Rates of propionate vary with the moisture content of the hay. But we don't recommend baling hay at more than 28 percent moisture. Spray propionic acid onto hay as it enters the baler. Here are some things to consider. Organic acids are highly corrosive to metal machine parts and paint.
Success depends on thorough and uniform application of the preservative to the hay crop.
Buffered propionates evaporate less, so you can apply them at slightly lower levels. Know the moisture content. Get a good-quality moisture meter, and probe through the diameter of a large, round bale. Hay bales will be heavier and more difficult to handle when the moisture content of the hay is high. You probably have the same complaint about your teens.