Wheat and barley increase phytate degradation and phosphorus digestibility in corn fed to pigs

Two experiments evaluated the additivity of phytate content and coefficient of total tract apparent digestibility (CTTAD) of phosphorus (P) in wheat-barley (WB) and corn (C) when soaked (exp. 1) or fed dry to pigs (exp. 2) in combination with or without microbial phytase (1250 FTU/kg as-fed). In exp. 1, WB (1:1, w/w) and C were soaked separately or together (W:B:C = 1:1:2, w/ w/w) without (WBC) or with microbial phytase (WBC + phy). Phytate content was affected by the interaction between diet and soaking time (P < 0.0001) as phytate decreased with time. The decrease was greatest for WBC + phy followed by WBC, WB and C resulting in a half-life of phytate (K) of 8.7, 15.8, 24.4 and 303.4 h, respectively. Soaking of WB and C together showed non-additive effects as the estimated (assuming additivity) phytate content at 2, 4, 8 and 24 h of soaking was 0.17, 0.34, 0.42 and 0.5 g/kg DM higher (P < 0.001) than the experimentally determined phytate content in WBC. Actually, phytate degradation in C was estimated to be 0.15, 0.30, 0.42 and 0.49 higher at 2, 4, 8 and 24 h, respectively, when soaked together with WB than when soaked alone. Accordingly, the estimated (assuming additivity) K-value for WBC was 148 h longer (P = 0.08) than the K-value derived from the experimentally determined phytate degradation in WBC. Exp. 2 comprised 24 pigs of 47 +/- 2 kg from 6 litters. Pigs were housed in metabolism crates and fed one of four diets (WB, C, WBC or WBC + phy) for 12 days; 5 days adaptation and 7 days total collection of feces and urine. The CTTAD of P was greatest for WBC + phy followed by WB, WBC and C (P < 0.0001). Feeding WB and C in combination tended to show non-additive effects as the estimated (assuming additivity) CTTAD of P in WBC was 7.7 percentage units lower (P = 0.09) than the experimentally determined CTTAD of P in WBC. In fact, CTTAD of P in C was estimated to be 14.6 percentage units higher when fed together with WB than alone. In conclusion, phytases present in W and B are non-specific and catalyze the degradation of phytate in C when soaked or fed dry together (ratio 1:1:2). Thus, when phytate rich cereals very different in plant phytase activity, are mixed, additivity cannot be assumed.