In short, we don't use any of it.  But please read on for a more complete understanding.

We used vaccines twice in our history, both now about 20 years ago.  The first was in chickens and the other was in beef.  A lady asked us to raise 500 laying hens for her because she didn't feel comfortable starting the chicks.  We said yes but when it came time for them to lay, she changed her mind and didn't want them.

We hadn't prepared for 500 extra chickens and didn't have anywhere for them to go.  That spring was cold and damp.  We kept them in the brooder house hoping the weather would clear so we could put them outside.  A week turned into two weeks.  They got crowded.  Not something you'd want to take a picture of—unless you were working for an animal welfare group.

By the time the weather broke enough to get them out, they were stressed.  They contracted a relatively weak virus called Marek's disease and we lost most of them.  Marek's disease is kind of a cross between respiratory infection and coughs, and this was a common problem in the early 20th century before the widespread use of vaccines and antibiotics.  It's highly communicable and we were unwilling to kill and incinerate every chicken on the place to start over. So for two or three years, we vaccinated our chicks against Marek's as a layer of protection until the virility of the disease could dissipate.  It did, and we quit vaccinating, and have never had a problem since.

The point?  The disease was our fault.

Case number two.  We rented a nearby farm property to get more pasture to expand our beef numbers. This place had acres of thick, 12-foot-high blackberry patches, some covering as much as 2 or 3 acres.  The landowner didn't want us to mow them down because they provided a wildlife habitat, but the vines were spreading and we had plenty of edge and steep spots for habitat. The place was not lacking for wildlife habitat.