For a human being. Why don’t we eat them?
If you’ll think about it for a moment, you’ll realize we eat lots of leaves.
All ruminants and horses too get by on mostly grass in the wild. And sometimes the bark from trees too.
Our digestive system isn’t equipped to get much out of “raw” grass. But when it’s cooked we can get nutrition from it.
In general, if there is a high percentage of cellulose, it “costs” more to process grasses (and partially at that) than you get back eating it. Nutritionally, you’re better off not eating them. Lettuce and such are mainly for water, roughage and a few vitamins (for leafy types) than for calories.
Leaves and grass contain large amounts of the polysaccharide cellulose in their cell walls. Humans cannot digest this compound because we lack the enzyme cellulase which breaks down the cellulose molecule. Ruminants (such as cows, sheep, goats) have special fermentation chambers where certain bacteria live. These bacteria have the enzyme cellulase. Hindgut fermenters (such as horses, rabbits) have a similar setup, but it is not as effective.
Okay, so I should forget about “giraffe” or “horse” as a possible career choice. I can live with that. 
Even animals that are designed to eat grass don’t get too much nutrition out of it. Notice how they spend most of their waking hours grazing, while carnivores can usually get by with one good meal every day or two.
Many vegetables are actually leaves - spinach, turnip greens, various forms of lettuce, and so forth.
And we DO eat grass, or rather a part of it – the seeds. Only it’s more commonly called wheat, rice, rye, oats, barley…
You forgot that many people smoke leaves and grass.
Absolutely - that’s what fed the cow that so generously, and selflessly, provided you with the hamburger you had for dinner.
We do eat it, as a matter of fact, the stuff is called ‘wheat grass juice’ & it tastes, well, try it yourself.
Cecil wrote an interesting article this week on whether plants poop or not & what part of
the plant is poopy & well, it seems to be those leaves. Tasty, eh?
This guy has lived on nothing but about 2kg of grass a day all his life.
http://padmasrinivas.blogspot.com.au/2007/02/flash-news-man-eats-2-kg-of-grass-every.html
I doubt the veracity of that claim
As do I. I can’t see how one could sustain oneself on the minimal calories and nutrition from even 4-1/2 lbs of grass. Sounds like bullshit to me.
Keep in mind the cattle etc. grazing on grass also get a bit of grass seeds (or wheat, or oats) and similar nutrients, and will seek out better food if they can.
But yes, the trick is to convert cellulose into real food, and ruminants let their friendly neighbourhood bacteria do the job for them…
Note the difference, cows leave a big wet pile of byproducts from broken-down cellulose, while less successful horses don’t ruminate on their food, need oats and wheat and such and leave a nice tidy ball of mostly undigested cellulose.
Wheat grass and other such sweets are IIRC the product of freshly geminating seeds, and as such the new plant still has a lot of the nutrients provided by the seed; often, I think, at the point where the starch is turning into sugar. So we get sugar, starch, chlorophyll, several other nutrient, and a dollop of undigestible fiber to do what fiber does best for us.
I thought at first, the article was implying he was eating ghaas ki roti (grass roti) - grass mixed with water and some kind of flour, then fried - that wouldn’t be so hard.
But it goes on to claim he’s just eating raw grass. Gotta be bullshit (although I don’t doubt that the man actually goes and makes a nuisance of himself eating grass in public)
Corn is basically an evolved grass. Still, we eat the seeds, which have been bred to be all huge and tasty and sweet and juicy.
Bamboo shoots are another form of grass commonly eaten. I don’t know how nutritious it is though. I was told that all grass shoots in North America are edible as well as all grass seeds (watch out for ergot though).
Rob
This data shows bamboo shoots comparing favourably to iceberg lettuce, nutritionwise: http://ndb.nal.usda.gov/ndb/foods/show/2820
Another grass used in food is lemon grass.