Environmental Engineering Reference
In-Depth Information
peppers (Canakci and Akinci 2005), but in heated Dutch
greenhouses the same crops may consume as much as 40
GJ/t (Dutilh and Kramer 2000) and the heating rate
may be several TJ/ha. These subsidies translate to about
3-4 GJ/t of Manitoba spring wheat, 5 GJ/t of Iowa
corn, and up to 7 GJ/t of rice. Although vegetables and
fruits need much higher energy subsidies per hectare,
their high yields translate into GJ/t rates that are similar
to those for cereals.
The high metabolic requirements of heterotrophs (see
chapter 4) mean that specific energy subsidies in animal
husbandry have to be considerably higher than those in
plant agriculture. Feedlot-fed beef, a product of large,
relatively long-lived domesticated mammals consuming
carbohydrate-protein mixtures, is the most heavily subsi-
dized animal food, whereas the meat of birds and pigs,
animals that are brought to slaughter weight in just a
few months, is less demanding to produce. Weight gains
are faster when feed consists of concentrates (cereals, le-
guminous grains) rather than roughages largely devoid of
proteins and lipids (grasses, straw), but the cultivation of
concentrates needs higher energy inputs. Eggs need less
energy than meat, and milk is the least energy-intensive
animal foodstuff; its low energy cost is rivaled or bested
only by aquacultured herbivorous fish.
Beef production, marginal in Asia and traditionally a
by-product of dairy industries in Europe, is the largest
livestock undertaking in the Americas. The North Amer-
ican system is a combination of intensive feeding based
on grain and soybeans, the continent's two main crops,
and extensive Western grazing. Even beef ranching—
with energy subsidies for pasture improvement, fencing,
pickup trucks, fuel, and production of hay—can be fairly
energy-intensive, with the published extremes of 18-130
MJ/kg of gain by weaner calves. Feedlot management
costs 3-11 MJ/kg of gain and is dwarfed by the cost of
feed, which depends on the length of feeding (60-150
days) and the share of grains in the ration (no less than
30%-40% of total feed energy). The rest comes from
roughages, mostly hay and silage corn.
Net energy needs during feedlot finishing of animals
that weigh 400-500 kg at slaughter are 50-65 MJ/kg
of daily gain, corresponding to 100-125 MJ/kg of
metabolizable energy. Depending on the roughage/
concentrate ratio, these gains require at least 8-10 kg
dry matter equivalents of 110-140 MJ/kg of gross feed
energy inputs (roughages at 8.5 MJ/kg, concentrates at
13.3 MJ/kg of metabolizable energy). When produced
by extensive cropping, 1 kg of feed may need 3 MJ, and
the feed would cost as little as 30 MJ/kg of beef. When
coming from irrigated fields, 1 kg of feed may need 10
MJ, and each kilogram of gain is then subsidized by
about 100 MJ (Smil 2000b). Case studies taking into ac-
count the needs of breeding animals have come up with
total energy subsidies of 60-150 MJ/kg of feedlot beef;
typical U.S. Midwest values are 80-110 MJ/kg of
dressed meat.
Pork is now produced in rich countries by intensive
feeding in total confinement, taking less than six months
from weaning to slaughter (Pond 1991; Taverner and
Dunkin 1996). Soybean meal supplies protein, grain corn
supplies carbohydrates. Year-round farrowing smoothes
the output, and breeding for a lower share of trimmable
fat has resulted in more meaty carcasses, quite distinct
from traditionally fatty Chinese pigs, which are still
largely fed wastes and take more than twice as long to
reach slaughter weight. Animals slaughtered after six
months of feeding need 75-90 MJ (5-6 kg) of concen-
trates to gain 1 kg of live weight, and these gains require
15-60 MJ of energy subsidies. All other energies (for
