Ensure that mobs don't spawn on farm land

I'm playing Minecraft with a mod called Advent of Ascension that has mobs that can spawn regardless of light level.

Is there a way to make sure that these mobs don't spawn on my crops or my grass blocks (so that the sheep can regrow their wool).

Even if mobs ignore light levels (like in the Nether or in your modded case all mobs), they still can't spawn inside blocks with a hitbox (with some buggy exceptions). One that affects not many other things is carpet, for example sheep can still eat through it.

Farmland was changed to be only 15/16 of a block high in 1.10.1, so mobs should already not spawn on it. If they do anyway in your modded game, you could try for example putting end rods above the crops, because usually mobs cannot spawn if there isn't space for them at least two blocks high, not even baby zombies or bats. If they do it anyway, even with crops on the field, there's probably no way to prevent it.