Persistently infected (PI) calves are created when a pregnant cow at 20-150 days of gestation is exposed to BVD virus. In a herd infected with BVD virus, the young calves are the most likely animals to be PI. Herds with a calving season longer than 90 days have cows calving after the breeding season has already started. PI calves born in this period would expose early pregnancies to BVD virus. Without a short calving season it will be difficult to test newborn calves and remove PIs before they have exposed pregnant cows to the virus.