I’ve been trying to replicate these types of fields/farms with Smooth Terrain, I have tried custom textures and grass however I have not been able to achieve a similar look to what is shown here.
Yes but not exact since Terrain doesnt support custom textures 
But you may make something similar
Can’t you just Paint the brown areas of terrain all one Material and the green areas a different Material, then recolour each Materials so they look like the image shown?