email from Emily Cassidy
Dear Ms. Lappé,
The livestock conversion efficiencies I used in the Foley publication and the more recent one (online here) are similar to the conversions in Smil's book 'Feeding the World: A Challenge for the Twenty-First Century' but I derived them with USDA data. I explain this more in the livestock conversion and sensitivity parts of my supplementary material that I have attached, but here's a brief explanation:
Often the 12 to 1 ratio is cited for beef, but that's 12 pounds of feed for a pound of live weight, which includes inedible parts of cattle. In order to get pounds of feed per pound of edible carcass, I used dressed weight proportions. Only about 60 percent of beef cattle is retail carcass, so the feed input to carcass weight is 21 to 1. But this still doesn't tell us the calorie conversion efficiency; for that we need calories in and calories out. According to the USDA beef cattle on feed (not during grass-fed stages) had feed compositions of 85 percent corn or soy and 15 percent grassy forage. So when we divide 21 pounds of feed that are 85 percent corn and soy by the beef carcass calories coming out, we get 30 calories of feed producing 1 calorie of beef carcass.
I'm happy to discuss this further and send a spreadsheet of the calculations if you would prefer. Thanks for your question and your great work!"
Initiative Institute on the Environment
University of Minnesota -- Twin Cities