To address the problem of manure-based environmental pollution in the pork industry, we have developed the phytase transgenic pig. The saliva of these pigs contains the enzyme phytase, which allows the pigs to digest the phosphorus in phytate, the most abundant source of phosphorus in the pig diet. Without this enzyme, phytate phosphorus passes undigested into manure to become the single most important manure pollutant of pork production. We show here that salivary phytase provides essentially complete digestion of dietary phytate phosphorus, relieves the requirement for inorganic phosphate supplements, and reduces fecal phosphorus output by up to 75%. These pigs offer a unique biological approach to the management of phosphorus nutrition and environmental pollution in the pork industry.

The objectives of this study were to determine true phosphorus (P) digestibility and the endogenous P output associated with corn for growing pigs using the regression analysis technique. Four barrows, average initial body weight of 25 kg, were fitted with a T-cannula and fed four diets according to a 4 x 4 Latin square design. Four cornstarch-based diets, containing four levels of P at 0.7, 1.5, 2.2 and 2.8 g/kg dry matter intake (DMI), were formulated from corn. Each experimental period comprised 8 d with 4-d adaptation and 4-d collection of ileal digesta and fecal samples. The apparent ileal and fecal P digestibility values in corn were affected (P < 0.05) by P contents in the assay diets. The apparent ileal and fecal P digestibility values increased from -51.0 to 33.3% and from -41.4 to 39.1%, respectively, as P content increased from 0.7 to 2.8 g/kg DMI. Linear relationships (P < 0.05), expressed as g/kg DMI, between the apparent ileal and fecal digestible P and the total intake of dietary P, suggested that true P digestibility and the endogenous P outputs associated with corn can be determined by the regression analysis technique. There were no differences (P > 0.05) in true P digestibility values (54.0 +/- 6.5 vs. 59.8 +/- 8.5%) and the endogenous P outputs (0.693 +/- 0.128 vs. 0.670 +/- 0.160 g/kg DMI) between the ileal and the fecal determinations. The endogenous fecal P output represented 12.3% and 25.8% of the daily total and available P requirements in growing pigs recommended by the National Research Council in 1998. The present literature data of apparent digestibility and availability underestimate the true digestive utilization of P in corn for growing pigs by approximately 35%. Current diet formulation on the bases of total, apparent P digestibility and availability values in corn inevitably leads to P overfeeding and excessive P excretion in pigs.

The objectives of this study were to determine true P digestibility, the gastrointestinal endogenous P outputs associated with soybean meal (SBM), and the role of the large intestine in P digestion in growing pigs. Four Yorkshire barrows, with average initial and final BW of 40 and 58 kg, were fitted with a simple T-cannula at the distal ileum and fed four diets according to a 4 x 4 Latin square design. The diets were cornstarch-based and contained four levels of P (0.098, 0.196, 0.293, and 0.391% on a DM basis) from solvent-extracted conventional SBM. Chromic oxide (3.5 g/kg of diet, as-fed basis) was included as a digestibility marker. Each experimental period consisted of 8 d with a 4-d adaptation period and a 4-d collection of representative ileal digesta (2 d) and fecal (2 d) samples. True ileal and fecal P digestibility values and the ileal and fecal endogenous P outputs associated with SBM were determined by the regression analysis technique. There were no differences (P > 0.05) in true P digestibility values (ileal, 59.0 +/- 8.3 vs. fecal, 51.3 +/- 7.9%, n = 16) and endogenous P outputs (ileal, 0.59 +/- 0.18 vs. fecal, 0.45 +/- 0.21 g/kg of DMI, n = 16) between the ileal and the fecal levels. The endogenous fecal P loss accounted for 8.1 and 17.6% of the NRC (1998) recommended total and available P requirements in growing pigs, respectively. In conclusion, approximately 51% of the total P in conventional SBM is digested in growing pigs. The large intestine does not play an important role in the digestion of P associated with SBM in the growing pig. The fecal loss of the gastrointestinal endogenous P is an important route of P excretion in the growing pig.

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 macromineral 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 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. The 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 weaning, growing, and finishing phases, respectively, than conventional Yorkshire barrows fed similar diets without supplemental P. In Exp. 3, CA and Yorkshire barrows 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 units 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 macrominerals, 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 macrominerals.

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Nonpoint phosphorus (P) pollution from animal manure is becoming a serious global problem. The current solution for the swine industry includes the enzyme phytase as a component in oil meal and cereal grain-based swine diets. A long-term approach is the production of transgenic phytase pigs that express phytase in the salivary glands and secrete it in the saliva. This study provides a detailed comparison of chemical structures of manure from conventional pigs and transgenic pigs that express phytase under growing and finishing phases using new solid-state NMR techniques. Spectral editing techniques and quantitative NMR techniques were used to identify and quantify specific functional groups. Two-dimensional (1)H- (13)C heteronuclear correlation NMR was used to detect their connectivity. Manure from conventional and transgenic pigs had similar peptide, carbohydrate, and fatty acid components, while those from transgenic pigs contained more carbohydrates and fewer nonpolar alkyls. There was no consistent effect from diets with or without supplemental phosphate or growth stages.

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Genetically modified pigs have been developed to enable them to use phosphorus (P) in the feed more efficiently and thereby reduce the amount of P excreted in the manure. However, there is no information available about how improved P digestibility affects nitrogen (N) and carbon contents in the pig manure and the subsequent dynamics that occur when the manure is applied to soil. An 8-week incubation study was conducted to determine the changes in inorganic N contents of two soils (a clay loam and a sandy loam) treated with pig manure (115 mg N kg -1 soil) from these transgenic pigs as well as from conventional pigs. In addition, both the transgenic and conventional pigs were fed either a low-P diet or a conventional P diet and the effects of diet type on N contents in the manure and the subsequent soil dynamics were examined. There was no difference in total manure N content between transgenic and conventional pigs. However, the ammonium content was lower and the soluble carbon level was greater in manure produced by transgenic pigs than by conventional pigs. The manure excreted by pigs fed with low-P diet contained more total N and ammonium N than the manure from pigs fed with conventional-P diet. Ammonium-N was nitrified completely in the first week of incubation in manure-amended clay loam, while the nitrification process took 4 weeks to complete in manure-amended sandy loam soil. Manure from transgenic pigs not only contained less inorganic N but this N was more rapidly immobilized and/or denitrified compared to the inorganic N produced by conventional pigs.