Agriculture Reference
In-Depth Information
tested in animal rations as leaf meal or as a
juice dried on maize. A poultry feeding trial
showed that this approach could halve the P
content of manure compared to feeding with
inorganic P supplements (Mueller
et al
.,
2008). Phytase expression can also be used
to improve P uptake: recently, a US-based
team developed a transgenic lucerne
overexpressing phytase in its roots, which
produced twice as much biomass as non-
transgenic lines when grown on natural soil
without P fertilization (Ma
et al
., 2012).
the price of the inorganic P currently used in
feed supplementation. Yet this additive is
rather inexpensive, despite its negative
environmental externalities. h us, the use
of feed with low phytate content could be
economically rational in countries where
environmental regulations controlling
phosphate leaching are in place, limiting the
use of conventional, high-phytate feed.
Once these limitations are overcome,
transgenic plants with high phytase content
would dei nitely enhance the uptake of
minerals and phosphorus by animals, while
at the same time reducing P pollution in
soils. When accounting only for savings in
feed costs due to the replacement of
inorganic P, Johnson
et al
. (2001) calculated
that the introduction of high-phytase maize
in diets could translate into an added value
of US$4.6/t and that approximately
55 million tonnes (Mt) of such a maize could
be consumed. h is would generate an
additional gross value of US$260 million for
US maize on world markets. h is estimation,
however, assumes that the production cost
for low-phytate maize will remain the same
as for conventional maize. It does not
consider any possible technology fee, which
will certainly determine the adoption of
such an innovation by maize growers. On
the other hand, the positive environmental
ef ects of a low-phytate animal feed should
also be considered as benei ts of this
technology, since it could lead to a reduction
of up to 85% of P waste when provided to
poultry, swine or i sh (Raboy, 2001).
Internalizing this external ef ect dei nitely
would contribute to the adoption and suc-
cess of such GM plants for animal nutrition.
Prospects and challenges for commercial
use of low-phytate GM crops
As demonstrated in the previous section,
the pipeline for low-phytate GM crops is
very active, with many research teams
involved and aiming to improve the most
important crops in the world. However, as
most of the studies point out, there are still
a number of challenges to overcome, which
may explain why, so far, few events have
reached a status beyond advanced develop-
ment. First, for plants with a low phytic acid
(
lpa
) trait developed by knocking out genes
involved in phytate biosynthesis, some
negative ef ects on seed and plant growth
have been described, leading to adverse yield
ef ects (Raboy, 2001). Second, the industrial
feed pelleting process requires heating of
the seeds in order to eliminate some anti-
nutrient compounds and to avoid possible
Salmonella
infections. h is implies that the
phytase enzymes present in the seeds must
be able to retain activity after this process in
order to be still useful when consumed by
animals. h e same limitations apply for
those cereals destined for human con-
sumption that require being boiled or baked.
Finally, the economic incentive for farmers
to pay a technology fee for a GM low-phytate
crop is another important issue. For farmers
raising monogastric livestock, it might be a
way to reduce feeding costs, especially if
they produce their own feed from self-
produced crops. Otherwise, the adoption of
these crops will depend on the premium the
feed industry will be willing to pay for the
modii ed crop, which in turn will depend on
Humans, as well as many farm animals, are
unable to synthesize certain amino acids,
called essential amino acids (EAAs) (see
Chapters 1, 7 and 10). Since plants are the
primary source of EAAs for animals, this is
why intensive breeding ef orts have focused
on elevating the amino acid content in
