I don’t know if it is classified as true inbreeding or not, but the genetic lineage of all current Holstein sires can be traced to only 2 bulls.      51% of Holstein sires born in this decade trace their lineage back to Elevation and 49% go back to Chief.

We do know that inbreeding or even close breeding can have several deleterious effects on dairy cows, including: 1. Impaired immune function, 2. Reduced longevity.  3. Lower milk production and 4. Diminished reproductive efficiency.

They may not be related, but in the light of the above facts, I find it curious that even while milk production seems to skyrocket, the average dairy cow in the US does not live to complete 2 lactations and 50 of the dairy cows calve with either a metabolic or infectious disease.  Just wondering ?!