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Total and water-soluble phosphorus excretion from swine fed low-phytate soybeans^1,2

W. J. Powers^*,3, E. R. Fritz^*, W. Fehr and R. Angel

^* Departments of Animal Science and and Agronomy, Iowa State University, Ames 50011; and Department of Animal and Avian Sciences, University of Maryland, College Park 20742

³ Corresponding author: wpowers{at}iastate.edu

A study was conducted to evaluate the impact of feeding soybean meal (SBM) from low-phytate (LP) or traditional phytate (TP) soybeans on performance and excretions from growing swine. Ninety-six crossbred barrows (initial BW = 18 ± 0.3 kg) were allocated by BW to 24 pens and fed 1 of 4 treatment diets: TP SBM without supplemental phytase; TP SBM plus 500 phytase units of phytase/kg, as-fed basis [Ronozyme P (CT) 2500; DSM Nutritional Products, Basel, Switzerland]; LP SBM (USDA-ARS breeding line CX1834-1) without supplemental phytase, and LP SBM plus phytase. All diets within a feeding phase were formulated to be isocaloric and have similar available Lys and nonphytin P content. Pens were assigned randomly to treatments at the beginning of each of the 4 feeding phases. An indigestible marker was added to the mash feed. Individual pig weights and fecal samples were collected, and feed disappearance by pen was recorded weekly. No phytase inclusion or SBM source effects were observed for pen ADG, ADFI, or G:F (P > 0.05). Total tract apparent digestibility of DM and OM was not different among treatment groups (P > 0.05). Apparent digestibility of P was greater for pigs fed diets containing the LP SBM (48.9 vs. 42.4%; P < 0.01) and less when diets included phytase (44.3 vs. 47.0%; P < 0.0001). Total P (tP) and water-soluble P (WSP) excreted were affected by dietary treatment (tP: 20.0, 18.0, 16.8, and 13.8 g/kg of feces DM, P < 0.01; and WSP: 10.9, 10.1, 9.1, and 8.5 g/kg, P < 0.01, for TP SBM without supplemental phytase, TP SBM plus 500 phytase units of phytase/kg, LP SBM without supplemental phytase, and LP SBM plus phytase diets, respectively). Inclusion of phytase decreased tP and WSP excreted (P < 0.01), as did use of LP SBM (P < 0.01). Diet effects on the fraction of excreted tP that was WSP were observed (P < 0.01); however, there was not a significant effect of SBM source. Inclusion of exogenous phytase in diets increased the proportion of tP that was excreted as WSP from 55% in diets without phytase to 59% in diets containing phytase. The findings suggest that there is a need for LP soybeans as a dietary component to minimize environmental impacts.

Key Words: excretion • low-phytate soybean • phosphorus • swine

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