What is most often used to track livestock on large farms and identify each animal...

RFID for Livestock: Radio Frequency Identification (RFID) tags are the most widely used technology for livestock tracking and individual identification on large farms.

Benefits:

Unique ID: Each RFID tag has a unique code.

Data Storage: Some tags store information about the animal.

Durability: Tags withstand outdoor conditions.

Automated Reading: Tags can be scanned quickly.

References

RFID (General): https://en.wikipedia.org/wiki/Radio-frequency_identification