You can kill an elk with a 45# bow, probably even a 35# bow.  It is all about shot placement. I relearned that lesson last year when I lost a cow elk hit square in the shoulder at 10 yards.   I was shooting a 57# Osage that has great cast, and a 625 grn arrow.  Maybe a 65# bow would have punched through the shoulder further.  But that arrow placed 4" back would have likely passed right through.