How many sheep are there on earth?

Allegedly: "Professor Richard Lancaster conducted a global census in 2008, concluding an estimated 24 billion sheep on earth, spanning six of the planet's seven continents."

However this number seems incredulous, and I cannot find any reliable sources to back it up. Or any information on Professor Lancaster and his work (other than the previous quoted answer).

More conservative estimates put the number at around 1.2 billion.