Energy – United States
Cotton agriculture has the potential to be energy positive
All energy used in U.S. cotton agriculture is direct energy in the tractor fuel and all of the energy “embedded” in the fertilizer and other inputs. This energy is about 50% less than the gross energy content in the cottonseed that was also produced with the fiber (Nelson et al., 2009). The energy stored in cottonseed can be captured directly, in biodiesel or other biofuels, or indirectly, as feed for dairy cows and aquaculture. Due to more efficient technologies in harvesting and ginning, as well as reduced tillage, U.S. cotton has reduced energy use by 38% since 1980 (Field to Market, 2016).
"U.S. cotton has reduced energy use by 38% since 1980."
– Field to Market, 2016
Fuel from cottonseed
About 700 pounds of seed are produced for every bale of fiber (a bale is 480 pounds). Like other oil seeds, it can be converted into animal feed co-products and biodiesel. The oil from this seed actually may have been the first vegetable oil ever used to create biodiesel, according to Van Gerpen et al., 2007. About 1,400 kilograms of seed are produced in the U.S. per hectare of cotton, which equates to over 220 liters of biodiesel per hectare – exceeding the direct fuel use in growing and ginning the crop (for average U.S. yields and assuming a typical cottonseed oil content of 14%).
There are other indirect energy costs associated with the inputs used in cotton agriculture, particularly fertilizers and materials used for crop protection; however, even when all energy inputs are accounted for, the total energy for cotton production is less than the gross energy content of the seed produced with the fiber.
The energy balance in cotton agriculture
A great deal of attention has been directed to the “energy balance” of agricultural production, especially in relation to biofuel crops such as soybeans for biodiesel and corn for ethanol. The energy input considers obvious factors such as the amount of fuel used by agricultural equipment, but also includes the energy associated with the manufacturing of inputs into the system such as fertilizers and crop protection products.
These methods have been applied to cotton agriculture in three different studies for cotton grown in the U.S. resulting in a range of 14 to 35 GJ of total energy per metric ton of fiber produced, and average value of 19 GJ per ton (Nelson et al., 2009, Matlock et al., 2008, and Field to Market, 2012). (1 GJ of energy is approximately equal to the energy contained in 7.5 gallons of gasoline. One metric ton is 2205 pounds.)
The gross energy of cottonseed produced with that ton of fiber is 30 GJ per ton (cottonseed has a gross energy content of 21.5 GJ per ton, and 1.4 ton of seed are produced with every ton of fiber). That means it exceeds the energy needed in the production.
The full energy of cottonseed has to be adjusted to account for its being processed into fuel. Notwithstanding, the oil alone from the seed can generate almost 20 gallons of biodiesel per acre, and that does not take into account the energy content represented by other parts of the plant (Holt et al., 2000). If cottonseed were converted to energy at an efficiency of 60%, the net energy requirements to produce an average bale of cotton in the U.S. is essentially zero.