Agriculture Reference
In-Depth Information
in the future as demand for human food increases
with rising population. Another consideration of
this assessment, not yet elucidated, is whether
the nutritional quality (e.g. dietary essential
amino acid profile) of plant-derived versus ani-
mal products for humans is similar or not, espe-
cially when evaluated per land and freshwater
use, or environmental or climate impacts.
Also, most P used in both plant and animal agri-
culture is from high-density sources (natural
mined deposits or industrial chemical synthesis),
transported and redistributed in different land-
scapes. When not managed properly it not only
causes environmental damage, but also is lost irre-
versibly for potential future use in plant and ani-
mal agriculture (Van Vuuren
et al
., 2010).
Nutrient excretion and broken cycles
Energy use
Intensification of agriculture (both plant and ani-
mal) in many developed regions has resulted in
major increases in amounts and fluxes of N from
application of synthetic fertilizers and manures.
Anthropogenic fixation of N far exceeded all
other disruptions of natural nutrient cycles more
than 40 years ago (Delwiche, 1970). Nitrogen
fertilization of cultured crops and consequent
emergence of reactive-N now far exceed all natu-
ral inputs into landscapes (Gruber and Galloway,
2008). Only a very small portion of this added N
actually appears in food products (plant or ani-
mal) with much flowing from farm systems to
air and water as ammonia, nitrate, nitrous oxide,
or nitric oxide, perturbing natural ecosystem
cycles until ultimately becoming inert-N
2
again
(Galloway
et al
., 2008). Animal agriculture is a
major source of reactive-N loss (Erisman and
Sutton, 2008). Typically, as much as 50-80% of
N ingested by animals is excreted (Chapters 2 and
3). Globally the amount of N excreted by animals
is roughly the same quantity as that in all syn-
thetic N fertilizers used (Bouwman
et al
., 2009).
Fifty per cent or more of ingested N is excreted
from feedlot cattle (McGinn
et al
., 2007; Erickson
and Klopfenstein, 2010) and dairy cattle (Kohn
et al
., 1997; Wilkerson
et al
., 1997; VandeHaar,
1998) and subsequently emitted as ammonia
into the atmosphere. Improvement in efficiency
of utilization of dietary N and more effective
management of excreted reactive-N represent
major challenges for animal agriculture in the
future. Nutritional and management approaches
are being studied and some reduction of N loss
from feedlot systems is possible (Erickson and
Klopfenstein, 2010). Animals also can excrete
large amounts of P (Bouwman
et al
., 2009).
When in excess and not properly managed in the
farm, P can move with surface water and con-
taminate natural and man-made waterways.
Long ago, intensified 'modern agriculture' was
recognized as 'the use of land to convert petroleum
into food' (Bartlett, 1978). In developed coun-
tries, use of relatively inexpensive (government-
subsidized) supplemental fossil fuel energy
has had a very significant influence on the large
increase in productivity of animal agriculture
in the last 50-plus years. For example, Hillel and
Rosenzweig (2008) estimated that about 35 kJ of
fossil energy were needed to produce 1 kJ of feed-
lot beef. Doubtless, worldwide the use of fossil
energy to produce livestock feed and animal
products will require greater and careful exami-
nation as future global energy supplies and
demands, forms of energy, and markets and gov-
ernments' policies evolve (Anonymous, 2010). It
seems entirely plausible, as recent trends suggest,
that less fossil fuel will be available for animal
production by 2050 and different forms and
cycles of energy utilization should be explored
both in developed and developing countries.
Climate change
Animal agriculture is identified often as a signifi-
cant contributor to global climate changes
(Chapters 9, 10) and also as a set of managed
biological processes that is likely to be substan-
tially affected by global warming in the future.
Recent estimates are that ruminant enteric
methane (with about 23 times the global warm-
ing potential of carbon dioxide) and nitrous oxide
(about 297 times) from manure N (from both non-
ruminants and ruminants) account for about 9%
of CO
2
-equivalents from anthropogenic emissions
(Gill
et al
., 2010). However, when changes in
other emissions resulting from altered land use
associated with animal agriculture are included,
the estimate of the net effects is about 18%
