Can we support a larger human population by eating grains and other primary foods directly,or by first feeding this food to cows(a secondary source),and then eating the cows.

Answer 1

As you go up the food chain, the amount of energy retained to be passed on to the next level decreases.

Adding a level by feeding secondary sources, then eating them, therefore decreases the energy humans would get with respect to primary foods.

So, which do you think supports a larger population?

Answer 2

Can humans digest cellulose?

Answer 3

This is about all I could find, it is on Dogs and Cats, but some human info. The issue in these comparision on food "efficiency" is energy vs quality amino acids. Energy is one item, if the animal can digest cellulose, but quality amino acid (protein) is another.
http://www.thepetcenter.com/imtop/contrast.html

Answer 4

And one other link, there are cellulose based foods that can supply needed protein (but still not complete)..
http://www.nutritiondata.com/facts/vegetables-and-vegetable-products/2708/2