Methane from the waste of cows is already being converted into electricity.
Now researchers say the two billion pounds of manure produced daily in California could be used to power motor vehicles that are made to run on natural gas.
Allen Dusault, an energy consultant, headed the U-S-D-A sponsored study.
"Certainly easily all the ones in the Central Valley and really all the ones in the state could be run on the fuel from manure."
Dusault says some countries in Europe are producing and distributing motor vehicle fuel derived from organic materials.
"Sweden primarily where they've been running their cars and trucks and buses on biomethane made from organic sources."
Diary farmers like the idea of converting manure to fuel. It could serve as another source of income, and help reduce the costly pollution generated by their herds.
Dusault says it remains to be seen if the private sector would be interested in committing the capital needed to produce and distribute fuel converted from cow manure.