Cattles have special process to digest food called rumination. This is where they break the cellulose with the help Clostridium (a type of bacteria).
However, in humans these bacteria are cause of diseases like blotted, stomach flu, diarrhea, vomiting and so on. We generally feel our stomach uneasy and say its upset.
So, by default, our body doesn’t contain such bacteria. Hence we are unable to digest cellulose like cattle can.
However, if we reverse engineer this, when our stomach is upset, grass feeding would helpJ.
In Siddha medicine, Arugam Pullu(in tamil) (Scientific name Cynodon dactylon) and generally known as Bermuda grass, has been used to treat stomach ailments. This correlates to the fact that these bacteria will work on the tough arugampul, and hence the good parts which are not supposed to be infected get rescued.
Animals like cats, dogs and other carnivorous are seen eating grass when they have stomach ailments. Animals seem to know better through experience.