Selenium is an element present in soils in varying amounts throughout the world, but volcanic soils such as those in New Zealand are particularly low in this important trace element.
This deficiency shows up in areas of volcanic soil with poor pregnancy rates in ewes and in high volcanic areas such as the South Island as white muscle disease in lambs and calves. These related, widespread issues are now prevented through farmers adding selenium to animal drenches, injections and fertilisers.
The United States has selenium levels that vary widely, same as New Zealand. This was established when scientists were looking for reasons of variations in human cancer levels which occur geographically. The only causative factor they could find was that areas with high cancer levels had low soil selenium levels and visa versa. To verify this observation, a ten-year trial was planned in a low selenium area.