First, elephants rear up to feed only when
conditions are such that they are forced to, meaning a drought.
On the only photo I've seen of a rearing elephant (in the wild at least),
the elephant is surrounded by green. Most of the photo is green. (As a whole
it's so beautiful they put it on the back cover of an ecology textbook. I'll
probably be able to get the citation; at present I only know where in the
library it is.) Had the elephant _wanted_ to eat grass, nothing would have
stopped it from doing so.