While using beef cattle of unknown source as recips is not without risk, I'm convinced that many beef seedstock operations introduced Johne's Disease into their herds by using dairy cows as recips.
We know that at least 25% of calves born to Johnes-infected cows are infected in utero, and those cows shed the organism in their milk/colostrum and feces... calves born to Johne's cows are 10X more likely to be/become infected than calves in the same herd born to non-JD-infected cows.
Our diagnostic tests for JD are still not great for those cows that have not yet begun to exhibit clinical disease or shed organisms in their feces. They may look clinically normal, and may test negative on serum and feces... but 6 months, 1 year, 2 years later... may be positive.