Relative stability of selenites and selenates in feed premixes as a function of water activity.

Sodium selenite is more hygroscopic than sodium selenate. It is, therefore, more likely to dissolve when dispersed in feeds of relatively high water activity. When dissolved, it may form selenious acid and disperse as a vapor. This is easily demonstrated by mounting a filter paper wetted with a reagent such as ascorbic acid over the subject feed, but not in contact with it. The paper turns brown as elemental selenium is formed from reduction of the vapor. Analysis of the paper ensures that the brown is indeed selenium. Though premixes are generally low enough in moisture content to ensure stability of the selenites, this is not true of many feeds. The water activities of a number of feeds, feed premixes, and feed ingredients have been determined instrumentally and compared to those of saturated solutions of sodium selenite and sodium selenate. There is no question that the selenite often dissolves with the potential to react and, in so doing, loses its nutritional function.