My goal was to find how close present farmers are to growing enough for the nutrition and wants of ten billion. The present food reaching people could not sustain the ten billion. If the feeding of farm animals were eliminated and people ate all the grains and oil-seeds now produced, the ten billion barely could survive on the calories. The protein in all the grains and oilseeds would, however, support them.
But a complete inventory of present agricultural production must include feed as well as food crops and items such as tea or rubber. It must allow some calories and protein from animal products and the consumption by draft animals because animals eat grass and plankton as well as crops. In my inventory I add feed grains and omit animal products; by this I imply that the ten billion will eat differently from the people today, which the changes in diets of large numbers of people actually hint could occur. I add the consumption by draft animals, implying horses, camels, and water buffaloes will be replaced by engines. By adding the inventory this way, I reckon that present agriculture on present land could sustain the ten billion.
Present farmers come within striking distance of growing enough for the nutritional needs and wants of ten billion. So future, eminently smart farmers might even sustain the ten billion better and spare more land for Nature-unless some global, physical limit stops the rise of yields/plot.