Little research has been done regarding the feeding of miniature horses. What has been done indicates that nutrient requirements will be the same as light horses based on age, physiological function, and activity level. In other words, intake of forages and any other feed should be based on body weight of the miniature horse, body condition score, age, physiological function, and activity. As always, it is important to feed a high quality forage and make up any nutrient deficiencies with a good quality concentrate to complement and balance out the nutrient intakes from the forage the miniature horse is consuming.
1-2% of body weight for a 100 pound horse means that you would feed 1-2 lbs of hay or grain per day. Since miniature horses have so little body weight to maintain, actual nutrient needs may be even less. Start your feeding program with 1% BW portions and adjust at the end of a month based on body condition score.