Comparative effects of two phytases versus increasing the inorganic phosphorus content of the diet, on nutrient and amino acid digestibility in boilers

Comparative effects of graded doses of two phytases on growth performance, nutrient and amino acid (AA) digestibility were investigated in corn-soybean meal-based broiler diets. A negative control diet (NC) with low phosphorus (P) content (1.8, 4.4 and 2.5 g/kg retainable, total and phytate P respectively) was supplemented with four doses of a Buttiauxella sp phytase (analyzed activity of 303 to 1046 FTU/kg) or an E. coli phytase (analyzed activity of 442 to 1811 FTU/kg), and tested against three positive control diets comprising the NC + 0.6, 1.2 or 1.8 g P from monocalcium phosphate (MCP)/kg feed and increased Ca content (+0.8 g/kg). A total of 1152 male Ross 308 broilers at 5 days of age were assigned to 72 cages with 16 birds/cage and 6 cages/treatment were given free access to pelleted diets until 21 days of age. Feed intake (FI), body weight and mortality were recorded and used to calculate body weight gain (BWG) and feed conversion ratio (FCR) during 5–20 days of age. Excreta was collected on day 18–20 to determine total tract retention of nutrients. Tibias (from 4 birds pooled per cage) and ileal digesta (from all birds and pooled per cage) were sampled at 21 days of age to determine tibia ash, ileal digestibility of nutrients and AA. Compared to NC, increasing phytase dose and MCP levels produced a stepwise increase in FI, BWG and reduction in FCR. Increasing phytase dose produced curvilinear increases in BWG, ileal P digestibility, retention and tibia ash (P < 0.01) but effects were greater for Buttiauxella vs. E. coli phytase. In contrast, only Buttiauxella phytase increased the ileal digestibility of protein and total AA (+3.7% on average vs. NC at 1046 FTU/kg, P < 0.05). Linear increases in ileal digestibility of all individual AA were seen with increasing dose of Buttiauxella (P < 0.05 in all cases) and were greatest for cysteine (+ 7.9% vs. NC at 1046 FTU/kg), whereas only 2 individual AA showed linear increases in digestibility in response to increasing dose of the E. coli phytase (cysteine and proline, P < 0.05). Ileal digestibility of Na (%) was markedly increased (less negative) by phytase, especially for the Buttiauxella phytase which produced a 96% increase in ileal Na digestibility at 1046 FTU/kg vs. NC. Across treatments, ileal digestibility of AA and Na were positively correlated (P < 0.001). These results provide insight into the ‘extra-phosphoric’ effects in broilers fed corn-soybean meal-based diets and suggest that the Buttiauxella and E. coli phytase had markedly different degrees of ‘extra-phosphoric’ effects on AA digestibility per standard FTU. The data showed that the AA digestibility response to increasing phytase dose do not follow the same response curve as for digestible P, and that the response curves are specific for different phytases.