Lots of folks have miniature horses and sometimes determining the amount to feed these horses can be difficult for two reasons. First, it is difficult to accurately estimate their weight as many of the tapes and formulas used for weight estimation are for larger horses and may not be accurate in miniature horses. Secondly, feed recommendations are established by studies on larger horses and so it is questionable if you can accurately interpolate to a smaller size horse. In other words, if the feed bag says feed 4 pounds per day for a 1000-pound horse, that may not mean you should feed a 250 pound horse 1 pound per day. And this is really important because miniature horses and ponies tend to be very efficient at using feed and because of this they are called easy keepers.
If possible, it is always a good idea to take your miniature horses to a scale at least once to have an accurate baseline weight.
If these horses are overfed, they will commonly develop laminitis and founder due to being resistant to the effects of insulin. So if you feed the same ratio of feed per pound of body weight to a regular size horse as you do to a 200-pound miniature horse, it is possible you will be overfeeding the miniature horse. To determine if it is possible to interpolate amounts of feed required from large horses to miniatures, a study was performed in Kentucky and published in the Equine Veterinary Journal. The study found that using large horse recommendations for miniature horses either overestimated or underestimated the energy intake required, depending on the feed chosen. Because of this, always consult with your vet for help in determining the amount of feed as the bag recommendations are not accurate for miniatures.