| This study was to investigate effects of microbial phytase and 1alpha-hydroxychole- calciferol (1α-OH D3) on phosphorus metabolism and to determine the regulation mechanism of NaPi-IIb cotransporter gene expression by dietary phosphorus and 1α-OH D3.Experiment one, two trials, was conducted to determine effects of microbial phytase and non-phytate phosphorus (NPP) on growth performance, parameters of tibia and plasma, and nutrient utilization of broilers fed a corn-soybean meal diet and to evaluate the equivalency values of microbial phytase for inorganic phosphate (Pi). Trial 1, on the day of hatch, 480, Arbor Acres chicks were allotted to ten treatments with four replicates of twelve birds. Ten diets with the same PP content of 0.24% were formulated. Diet 1 (basal diet) to 4 contained NPP of 0.21, 0.29, 0.37, and 0.45%. Diet 5 to 10 was formulated by supplements of 250, 500, 1000, 2000, 4000 and 8000 U/kg microbial phytase to the basal diet, respectively. Trial 2, four hundred and eighty Arbor Acres broilers, 21-d-old, were allotted to ten treatments with four replicates of twelve birds. Ten diets with the same PP content of 0.22% were formulated. Diet 1 (basal diet, no Pi addition) to 4 contained NPP of 0.13, 0.20, 0.27, and 0.35%. Diet 5 to 10 was supplemented microbial phytase 125, 250, 500, 1000, 2000, and 4000 U/kg to the basal diet, respectively. Results showed that growth, tibia parameters, plasma Pi, and total P retention increased with phytase or dietary NPP. Microbial phytase increased but dietary NPP decreased PP digestibility. About 94.84% and 75.88% Pi was released for 1 to 21 day broilers fed diet with 8,000 U/kg phytase and for 21 to 42 day boilers with 4000 U phytase, respectively. Growth performance of 1 to 21 day broilers fed 8,000 U phytase or 21 to 42 day broilers fed 4000 U phytase was similar to those of birds fed the normal-NPP diets. The model to predict inorganic phosphate released from phytate phosphorus by microbial phytase for 1 to 21 day broilers was as follows: y = 0.0425×log2(1 + x/250) + 0.0283 (n=7, R2 = 0.9153, P = 0.0007), and that for 21 to 42 day broilers was y = 0.0284×log2(1 + x/125) + 0.0373 (n=7, R2 = 0.8233, P = 0.0048), where x is the phytase activity (U/kg) and y is the Pi equivalency value (%). These results indicate that high inclusion level of phytase could hydrolyze almost all phytate phosphorus and the model could be helpful for poultry diet formulation.Experiment two, two trials, was to investigate effects of 1α-OH D3 on growth performance, parameters of tibia and plasma, breast and thigh meat quality, and nutrient utilization of broilers. In trial 1, 96 Arbor Acres broilers, 1-d-old, were allotted to treatments of ten per cage with four cages per treatment. Diet one and two was supplemented 1α-OH D3 0 and 5μg/kg to the basal diet (NPP 0.21%). Trial 2, ninety six Arbor Acres broilers, 21-d-old, were allotted to treatments of twelve per cage with four cages per treatment. Diet one and two was supplemented 1α-OH D3 0 and 5μg/kg to the basal diet (NPP 0.13%). Results showed that growth, tibia parameters, plasma Pi concentration, tP and NPP utilization, and lightness and yellowness of breast and thigh meat were increased, while shear force, water holding capacity of thigh meat, and mortality decreased by 1α-OH D3 in 1 to 21 day broilers. Parameters of tibia and plasma in 21 to 42 day broilers were improved by 1α-OH D3, but not growth. These results indicate that 1α-OH D3 improves performance, tibia, and meat quality by increasing digestibility and retention of phosphorus for 1 to 21 day broilers. However, 1α-OH D3 has a lower capacity to improve growth of broilers in grower phase than starter phase.Experiment three was to determine effects of interaction between 1α-OH D3 and phytase on performance, plasma and tibia parameters, meat quality, and nutrient utilization of broilers. Two 2×2 factorial trials were designed to test 0 and 5μg/kg 1α-OH D3 in combination with 0 and 500 U/kg of phytase. In trial 1, 240 Arbor Acres broilers, 1-d-old, were allotted to treatments of ten per cage with four cages per treatment. The basal diet contained NPP 0.29%. A fifth treatment was also included as a positive control (NPP 0.45%). In trial 2, 21-d-old broilers were used and the NPP levels for the basal diet and positive control were 0.20% and 0.35%, respectively. Results showed that 1α-OH D3 and phytase supplemented together at the dietary NPP level of 0.29% in 1 to 21 day broilers or at NPP level of 0.20% in 21 to 42 day broilers increased tibia ash and P content, but did not improve growth performance. These results indicate that 1α-OH D3 and phytase had positive interaction in tibia parameters.Experiment four, three trials, was to study effects of dietary phosphorus and 1α-OH D3 on intestinal abundance of NaPi-IIb mRNA. Trial one was conducted using 11 day broilers fed low (NPP 0.13%) and normal (NPP 0.45%) phosphorus diets. Trial two 21 day broilers were fed low (NPP 0.21%) and high (NPP 0.65%) phosphorus diets. In trial three, diet one and two was supplemented 1α-OH D3 0 and 5μg/kg to the basal diet (NPP 0.21%). The NaPi-IIb mRNA abundance was determined by real-time RT-PCR. Results showed that the highest expression of NaPi-IIb gene was present in the duodenum, followed by the jejunum and ileum. Compared with low NPP diet, normal dietary NPP increased NaPi-IIb mRNA abundance in duodenum, jejunum, and ileam of 11 day broilers. However, higher dietary NPP decreased duodenal NaPi-IIb mRNA abundance in 21 day broilers. When 1α-OH D3 was supplemented in low NPP diet, duodenal, jejunal, and ileal NaPi-IIb mRNA abundance in 21 day broiler increased. These findings indicate that dietary phosphorus and 1α-OH D3 can regulate NaPi-IIb cotransporter gene transcription.The findings from this study suggest that high inclusion level of phytase could hydrolyze almost all phytate phosphorus. NaPi-IIb cotransporter gene expression was regulated by dietary phosphorus and 1α-OH D3. Phosphosrus absorption, retention, meat quality and growth were increased when 1α-OH D3 supplemented, and interaction between 1α-OH D3 and phytase improved tibia quality. |
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