| Four experiments were conducted to sdudy the effects of high doses of microbial phytase supplementation in diets on growth performance, tibia development, serum biochemical indices, immune organic indices and nutrient utilization of energy, protein, Calcium, phosphorous, amino acid in broiler chickens. The mechanism of phytase improving the nutrient utilization and the quality of nonconventional protein meal was determined with Two base diets of corn-soybean meal diets and corn- cottonseed meal.Experiment 1 was conducted to determine the effect of high doses of microbial phytase supplementation in corn-soybean meal diets on growth performance, tibia development and serum biochemical indices. Experiment 2 was conducted to determine the effects of high doses of microbial phytase supplementation in corn-soybean meal diets on nutrient utilization of AME, protein, Calcium, phosphorous, amino acid and immune organic indices in broiler chickens. The same diets and treatments were carried out in Experiment 1 and experiment 2. A total of 576 one-day old male Arbor Acres broilers were assigned to 6 treatments, consisting of 8 pens of 12 birds each, and were fed two periods experimental diets from 1 to 21d and 22 to 42 d.The diets included a positive control (0.69%tP, 0.45% nonphytate P for 1to 21d ; 0.59% tP, 0.35% nonphytate P for 22 to 42 d), a negative control (0.60%tp, 0.35% nonphytate P for 1-21d; 0.49%tP, 0.25% nonphytate P for 22-42d), and four experimental group (with 500 FTU/kg, 2000 FTU/kg, 8000 FTU/kg, 32000 FTU/kg phytase in negative diets).Experiment 3 was conducted to determine the effect of high doses of microbial phytase supplementation in corn-cottonseed meal diets on growth performance, tibia development and serum biochemical indices. Experiment 4 was conducted to determine the effect of high doses of microbial phytase supplementation in corn-cottonseed meal diets on nutrient utilization of energy, protein, Calcium, phosphorous, amino acid and immune organic indices in broiler chickens. The same diet and treatmen were carried out in Experiment 3 and experiment 4. A total of 624 one-day old male Arbor Acres broilers were assigned to 8 dietary treatments, consisting of 6 pens of 13 birds each, and were fed two periods experimental diets from 1to 21d and 22 to 42d. The diets included a positive control (0.73% tP,0.45% nonphytate P for 1-21 d ; 0.62% tP, 0.35% nonphytate P for 22-42 d), a negative control (0.63% tp, 0.35% nonphytate P for 1-21d; 0.52% tP, 0.25% nonphytate P for 22-42d), and six experimental group (with 500 FTU/kg,2000 FTU/kg,8000 FTU/kg, 16000 FTU/kg, 24000 FTU/kg ,32000 FTU/kg phytase in negative diets).The results showed that:(1) High doses of phytase significantly improved the ADG, FBW, FCR (P <0.05), while the feed intake had no significantly differences (P>0.05 ). The addition of exogenous phytase above 500 FTU/kg improved the weight gain as the same effect of positive control. 8000FTU/kg phytase improved the performance higher than other groups compared with the negative control diet. Compared 500 FTU/kg and 8000FTU/kg with control groups, the results as follows: With corn-soybean meal diet, ADG respectively improved 0% at 21d and 0% at 42d compared with positive control, and improved 2% at 21d and 3% at 42d compared with negative control at the dosage of 500FTU/kg; ADG respectively improved 2% at 21d and 20% at 42d compared with positive control, and improved 4% at 21d and23% at 42d compared with negative control at the dosage of 8000FTU/kg.While with Corn-cottonseed meal diet, ADG respectively improved 1% at 21d and 2% at 42d compared with positive control, and improved 4% at 21d and 9% at 42d compared with negative control at the dosage of 500FTU/kg; ADG respectively improved 10% at 21d and 17% at 42d compared with positive control, and improved 13% at 21d and 25% at 42d compared with negative control at the dosage of 8000FTU/kg.(2) The level of serum Ca, BUN and AKP significantly be decreased, serum P significantly increased with high doses of phytase. BUN reached the lowest level at the dosage of 8000FTU/kg.(3) High doses of phytase significantly promoted the tibia development. With the increasing of phytase level, Tibia strength, weight, length and ash significantly increased, the Ca: P ratios tended to be narrow. There were significiant positive relationship between the phytase levels and tibia strength and ash.(4) High doses of microbial phytase significantly improved the utilization of AME and protien(P <0.05), especially at 42d.At the dosage of 32000FTU/kg in Corn-cottonseed meal diet, the utilization of protein respectively improved 11% at 21d and 27% at 42d, and the utilization of AME respectively improved 2% at 21d and 11% at 42d compared with negative control.At the dosage of 32000FTU/kg in Corn-soybean meal diet, the utilization of protein respectively improved 1% at 21d and 7% at 42d, and the utilization of AME respectively improved 2% at 21d and 2% at 42d compared with negative control.(5) High doses of dietary phytase increased the utilization of essential amino acid, nonessential amino acid and total amino acid, especially at 21d. High doses of microbial phytase significantly enhanced the utilization of limiting amino acid, included Met, Lys and Cys, in Corn-cottenseed meal diet.(6) High doses of microbial phytase significantly affected the utilization of Ca and P (P<0.05).The utilization of Ca were increased, and the utilization of P were decreased with the increasing of phytase level between 1d and 21d. The utilization of the Ca and P were increased at the 500 FTU/kg level, however, the utilization of the Ca and P had reduced above the addition dosage of 2000FTU/kg between 22d and 42 d., this result indicated that the NPP level of in experiments diet were excessive.(7) High doses of microbial phytase significantly improved immune organic indices (P<0.05), especially for the thymus index. And this effect was higher in corn-cottonseed meal diet than corn-soybean meal diet.This study suggested that high microbial doses dietary phytase improved the growth performance and the utilization of energy, protein, calcium, phosphorous, amino acid in broiler chickens. And current recommendation of phytase 500 FTU/kg and diet nutrients with the poultry industry may need to be reevaluated. When 8000FTU/kg microbial phytase of the recommended dosage added in diets, the level of NPP would be advisably reduced by NRC.
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