Re: How can herbivores eat grass but human cannot?

Grass is a very tough food to live on.  Not only is is literally tough to 
bite off and chew, but the leaves contain hard particles which are very 
abrasive to the teeth.  Also, like most plant leaves, it is nutrient-poor, 
and you have to eat a lot to survive.

Herbivores, like deer or horses, have specialised adaptations to suit them 
for this diet.  They have large rows of very rough grinding teeth, with 
very pwerful jaw muscles, so that a large proportion of the plant cells are 
burst, releasing the nutrients within, before they are swallowed.  

Herbivores have very large bellies.  This is because their stomachs and 
intestines are not simple tubles like our own, but have blind-ending 
sections where the ground-up plant food is held so that micro-organisms can 
get to work on it.  Because plant material consists largely of cell walls 
made of cellulose, which mammals cannot digest, the guts of herbivores 
contain a mini ecosystem of micro-organisms which can digest it, and are 
then digested by the mammal.

If you wanted to live on grass, you would have to spend several hours a day 
eating (sheep and cattle do).  Your teeth would quickly wear out, and you 
would still get very thin because your gut would be unable to extract much 
nutrient from the grass. 

Hoper this helps

Will Higgs