So, if you're like me and you see this deal on craigslist, you get temporarily excited. Feed if $400 ton or more, this is just a little over a $100 a ton. I actually called up one of these craigslist guys a few years ago, got all ready to go out and pick up a barrel, then took a second to screw my head back on my shoulders. Here is the issue: let's do a simple comparison of using spent grain as a feed to plain old unspent barley. (Most of the time, spent grain is fermented barley). A quick look at the nutrition charts for spent grain and barley will tell you that spent grain is 26% protein and barley 12% protein. Again, at first glance, this seems like a great deal. Looking a little farther, we get some worse news: spent grain has 11 MJ/kg DM of digestible energy, while barley has 15 MJ/kg DM. That doesn't sound so bad, spent grain is a little lower in energy (i.e calories), but it's so much cheaper. The key is the DM. That means dry matter. Feed nutrition is always reported on dry matter basis, and if we scroll back up, we see that barley is 87% DM and spent grain 25%. Uh oh. When you multiply the energy by the dry matter basis, you see that on a per pound basis, wet spent grain (the way these Craigslist guys are selling it and the way it comes out of the brewery) has 4.7 times less energy than regular old barley. Therefore, you need to feed 4.7 times more spent grain than barley, and the equivalent amount of barley would need to cost $523/ton. Therefore, instead of costing a $111 ton, you really need to think of that spent grain as costing $523/ton. Regular old pig feed is costing about $450/ton picked up at the feed store right now, and it lasts much longer than spent grain, is much more nutrient dense (animals don't have infinite stomach and intestinal space to be able to eat the amount of that bulky feed they would need to equal pig feed or barley), and is a complete ration.
The moral of the story: don't buy those $25 barrels of spent grain. Unless you can get it for free or near free, it's not worth the time.