Answer

By Your Horse

Any concentrate meals greater than 2kg in weight will increase the chance of undigested starch reaching the horse’s hindgut.
This is rapidly fermented by the fibre-digesting bacteria, and lactic acid is produced as a result. Lactic acid is poisonous to the bacteria in the hindgut and they begin to die, releasing toxins into the gut. This can lead to conditions such as colic, laminitis and tying up.