Just to follow up on Shqippy's speedy reply...the article he links to cites

Hendrickson, John. 1996. "Energy use in the U.S. food system: A summary of existing research and analysis." Sustainable Farming-REAP-Canada. Ste. Anne-de'Bellevue, Quebec. Vol 7, No 4. Fall 1997.

as the original source of the data. Unfortunately the journal isn't held in libraries near you.

You'll have to get it via Interlibrary Loan if you want to read the original study.

fyi, the journal title may also be listed as Sustainable farming : the quarterly magazine of resource efficient agricultural production.