you can set up something like, you put two grass blocks with pressure plates on them and the cows will still spawn on the pixels of grass that are left, like the pressure plates up to Pistons a block higher and it will push the cow down,

It would look like this

[] Spawner []
-- Spawner --

[]= Piston
--=Pressure plate

You can set up something like, you put two grass blocks with pressure plates on them and the cows will still spawn on the pixels of grass that are left, like the pressure plates up to Pistons a block higher and it will push the cow down.

It would look like this:

[] Spawner []
-- Spawner --

[]= Piston
--=Pressure plate