Environmental Engineering Reference
In-Depth Information
feed preparation, farrowing, feeding, and cleaning opera-
tions, equipment, and buildings) add about 10 MJ/
kg, for a total of 25-70 MJ/kg. In terms of dressed
meat, the U.S. mean is 40-45 MJ/kg, but Dovring
(1984) put the cost of Midwestern dressed pork at 68
MJ/kg.
Broiler chickens are the most efficient converters of
feed into lean meat; their efficiency of feed conversion
has improved with breeding. Feeding rates for U.S. hogs
have shown no declining trend, and feeding rates needed
to produce milk and eggs have remained virtually identi-
cal since 1910. But by the year 2000 about 2 kg of grain
were needed to add 1 kg of broiler live weight, only
about 60% of the 1935 rate of 5.3 kg (Smil 2000b).
Broilers thus need generally less than 10 MJ/kg of en-
ergy subsidies for their feed. Energy used for incubation,
heating, ventilation, and lighting ranges from about 6
MJ/kg of live weight in buildings with good insulation
to triple that rate in inferior structures.
Other costs, including feed preparation, buildings,
equipment, and a 2.5-5% markup for flock mortality,
add up to 10 MJ/kg. Published studies quote total costs
per kilogram of live weight at 30 MJ for U.S. northern
states, 27 MJ for U.S. southern states, and 33 MJ in En-
gland (Ostrander 1980); typical rates of the 1990s were
10%-20% lower. Egg production starts with rearing pul-
lets to the point-of-lay (110-140 MJ/bird), and a laying
hen requires 38-42 kg of feed per year, or 2.5-3.5 kg of
feed per kilogram of eggs (annual output is 200-250
eggs). Feed costs are thus at least 230 MJ/a, and
the overall rates are 450-500 MJ/a. Milk is the least
energy-intensive animal food; only about 1 kg of concen-
trate feed is needed to produce 1 kg. Mixtures of grazing
and grain feeding translate into very different energy sub-
sidies. Other inputs, dominated by milking and water
heating, are usually no more than one-third of the total
subsidy of 5-7 MJ/kg of milk.
No single measure can serve as a clear yardstick for
comparing energy efficiencies of animal food production
(fig. 10.11). In rich societies no animal food is eaten pri-
marily for its overall energy content. In fact, fat, the most
energy-dense component of these foods, is frequently
avoided or discarded as people buy skimmed milk and
low-fat cheeses and throw away trimmable fat before
cooking meats. High-quality protein is the most desirable
nutrient in animal foods, and young lean birds are its best
source compared to pigs or cattle. But ruminants can di-
gest cellulosic phytomass and convert otherwise unusable
low-quality crop by-products and forages into high-
quality protein.
Gross energy in feed is converted to dressed meat with
efficiencies of about 5% in cattle and roughly 10%-15% in
pork and poultry, and these respective shares drop to less
than 2%, 5%, and up to 7% for cooked edible meat. Feed
energy is converted to eggs with 15%-20% efficiency and
to milk with 20%-25% efficiency. Production of 1 g
of protein requires about 2 MJ of feed in beef, at least
500 kJ in pork, 300 kJ in poultry, eggs, and milk (Smil
2000b). These differences are not perfectly mirrored by
energy subsidies going into animal protein production
because the relatively high energy use in broiler and bat-
tery egg operations narrows the gap. While 1 g of beef
protein needs at least 600 kJ of energy subsidies, pork
protein can be grown with 400-500 kJ/g, chicken and
egg protein with 300-350 kJ/g, and milk protein with
200 kJ/g. It is just a coincidence that the gross feed en-
ergy required to produce 1 g of poultry protein is identi-
cal with the energy subsidies invested in the process.
Much of ocean fish protein costs as much as beef or
pork protein. Modern fishing is totally dependent on liq-
