Hay will never be in a better nutritional state than the moment it is cut. It simply deteriorates from that point. The only thing you can then control is getting the hay baled, out of the field and into good storage conditions as rapidly as possible. However, it is important to be certain that hay reaches a safe moisture level and internal temperature before moving it to storage to avoid spontaneous combustion. Otherwise, fire can occur in hay stored outside or in a barn. Forage-Livestock Quotes and Concepts, vol. 2 is available here.