CarolC,
Intensive livestock operations, or rather those where the manure is easily collectible, (I'm not too keen on feed-lots) can certainly generate a lot of methane. All the stats are available i.e. so many cows/pigs/whatevers produce so much manure per day which gives so much gas.

We had the manure from 63 cows fuelling two digesters and used a lot of gas for hot water, cooking etc. and still had more to spare.

Methane digesters, however, aren't primarily targeting gas. You have a lot of manure to get rid of. Putting it through a digester gets rid of the offensive smell, flies etc. and the end product is excellent feertiliser. I used to sell it at US50 cents a gallon which wasn't bad considering what we were getting for milk.

The methane gas is a bonus. It's easy to run a pipe from the digester to your home or a village. Compressing it for transport is more complicated.