I knew that for all of the bird-themed projects I wanted to work on, I was going to need some seed, both 3D and some 2D textures. So, to make life a bit simpler for the long run, I first modeled some 3D seed, generated particle systems to drop it and spread it in layers, and then used the regular camera to render out color, depth, and normal layers. This allowed me the freedom to create several different mixes by adjusting the rates of emissions for each of the different seed emitters.
These layers of renders, I then composited in Substance Designer, in order to create these seed-mix Substances.
As a bonus, I can take my original seed models and use them to add finer details in my projects by scattering real 3D seed over the Substance materials. Win-win! (This is something you can already see in my bird feeders and seed balls.)