Dunno how efficient it would be, but you might be able to use runtime virtual textures for this?
RVT can have meshes baked into them that aren’t visible in the main pass, so in theory one could throw a bunch of different colored planes around the map, render them to RVT, then use that as a mask to sample different types of grass for the crops.
If you’re already using RVT for the primary landscape material, a second layer could get expensive. But since grass samples are rather blocky either way, the RVT could be pretty low rez.