The cellulose must first be broken down into it's component sugars, which is something mankind has yet to master.
*Picture of modified cow snipped*
There you go. A Cellulose to Methane ( byproduct ) converter named Daisy.
Now turn the methane (1 carbon, 4 hydrogens) into sugar (6 carbons, 12 hydrogens, 6 oxygens).
Done yet?
How about now?
Still waiting...
-Blast
p.s. where does the tubing enter the cow?