| Two trials were conducted to investigate the true phosphorus (P) digestibility and the endogenous P output of growing pigs using the regression analysis technique. In trial one ,eight crossbred barrows (D L Y), average initial body weight of 30.15kg, were fitted with a T-cannula at the distal ileum by surgeon and fed experimental diets according to two 4X4 Latin square designs. Four cornstarch- based diets, containing four levels of P at 1.96, 2.43, 3.58, 4.15g/kg dry matter (DM), were formulated with soybean meal (SBM). The other four cornstarch-based diets, containing four levels of P atl.81, 2.56, 3.57, 4.26g/kg dry matter (DM), were formulated with wheat-middlings. Each experimental period was comprised of 8 d with 4-d adaptation and 4-d collection of ileal digestaand fecal samples. The results indicated that the apparent ileal and fecal P digestibility values in feeds were affected (P < 0.05) by dietary P contents. The apparent ileal and fecal P digestibility values in SBM increased from -3.29 to 26.37%and from 20.86 to 38.11%, respectively, as P content increased from 1.96 to 4.15 g/kg DM. And the apparent ileal and fecal P digestibility values in wheat-middlings increased from 9.18 to 37.85% and from 32.00 to 50.89%, respectively, as dietary P content increased from 1.81 to 4.26 g/kg DM. 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 SBM and wheat-middlings can be determined by the regression analysis technique. There was no significant difference (P > 0.05) between the true ileal and fecal P digestibility values in both SBM and wheat-middlings(50.15%±5.85% vs.5I.87%±3.43%, and59.23%±0.18%vs.63.70%± 3.74%) . Apparent digestibility values under-estimate the true digestibility values of P by ~ 25%. True rather than apparent P digestibility values should be determined and used in diet formulation for pigs. The endogenous P output was higher (P < 0.05) in ileal digesta than in feces (0.91 ±0.01 vs. 0.67±0.02 g/kg DM diet intake). The endogenous fecal P output represented 11.30-11.78% and 24.57~25.61% of the daily total and available P requirements ingrowing pigs (20~45kg) recommended by the National Research Council in 1998. In addition, this study shows that the gastrointestinal endogenous P output is important in total P requirement and homeostasis.Based on the above results, the second trial, using two 4 4 Latin square designs and applying the method of substituting a corn-starch and soybean meal diet, was designed to determine the P digestibility of Corn, Barley, Wheat, Wheat-bran, Paddy ,Rice hull, Pvapeseed meal, Cottonseed meal for growing- pigs and to analyze the correlations among the total phosphorus (TP), Phytate- phosphorus (PP) contents ,the phytase(E.C.3.1.3.26) activity and the contents of true digestible phosphorus in the feeds. The results of the true P digestibility in ileal and feces were as follows: Cottonseed meal: 30.21%±2.00%, 30.22%±4.61%; Rapeseed meal: 37.22%±1.78%, 39.30%±2.50%: Paddy: 48.37%±4.49%, 43.65%±3.88%; Rice hull: 23.69%± 1.68%, 23.42%±3.44%; Corn: 47.45%±2.19%, 49.15%±4.16%; Wheat: 47.65% ±2.34%, 49.74%±4.92%; Barley: 58.30%±5.17%, 56.68%±3.56%; Wheat-bran: 56.45%± 5.53%, 56.82%±1.59%.The results of the true digestible phosphorus in ileal and feces were as follows( g/kgDM): Cottonseed meal: 3.87, 3.87; Rapeseed meal: 4.17, 4.40; Paddy: 1.50, 1.35; Rice hull: 1.26, 1.26; Corn: 1.57, 1.62; Wheat: 3.34, 3.48; Barley: 2.80, 2.72; Wheat-bran: 7.00, 7.05.The results show that the total phosphorus content(X,g/kgDM), Phytate- phosphorus content(Y,g/kgDM),and the phytase activity (Z, FTU/kgDM) have a significant effect on the true digestible phosphorus content (Y,g/kgDM).The true digestible P estimation equation based on total P content,phytate P content and phytase activity in plant feeds is as follow:TDP=1.2151X-1.2513 Y+ 0.00383Z -0.3651...
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