Kentucky Equine Research indicates that reputable feed manufacturers blend fresh ingredients to create palatable concentrates for horses and then provide valuable nutritional information about the products on bags and tags.  Aside from the nutritional guarantees and ingredients, some manufacturers will stamp their products with useful information pertaining to manufacture date and best use practices.  Unlike many products for human consumption, bags of horse feed are not required to have manufacture or best by dates, although many feed mills do list this information.  Although the manufacture date is important, the best by date is pretty vague because the manufacturer doesn’t really know the best by date. The date has to do with the conditions in which the feed is stored after purchase.  If products are stored in a cool, low-humidity pest-free atmosphere they might be good to feed beyond the best by date. 

However, those conditions are rare in climates where it is hot and humid with lots of insects, so the feed might not even be good until the suggested best by date.  Feed stored in hot trailers with exposure to external moisture and left open for insects is certainly going to decrease the time you can use that feed.  There are differences in the various kinds of feeds as straight grains have the longest shelf life, followed by extruded feeds that are subject to heat processing during manufacture. Both contain little moisture.  Pelleted feeds undergo some exposure to heat and low moisture during processing so shelf life is moderate. The feeds with the shortest shelf life are those textured feeds containing molasses as they are prone to mold and spoilage.