To collect such gases from grazing animals would be tricky. First, you would have to apply a sort of gas mask and a sensor which would detect the arrival of a burp. Between then, you would have to supply the grazing animal with oxygen. Simultaneous to that, the animal would have to graze. Nobody in their right mind would attempt doing it that way.
The way it's usually done is to wash stall waste into a container and let bacteria convert the entire sluice into gas and broken up waste. The same can be done with domestic waste such as sluice and food waste. To obtain more methane per capita, you would have to drive peasants out of rural areas and into urban centers which have the infrastructure for collecting all that waste. This has already< happened, by accident, in China. Whether or not they are collecting gas or not is unknown to me.
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...and the link that you provided is silly (and I believe that you only found it in the last hour)...
The best links are in German where they have more experience with these issues:
https://www.stadtwerke-hameln.de/umwelt/biomasse/biogasanlagen.html
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....1 acre of "whole crop maize"...
Using foodstuffs for replacing gasoline and organic waste isn't an idea popular with everyone. Communities are presently collecting food waste and are extracting methane from it.
This particular plant produces enough energy for powering 3000 households with 10,000,000 kwh per year:
http://www.badische-zeitung.de/neuried/badenova-kauft-biogasanlage--19005082.html
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...Die Neurieder Anlage bringt es jährlich auf zehn Millionen Kilowattstunden und versorgt 3000 Haushalte...
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a coal station would do that with 16 tonnes of coal....(80 205 litre drum of the stuff)...
Coal could burn much cleaner, if it was pulverized to dust. But, to pulverize it would require amounts of energy which would call into question of it being feasable to even do so