3% of feed calories end up as beef calories humans consume
"Only about 3% of the feed energy consumed by livestock remains in edible animal tissue.[Powell] Thus, animal-based agriculture needs considerably greater areas of land to output product of equivalent energy value; for example while one hectare of land is needed to produce sufficient rice or potatoes to feed 19 to 22 people per year, the same area would produce enough lamb or beef to supply only one or two people."
Jonathan Foley, "Typically, grain-fed cattle operations use 30 kilograms of grain to make one kilogram of edible, boneless beef."
Emily S. Cassidy et al., “Redefining agricultural yields: from tonnes to people nourished per hectare,” Environmental Research Letters, 8, no. 3, (August 1, 2013): 3, http://iopscience.iop.org/1748-9326/8/3/034015/article.
"Global food: waste not, want not," Institute of Mechanical Engineers, January 2013, 10. http://www.imeche.org/docs/default-source/reports/Global_Food_Report.pdf...
Source cited (unavailable without purchase):
Powell, T.W.R. and Lenton, T.M., Future carbon dioxide removal via biomass energy constrained by agricultural efficiency and dietary trends. Energy Environ. Sci. DOI: 10.1039/c2ee21592f (2012)