If you are talking about out of date beer from the sealed casks it should be okay, check with trading standards or FSA. If you referring to wasted served beer I think they would have a problem with possible contamination. I feed spent Brewers grains from a local micro brewery along side their home mixed feeds, very beneficial as a vast amount of nutrients, protein and fibre are left behind in the grains after brewing, though I am not sure how benficial the beer would be.