feed on

We did this one a long time ago, Vincent. I wish I could find the post, but I don't remember which forum site it was at.

so, which is the best answer?

If it is a general statement, then:
The animals feed on grass

That means:

The animals feed on the grass. (wrong?)

Can I say,

(a) The animals are feeding on the grass.

There is some overlap between your two original examples, but broadly speaking:

(a) The animals feed on grass = the animals eat grass wherever they find it - a general statement, like saying monkeys eat bananas.

(b) The animals feed on the grass = the animals eat the grass in this field

Your new example, "the animals are feeding on the grass" implies you are watching som

That the animals are feeding on the grass sounds like there are other things they can feed on but they chose THE GRASS

Can I say,

(a) The animals are fed on grass.

(b) The herbivore animals feed by some grass.

a is possible but it means differently

b is not possible as the animals can't feed themselves by the grass since they don't have a hand But you can pay something BY cash because you do.