How many homes could be heated by cow muck?
It is not a question I have thought about before, but as Britain searches for sustainable sources of heat, many farmers are looking, well, right under their noses.
Cow manure can be treated to release biomethane, a renewable gas which is already used to power central heating boilers in about a million homes.
But there is "huge potential" to increase that to 15 million homes, according to the campaign group Green Gas Taskforce.
One dairy farmer who turns his cattle waste into biomethane, describes cow muck as "nature's crude oil".
