What kind of Barley mash is it? Is this something that has been developed just as a feed stuff or is it from a brewery, distiller? I use my "tailings" for feeding, but without pigs at the moment, I dry the tailings from my fuel production mash before feeding it to my goats and
chickens, who love it by the way. I also use some of it in my composting. A brew or distilling mash will have been converted to sugars and yeast will have been added, and there will be a slight amount of alcohol left in, which is not a concern for feeding to most livestock. If it is a mash that has been solely developed for feed, it probably will not have had yeast added but rather only have wild yeast that found its way in, and you might need to do some testing, since some wild yeasts create toxins (other than alcohol) to eliminate competing organisms. In any case, to easily store a mash product you will need to dry it out. If you don't it will mold rather quickly.