Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
Welcome to my forum.
Here you will find more than 6 years of questions and my answers. It is searchable and offers a great deal of information.
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All the best,
Dr. Getty
Hi Ashley,
A prebiotic is different than a probiotic because a prebiotic does not contain any live microbes. Instead, it contains fermentation products that feed the existing bacteria in the hind gut, making them healthy, in larger numbers, and better capable of digesting forage and producing B vitamins.
Probiotics (live microbes) are useful in situations when the number of microbes in the hind gut are diminishing. This can occur when a horse is taking antibiotics, for example.
So, to keep the hind gut healthy, I would go with a prebiotic such as Ration Plus.
Regarding bran -- if you are feeding a bran product that does not have added calcium, you may be providing too much phosphorus in the diet. It is very important that there be more calcium than phosphorus in the diet and since you are not feeding alfalfa, this could be a concern. I would be happy to crunch some numbers if you think you'd find that service helpful.
Also, if you are going to feed bran, make sure it's on a daily basis, not once in a while or every few days. Consistency is the key toward keeping the bacterial flora in the hind gut healthy. Some horseowners like to give a bran mash every few days, but this can be dangerous and lead to colic.
Thanks for writing!
Dr. Getty 
Where are you from? Bayfield, CO