[English] plural heifers

I have seen several definitions of a heifer. They vary from 1) a female cow aged 8-20 months; 2) a young cow, that has not had a calf (OED); 3) a young female cow of over 12 months old which may or may not have had a calf (Letham Shank Farm); to 4) a young cow aged up to having her second calf (Lackham Farm). My brother says "A heifer is a young female and remains so called until giving birth to her 2nd calf" and that is the correct definition.

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