The more I look in to this the more fascinating the topic gets. Rather than pay the exhorbitant prices for probiotics for your horse/s you an very inexpensively make your own. A culture must be obtained that is known as Water Kefir. There is also Milk Kefir but we're dealing with Water Kefir. It's a beneficial bacteria that looks something like rice. Powdered kefir can be purchased but it's better to get them from someone else as once you have them and you look after them (a very small task) you will have a steady supply of probiotics for your family and animals. There is much info on youtube so I won't go into it hear. I just obtained Milk Kefir from a gal with goats and it is now basking in a cup of milk in a pint jar with a coffee filter secured to the top to allow any pressure to escape and keep a meandering fruit fly out of it. This will also contain many probiotics. No need to refrigerate for the first 24 then it's time to remove the "grains" and add them to new milk. This can then be refrigerated to slow them down or left out if used in about 24 hrs.