9 CFR §2.51

1. (a)The official tag shall be made of a durable alloy such as brass, bronze, or steel, or of a durable plastic. Aluminum of a sufficient thickness to assure the tag is durable and legible may also be used. The tag shall be one of the following shapes:
   1. (1)Circular in shape and not less than 1 1/4 inches in diameter, or
   2. (2)Oblong and flat in shape, not less than 2 inches by 3/4 inch and riveted to an acceptable collar.
2. (b)Each tag shall have the following information embossed or stamped on so that it is easily readable:
   1. (1)The letters “USDA”;
   2. (2)Numbers identifying the State and dealer, exhibitor, or research facility (e.g., 39-AB); and
   3. (3)Numbers identifying the animal (e.g., 82488).
3. (c)Official tags shall be serially numbered. No individual dealer or exhibitor shall use any identification tag number more than once within a 5-year period.