I used to doing quite a bit of home brewing and tried using my spent grains for feed. I have not doubt it is a good filler to add to other feed. I would not depend on it for a sole source of feed though and here is why. When the grain is used in brewing they extract all the sugars and other goodies from the grain. This is the primary goal of whole mash brewing. The “wort” is the extracted fluid from the grain and is the basis of the beer/ale. What you are left with in the grains after this is a very minimal source of feed. You can tell by watching the extract process. The runoff of the wort goes from very dark to almost clear at the end indicating all the sugars are extracted from the grains.
Once again, as a filler it would be okay but plan on supplement with grazing are high quality feeds.
JMHO.
Thanks