>until the antibiotic washes out.

This is part of the problem. If animals are given the antibiotics, then their waste products contain them, and piles of manure or water runoff/storm drains/wastewater ponds form a giant evolutionary experiment for development of drug resistant organisms.

They're not just a problem while they're in animals, they're a problem if they're used at all.

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...

That’s often irrelevant. If a cow is going to be slaughtered at 36 months, you can use antibiotics for 95% of their lifetime and still have a 50 days window at the end.

> until the antibiotic washes out.

while I don't want surprise antibiotics in my food, I'm probably more concerned about the antibiotic resistance that antibiotic exposure generates. The flora of those animals _will_ have drug resistance genes that doesn't just "wash out". That resistnce can be passed to other animals or human by direct contact, environment or otherwise.

Does that make any practical difference for animals meant for consumption like beef cattle, hogs, or chickens? You can give them all the antibiotics you want until a few days before slaughter.

It does. The animals will be colonized by bacteria that have "learned" to deal with all those antibiotics you gave. That resistance is easily transmitted.

I understand the problem with antibiotic resistance, but telling a farmer that an animal treated by antibiotics can't enter the food supply until after treatment ends doesn't really solve the problem when it comes to animals raised as food -- the farmer can treat the animal with antibiotics for nearly all of its life, only stopping the treatment just before slaughter.

Totally agreed. I'm naive to much around Ag/Husbandry. If that's what farmers do, that is a concern. From a microbiology point of view, the concept that "washing out" the drug before slaughter improves anything is flawed. It will be a reservoir of smoldering antibiotic resistance as long as that practice persists.