The link above is for a fact sheet about a recent experience in Yates County dealing with stored manure that included gypsum used for bedding. Hydrogen Sulfide gas, which can be deadly, can be produced from stored manure. The addition of gypsum (sometimes used as bedding) can elevate hydrogen sulfide levels to very dangerous extremes. Gypsum is banned for bedding use by some European countries because of this great danger to people and animals.
