In which of the common animal identification number systems is the birth year represented by a letter?

In the context of animal identification number systems, the representation of the birth year by a letter pertains specifically to the system where letters correspond to specific years. This system is designed in a way that each letter corresponds to a designated year within a cycle, making it easier to identify the birth year of an animal at a glance.

For instance, if the letter 'A' is designated for the year 2020, next year would be 'B' for 2021, and so forth. This method is particularly advantageous because it simplifies tracking the age of livestock and facilitates breeding records by visual recognition rather than numerical interpretation.

In contrast, other systems may use numbers or different methods that do not incorporate letters to indicate birth years, which would not fulfill the specific criteria set forth in the question. The correspondence of letters to years in this particular system enables effective communication and record-keeping in livestock management.