Maybe you don't....it depends on your state.
In many cases, it depends on whether you plan to sell pigs and sell them across state lines.
If so, then the USDA requires your farm to be on record and have a specific premise ID or location identifier assigned to it.
Why? This is for tracking purposes should disease management be necessary.
Is this ID costly or difficult to get? The good news is that your premise ID is easy to obtain and Free of Charge.
I have a State Vet that's different from my vet that comes to my farm? Yes! Your state veterinarian is separate from your local vet. A quick internet search can point you in the right direction on their contact info.
When would I NOT need this tag? If you plan to keep all your pigs for your own consumption, then this tag may not be necessary. For accurate information on your state's requirements though, contact your State Vet to be sure.
Will the state vet help me with my health certificates when I do sell a pig and need one? No. They won't. That's your local vet. A copy of that health cert is sent to each state vet receiving your pig when a health cert is written though.
Not sure where to start? Use this. Here's a quick link to click on your state through a USDA website to determine your requirements: USDA Premise Identifier Link By State
It may sound complex and complicated, but it's really not. Once you're established, that's it. You're good to go.
Just hold on to your Premise ID for ear tags & health certs.