oats cannot digest easily

Question
hello ma'am. may i know why cant horses digest grains easily when compared to roughage?

Answer
Hello Caine,

Physiologically, horses digestive systems have limited capability to digest grains.  They small grain storage and digesting space, and fast rates of passage when fed large amounts of grain or if they are exercised excessively.  Conversely, the horse's digestive system is ideally designed to provide adequate space and capacity to digest forages.  

Thanks, Corlena