Part of this beneficial affect of minimizing hay losses is from the clearance between the ground and the hay bale that allows for air circulation and drying. This statement is supported by research that shows storing bales on old tractor tires is not as good as on rails in preventing nutrient losses. Old tires will keep the hay from direct contact with the ground, but does not allow for air circulation. The only outside hay storage system that would approach barn storage based on research data would be the net wrap hay and above ground storage using rails. This process of net wrapping and placing hay bales on rails is somewhat expensive and is time consuming to place the bales on rails.
Total cost are probably equal, or cheaper for barn storage.
The final factor that appears to favor the economics of barn storage, is the cost of replacing the nutrients lost during the hay storage process outside. In an example of the money saving by barn storage, assume a 1200-lb bale that is stored outside on the ground. Or, if you have ample hay supplies, there will still be losses in crude protein and TDN because they are the most water-soluble portions of the hay, and outside storage will result in the need to have hay supplemented with energy, protein or both. When all of these losses in amount, protein, and energy are added together, barn storage becomes a viable option in high rainfall areas. Hay storage losses can range from minor to substantial.