The problem with this on a large scale is that grain fed animals, beef especially, is seen as an inferior product. And I know the culinary type Americans I've talked to have much prefer our grass fed beef to their own. Also, grain fed animals are much more associated with factory farming rather than pasture farming. It allows the farmer to pack animals into sheds. This in turn means that the animals are in too close a proximity and disease becomes an issue, so they get pumped full of anti-biotics, which is one of the biggest concerns in the world wide food industry at the moment. So while it's admirable for a small farmer who wishes to be economical, I'd be wary of this if I ever saw it coming from a very large commercial entity purely because grain fed animals' manure is associated with unhealthy animals.
Fair play to yer man though. He seems to be running a "total business." With every output of his process feeding in as an input to another part of his business.
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