Google is allowing the users to take their data out, but the discussions are being shut down.
Beside that, bio-energy does not count the CO needed for stuffing the animal with food, so you might to count all the chemicals, fuel and machinery a farmer will use to grow that animal into account.
Given those unreliableness, you would have to have a long time backup energy for that (like it would take time to get new, uninfected animals in case of an disease).
That given in account I would'nt go for poo-energy and stay with an alternative mix of green energy.
How many weeks are there in a light year?