Apparently, most antibiotics don't undergo metabolism in animals, and 40-90% of ingested antibiotics in livestock are excreted in urine and feces[1].

Those antibiotics go on to contaminate groundwater, and as you say, contribute significantly to antibiotic resistance[2].

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4220964/

[2] https://en.wikipedia.org/wiki/Antibiotic_use_in_livestock#An...