What the Heck Is grain free forage replacer UK?

https://balanced-horse-feed827.cavandoragh.org/horse-feed-for-pelleted-feed-a-handy-uk-guidebook

Gentle horse feed for sensitive stomachs, designed to support comfortable digestion, hindgut health and efficient nutrient uptake. A low sugar fiber-rich formula with naturally sourced ingredients to help maintain calm digestion and overall wellbeing.