What do cattle wear as an external form of identification?

Cattle wear ear tags as an external form of identification.

Explanation:

There are various ways of cattle identification through several numbering systems; these are typically alphanumeric, or a simple combination of the current year and date of birth of each individual. This allows for tracking, keeping inventory and proper records on livestock at a farm. Commonly, tags are applied to the ears of cattle with applicator guns for a unique, highly visible means of identification.

These may be one of several types...

Also, a secondary tagging system may be applied to avoid misidentification, particularly in larger herds such as:

✿   this landform has a high elevation and a high relief

✿   this landform is a mountain