"Usually hemp had been grown for fiber, thus most of the THC free varieties are not very good in seed/oil production. There are only a few (Finola and some Australian variety) seed carrying germinations which easily hover in the 2t/hectare range. Andreas Sommer"
from:
Hemp Oil Yields? Industrial Hemp Q&A
a hectare is 100 acres. this means that an acre only produces about 20 pounds of hemp seeds. oil generally weighs about 7 pounds per gallon. This means that even if all the weight of the hemp seed was oil, which it is obviously not, it would only yield about 3 gallons of fuel per acre, which, in industrial farming, is less than it would take to grow the same amount.
Anyone who says that hemp and marijuana are different plants is just ignorant. They are both cannabis sativa.
edit:"According to the research from Cornell, you need about 140 gallons (530 liters) of fossil fuel to plant, grow and harvest an acre of corn. "
from:
http://answers.yahoo.com/question/in...0070222AAHJotZ