Using a molasses based 40% protein liquid, we will put 10% of the bale weight on.  That would be 120# on a 1200# bale, which is 10 gallon.  It all soaks in - it absorbs so well that if we use less it won't make it to the bottom of the bale and the cows won't eat the bottom of the bale.
This will raise the protein in the bale 4 points (so it'll take 6% hay to 10%), increase the digestibility of the hay (increasing the bale's energy yield) and also add energy in the form of sugar (the liquid is 80% the energy of corn). The cows will eat it better and waste much less of it.