Three sets of identical twin cows were used to quantitate the excretion of selenium in feces, urine and milk in response to increments of dietary selenium. The first trial consisted of three test periods, each 8 days long, separated by recovery periods of 5 days. A solution of sodium selenite was applied to the grain portion of the ration to provide 0, 6.0, 12.0, 24.0 and 48.0 or 100.0 mg of sodium selenite per day. Samples of milk, urine and feces were collected on the last 2 days of each treatment period and analyzed for selenium. There was a significant, positive linear (P < 0.05) response between excretion of selenium in feces and increments of dietary selenium. There was a marked but variable response in selenium content of urine to dietary intake but selenium levels in milk were not increased above those of control animals. In a second trial, sodium selenite was fed at the rate of 170 and 260 mg per day for 10 days. These theoretically toxic levels resulted in 10- and 40-fold increases in the selenium levels of feces and urine, respectively, but only a 3-fold increase in selenium content of milk. At these high levels of selenium supplementation the amounts of selenium in the kidney and liver were increased above the normal values. This study shows that selenium in the form of sodium selenite would have to be fed at levels toxic to the cow before there would be any measurable increase of selenium in milk.
Selenium and Thyroid Disease: From Pathophysiology to Treatment
