Edible and non-edible leaves
For grasses leaves tend to be inedible due do with the high quantity of silica deposited in the leaves to form structures known as phytoliths Wikipedia.

While this has been shown to inhibit digestion by insects (Hunt et. al., 2008, Massey et.al., 2006), I'm not aware of any studies on how this affects digestibility for humans.  There are however anecdotes about people starving to death with grass stains around their mouths during the Irish Potato famine.