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Opportunities and limits of phosphorus reduction in the feeding of pigs and poultry
Institut für Tierernährung, Fachbereich Veterinärmedizin
Freie Universität Berlin
Although the phosphorus contents in rations for pigs and poultry were reduced substantially
during the last two decades by advanced feeding strategies including microbial phytases,
further reductions would be of economic and ecologic relevance. This contribution attempts to
point out fields, which are promising to contribute to that goal but also to show possible
The following possible ways for reduction of phosphorus in rations for pigs and poultry were
- More precise estimation of requirements and defined recommendations for phosphorus
- Application of phytases as feed additives and development of phytases with improved
properties (screening, selection, genetic engineering)
- Plant breeding (GMO) aiming increased content of non phytate phosphorus (NPP) at the
expense of phytate phosphorus. A further target might be an elevation of phytase activity
in feedstuffs of plant origin.
- Transgenic animals, able to express and secrete phytases into the digestive tract.
With regard to recommendations for nutrient supply, one contribution could be the consistent
application of the knowledge on P-requirements leading to phase feeding systems. While the
recommendations for pigs are based on the digestible phosphorus (GfE 2006) the
recommendations for poultry are based on NPP (GfE 1999). The latter ignores the indigenous
plant phytase activity and possible microbial activities in the GIT and thus underestimates the
P-utilization. Therefore, recommendations based on data derived from animal studies (Putilization
or praecaecal digestibility) should be elaborated.
Application of phytases as feed additives in diets on feedstuffs of plant origin contributes
considerably to improvements of P-digestibility. On the other hand, in practice the Pdigestibility
does not exceed 60 to 70 %. This is mainly due to incomplete dephosphorylation
of inositolphosphates in the GIT. Limitations for the enzymatic action may be inactivation
during feed processing, narrow pH profile, limited capacity for dephosphorylation of
inositolphosphates by a particular phytase, proteolytic inactivation in the GIT and short
retention time of digesta in the GIT. Many attempts are made to develop phytases with
improved properties. Some of these developments are promising and include screening for
new phytases, genetic modifications and combinations of enzymes.
It was shown that the selection of plant mutants or molecular biology techniques may
effectively lead to low phytate varieties of e.g. corn and barley or to high expression of
phytases in plants. Both result in an improved digestibility and utilisation of phosphorus. It
seems that the applicability of these varieties is limited at present due to impaired germination
rates of the seeds and reduced productivity of the crops.
Transgenic pigs transformed for exprimation and secretion of phytases into the digestive tract
show a highly effective P-digestibility. A problem associated to such transgenic lines seems to
be the very high variation in phytase expression among the animals. Furthermore, the costs of
production and maintenance of a transgenic line (of pigs) are not indicated, therefore an
assessment of the practicability is not possible at this stage.
Because many measures described above include genetic modifications in micro-organisms,
plants or animals, also public concerns have to be taken into consideration.