Probiotics; Are they necessary?

Probiotics are common in today’s equine industry and even human health.  They are heavily marketed for supporting gastrointestinal health on many levels, but are they really needed and are they really beneficial?  There are many types of probiotics promoted in various products, but unfortunately, they are all lumped together and promoted as being beneficial, which can further muddy the waters. So how do we know if we need to use them and if so, what types are most beneficial? The answer to this question lies not only within research but also through clinical experience.