Well, you aren't needing to 'produce' the crop, you just have to have the production. So...you don't have to have 60 accumulated crop to build them, you just need to have the crop procusion high enough to support the new investment. Thus, you don't have to have it built and then wait fo the crop to come rolling in, and if you had built a ton of crop first to account for future need in production, then you are going to have a ton of it in resources just sitting there for nothing but being plundered.