Ear notching is done in large farm to aid in data gathering or farm monitoring.
In some sytem they just notch the animal number .
In some the left ear is for the animal number and the right is for farrowing day.
Every farm have different notching system depending on their use.

So, how is it done?
For example the piglet's number is 4, on the left ear (animal facing you/ your right side) cut on the portion of 3 and 1 .
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i have to edit lang for clarification