The objective of this project was to investigate the dose–response relationships between on-farm antimicrobial use and antimicrobial resistance (AMR) in E. coli from grow-finish pigs. It was found that although this study considered E. coli grow-finish pigs only, AMR was associated with antimicrobial use in all production phases. This indicates that antimicrobial use in pigs distant from market may have food safety repercussions. Five of the resistance outcomes were associated with exposure to unrelated drugs, suggesting co-selection. Although co-selection is familiar to scientists, producers and veterinarians need to understand this phenomenon before AMR can be addressed at the herd level.