antibiotics are used in farmed animals in order to control infections that can be caused by the intense confinement they are subjected to and to promote growth. Approximately 80% of antibiotics sold in the US are used in animal agriculture.

This antibiotic use is a major contributor to the evolution of antibiotic-resistant bacteria ("superbugs") such as MRSA. For more information on how this happens, check out this 
CDC page.

The table below shows the USDA's most recent list
of antibiotic residue repeat offenders,
livestock producers who have sent animals to slaughter
repeatedly with higher than allowed levels of certain drugs
in their systems. Sometimes hundreds of times the legal limit.