[giarc]

It's not only the living conditions that is promoting the use of antibiotics in agriculture. The use of antibiotics in agriculture is so high due to the idea of "growth promoters". Animals given long term antibiotics actually grow quicker, and therefore become profitable faster than animals not given background levels of antibiotics. The problem with this activity is that low levels of antibiotics selects for resistance as we have seen.

In addition, drugs are given "label use" directions for particular species (ie. ovine, swine, equine) but only for major economical species. For example, the farmer might have a bottle of long acting penicillin that says give horse X mg/kg of body weight. However, this farmer might be giving these drugs to sheep and the maker of the antibiotic doesn't have label use instructions for sheep. This results in what's called "extra label drug use" and the farmer has to basically guess at what dose to give. Too high could be harmful to the animal (toxicity, clearance rate etc), too low and you promote resistance.