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Expert talk: Zinc oxide in phytase-low phosphorus diets

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In many countries, a pharmacological dose of zinc oxide (ZnO) is routinely fed to newly weaned piglets for growth promotion. EU legislation limits the content of Zn in swine feeds to 150 ppm to reduce its impact on the environment.
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Still, in some EU countries its use is allowed as a therapeutic agent in medicated feeds to prevent
piglet diarrhoea in the first two weeks after weaning. Diets are based on cereals and legume seeds, and therefore contain substantial amounts of phytate-phosphorus which, to become available require previous hydrolysis by phytases. Thus, diets are commonly supplemented with phytases to improve P digestibility, and reduce the use of inorganic-P and the excretion of P. In vitro, phytic acid can readily form chelates with divalent minerals, like Zn, thereby reducing phytase’s accessibility to phytate-P. If the same occurred in vivo, the efficacy of phytases may be compromised in diets supplemented with high doses of ZnO.
 
That hypothesis was investigated in a trial where inorganic-P was reduced at the expense of phytases, in combination or not with a pharmacological dose of ZnO, by Lizardo and others in 2004. Results showed that despite using phytases, the combination of reduced inorganic-P supply and ZnO, decreased feed intake, growth performance and serum P concentration (see Table 1). The withdrawal of ZnO increases the piglet’s feed intake, they grow better and recover serum Ca concentration. Nevertheless, the serum P did not reach the normal concentration.
 
Zn at high concentrations may bind to phytate and less P may be recovered with the use of phytases. Then the piglets may become P deficient and their bone mineralisation and growth performance may be impaired. Although the use of phytases in diets for early-weaned piglets may be limited, these results confirm the risk of reducing the inorganic-P supply in medicated feeds containing pharmacological doses of ZnO.
 

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by Dr Rosil Lizardo, IRTA - monogastric nutrition, Constanti, Spain

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2 comments

  • # 1

    RAFA DURAN

    Hello there. I knew this piece of work and it is stg. which comes back and forth. I want to underline the treatment with just phytase did work pretty well. On top of this, phytases do their job at a pH < 4 and even lower, at that pH, phytate molecule is generally reacting - chelating - proteins, only at more basic pH, already in the small intestine, the phytate will chelate with positively charged minerals (Zn, Ca). But the role of phytase at that level should have been already done. So if the phytase is active at pH <4, even at pH of 2-2,5, I don�t think this negative eefect of Zn would be found. So it is important the pH at which the phytase is effective and I am afraid the phytase used in that trial was of fungal origin (2004), not very effective at low pH. Thanks anyway to Rosil for his work.
  • # 2

    Dr Nikolaos Kotrotsios

    Dr Rosil Lizardo,
    I am honored to share your scientific positions. Many greetings and thank you very much for the valuable assistance offered me the completion of my PhD thesis. Nice to see you again!

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