Every animal is known
In Britain, sheep, goats and cattle wear official ear tags (pigs have their own marks and tags). The number shows which farm the animal was born on, and each animal has its own number.
Tracking every move
Whenever an animal moves between farms, to a show or to market, the move is reported to a government database. In Wales, sheep and goat moves go to EID Cymru.
Why bother?
If there’s ever a disease outbreak, vets can quickly trace which animals have been where, and stop it spreading. It also means food can be traced back to the farm it came from.
Electronic tags
Most sheep tags now have a tiny chip inside, which a reader can scan in a second. It saves a lot of squinting at numbers in the rain.