Thought someone might like to hear the epilogue to this story, or at least the conclusion of chapter 1:
I decided to get my little handful of malt, grown from the feed barley as per the OP, and brew it up (JUST that grain, no other ingredients except a little hops), just for the hell of it. The amount I had was good for a maximum of a litre of beer, if I wanted any sort of gravity, but I wasn't really interested so much in whether I get beer at the end, as seeing what the efficiency of the entire process is. Yes, brewing up a litre of beer is laughable, but I thought it might at least give me a rough idea of whether I am actually getting anything viable out of this home-malted feed barley.
Well, to cut a long story short, disastrous! My gravity ended up at about 1.017, which according to Brewsmith (as compared to a standard 2-row pale malt) is consistent with an efficiency of about 30%
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Now I know I can't possibly get any sort of accurate or efficient process with mashing 200g of malt, but still, as a wild ballpark indication, it still doesn't look good.
My guess is that the malting process is mostly to blame (unless I have bungled up a calculation somewhere), since mashing such a small amount ought to be done pretty efficiently - mashing is easy, sparging is easy, you can swoosh the grains around and drain them again, and all sorts. On the flipside, it does get much harder to regulate water temperatures properly...
However, I am thinking that probably the barley isn't THAT suitable to begin with, maybe by virtue of some properties it has that make it more suitable for animal feed. Also, probably inconsistent germination (my fault) leading to under-/over-conversion, too many unviable seeds, too much other crap in there that I couldn't be bothered to pick out.
So I have to rethink this, or else using 3 times more malt for my future AG projects
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I still pitched some S04 in there to see if I get anything remotely resembling beer - I do actually have a nice sweetish wort that I can sort of imagine as a very weak beer in the not-too-distant future!