Knowledge for better food systems

Paper: Phosphorous efficient Enviropig

Science Daily summarises the findings of a paper which reports on recent successful attempts to transgenically breed a pig that utilises  phosphorous more efficiently.  The pigs have genetically modified salivary glands, which help them digest phosphorus in feedstuffs, thereby reducing phosphorus pollution in the environment.

Science Daily’s summary is here.

The citation and abstract is as follows:

Meidinger R. G., Ajakaiye A., Fan M. Z., Zhang J., Phillips J. P., and Forsberg C. W.  Digestive utilization of phosphorus from plant based diets in the Cassie line of transgenic Yorkshire pigs that secrete phytase in the saliva. Journal of Animal Science, 2013; DOI: 10.2527/jas.2012-5575

Abstract

A line of transgenic Yorkshire pigs referred to as the Cassie (CA) line was generated, which possessed a stable, low copy number phytase transgene insertion that enabled phytase secretion in the saliva. This study was conducted to assess growth and efficacy for improving P, Ca, and other macro-mineral utilization in the CA pigs receiving diets typical of those used for commercial swine production. In Exp. 1, 12 CA boars and 12 CA gilts were fed diets without supplemental P gained weight and exhibited feed efficiency similar to conventional age-matched 12 Yorkshire boars and 12 Yorkshire gilts raised on similar diets with supplemental P. Serum concentrations of P and Ca were similar for CA and Yorkshire pigs during the growing and finishing phases, indicating that the CA pigs were not P limited. In Exp. 2, 6 CA (13.1 kg BW) and 6 Yorkshire barrows (8.8 kg BW) were fed 3 diets (control, low in Ca and P, and low in Ca, P, and CP) over 3 phases, and CA barrows fed the diet without supplemental P retained 25 to 40% (P < 0.001), 77 to 91% (P < 0.001), and 27 to 56% (P < 0.001) more P during the weanling, growing, and finishing phases, respectively, than conventional Yorkshire barrows fed similar diets without supplemental P. In Exp. 3, the CA and Yorkshire pigs of similar ages weighing 66.2 ± 1.7 kg (n = 10) and 50.0 ± 1.0 kg (n = 10), respectively, were used. The P retention of CA finisher barrows fed a diet without supplemental P was 34% greater (P < 0.001) than conventional Yorkshire barrows fed the same diet with 750 unit of exogenous phytase/kg diet. Urinary Ca to P ratio in the CA pigs was 0.27, whereas that for the Yorkshire barrows was 30, thereby, indicating that the Yorkshire barrows suffered a P deficiency. Furthermore, digestive utilization of major electrolyte macro-minerals, K and Na, was improved (P < 0.05) by 18 and 16%, respectively, in the CA finisher pigs compared with the conventional Yorkshire finisher pigs fed phytase; however, only K exhibited enhanced retention. In conclusion, the CA line pigs secrete sufficient phytase from the salivary glands to enable efficient digestion of plant P, Ca, and major electrolyte macro-minerals.

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Tara Garnett's picture
Submitted by Tara Garnett on

Background

  • Much of the phosphate in plant derived animal feed is locked up in a relatively inaccessible form called phytate.
  • Phytate is inositol hexaphosphate phosphate  (inositol is a sugar and it has six phosphate groups bound) 
  • Phytate is a major storage form of phosphate in plant cells and therefore animal feed derived from plants (grain) is rich in phytate
  • Phytate is a major cause of pollution because animals cannot efficiently digest phytate.  It is therefore passed through the gut into the excrement. 
  • Pig muck therefore can contain very high levels of phytate.
  • Phytate in  manure is then broken down by bacteria. This  releases large amounts of phosphate into the soil and ground water. 
  • A scientist from Dow AgBiotech told me that being able to get rid of phosphate/phytate-rich manure is the major limitation for pig production. 
  • Farmers tend to spread this manure on their land and there is therefore an upper limit on what they can spread. 
  • Decreasing phytate concentrations is feed grain is an important trait for cereal breeding - low phytate containing maize have been bred (I'm not sure if they are commercially used)

 

Results reported in this paper:

This paper describes an alternative approach – generating transgenic animals that can break down phytate. The technology used seems to be pretty routine. An E. coli (bacteria) gene encoding the enzyme phytase which releases phosphates from inositol hexaphosphate was  introduced into pigs. This gene was expressed in the salivary glands resulting in the expression of phytase in the saliva.They showed that these animals can access sufficient P in their diet in the absence of dietary P. That is they can get their P requirement from phytate.

NB: as regards the ethics of GM breeding in relation specifically to animals, any thoughts, or links to relevant papers -on all sides of the debate - would be much appreciated.