Where can i find a website for the percent yield of food?

Check out the site about worldwide food quarterly. It may help somehow. http://www.cgfi.org/gfq/gfq_index.htm.
ask the query again
what do you mean?
Do you want the nutritional content of foods
or
the dressing percentage of animal carcass
or
the useful percentage of food from a given amount of agricultural product
or
how much the cultivator makes out of the cost of a loaf or bread??