There are several extant herds that have been genetically tested and proven "pure", although they're the minority
Do you recall anything about where those herds are, because I’ve only seen research showing that all treated herds have identifiable cattle DNA, i.e., https://www.nature.com/articles/s41598-022-09828-z
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Do you recall anything about where those herds are, because I’ve only seen research showing that all treated herds have identifiable cattle DNA, i.e., https://www.nature.com/articles/s41598-022-09828-z