| The aim of this study is to investigate the effective utilization of phosphorus in common feedstuff was used in duck diets. Total phosphorus, phytate phosphorus content and true availability of phosphorus in common feedstuffs were determined and the regression models of available phosphorus were established. According to true available phosphorus of feedstuffs, the diets were designed at different levels of available phosphorus and supplemental phytase based on corn-soybean-rapeseed meal diet. The equivalent replacement relationship between phytase and available phosphorus was explored according to duck growth performance, serum parameters, tibia properties, apparent utilization of calcium and phosphorus. The phosphorus equivalency value of phytase was determined according to regression analysis, which could provide theoretical guidance for phytase utilization in duck production.The objective of trial 1 was to evaluate the correlation among TP, PP and true availability of phosphorus in adult Cherry Valley Ducks. The contents of total phosphorus and phytate phosphorus were determined by chemical composition analysis, and true availability of phosphorus was determined by fasting-force feeding. True available phosphorus content was calculated, and the regression models of available phosphorus in all feedstuff were established. The results of trial 1 were as follows:(1) The ratio of AP to TP in corn, wheat by-products, soybean meal, rapeseed meal and DDGS samples were 80.46%,73.48%,44.16%,62.11% and 32.47%, respectively. There was a significant correlation between TP and PP content in all feedstuffs (r=0.81, P < 0.05). Remarkably, the correlative relationship between TP and PP in energy feed (r=0.98) was more significant than that in protein feed (r=0.82).(2) True availability of phosphorus in corn, wheat processing by-products, soybean meal, rapeseed meal and DDGS samples for ducks were 80.6%,39.01%, 53.03%,38.69%, 45:55%, respectively.(3) The content of true available phosphorus was predicted according to content of total phosphorus and phytate phosphorus in feedstuff, significant prediction models and high correlations were observed from corn, wheat by-products and soybean samples, and the regression equations among true available phosphorus (Y,%) and total phosphorus (X1,%) and phytate phosphorus (X2,%) were Y=0.008+1.226X1-0.595X2 (R2=0.98, RSD=0.004, P<0.05), Y=0.047-0.148X1+0.640X2 (R2=0.77, RSD=0.060, P<0.05) and Y=-0.141+2.779X1-4.610 X2(R2=0.85, RSD=0.084, P<0.05), respectively. These results suggested that TP and PP were the appropriate indicators for the prediction of the AP value of corn, wheat by-products and soybean meal for ducks.The objective of trial 2 was to explore phosphorus equivalency of phytase with various evaluation indicators at different growth stages of Cherry Valley duck based on corn-soybean-rapeseed meal diet. The trial diets were designed into four available phosphorus levels and four supplement phytase levels. Available phosphorus levels were 0.25%,0.32%,0.39% and 0.46% in 0-14day,0.20%,0.27%,0.34% and 0.41% in 15-35day, respectively. Phytase was supplemented in low phosphorus basal diet (treatment I) with different levels, phytase supplement levels were 300U/kg,600U/kg, 900U/kg and 1200U/kg, respectively. Other nutritional compositions were adequate for ducks, and the ratio of calcium and phosphorus was adjusted to 1.3:1. Three hundred and twenty Cherry Valley Ducks were randomly assigned to eight treatments (each treatment consisted of five replicates of eight ducks according to initial weight). The whole trial stage lasted 35 days. Metabolic trials carried out from 14d to 19d, and 35d to 40d. One duck from each replicate were slaughtered at 14d and 35d. The results revealed that the choice of evaluation indicators greatly affected the results of phytase phosphorus equivalency. The results of trial 2 were as follows:(1) When feed intake was used as an evaluation indicator,500U/kg or 1000U/kg of phytase supplemented in the diet was equivalent to 0.03% or 0.07% available phosphorus in starting period and 0.02% or 0.14% available phosphorus in growing period of ducks. When body weight gain was used as an evaluation indicator,500U/kg or 1000U/kg of supplemented phytase in the diet was equivalent to 0.04% or 0.09% available phosphorus in starting period and 0.02% or 0.04% available phosphorus in growing period of ducks.(2) 500U/kg or 1000U/kg of phytase supplemented in the diet was equivalent to 0.02% or 0.06% available phosphorus in starting period and 0.02% or 0.04% available phosphorus in growing period of ducks, when tibia ash content was used as an evaluation indicator.500U/kg or 1000U/kg of phytase supplemented in the diet was equivalent to 0.01% or 0.02% available phosphorus in starting period and 0.02% or 0.05% available phosphorus in growing period of ducks, when tibia calcium content was used as an evaluation indicator.500U/kg or 1000U/kg of phytase supplemented in the diet was equivalent to 0.02% or 0.07% available phosphorus in starting period and 0.01% or 0.02% available phosphorus in growing period of ducks, when tibia phosphorus content was used as an evaluation indicator.(3) When apparent utilization of calcium was used as an evaluation indicator, 500U/kg or 1000U/kg of phytase supplemented in the diet was equivalent to 0.08% or 0.09% available phosphorus in starting period. When apparent utilization of phosphorus was used as an evaluation indicator,500U/kg or 1000U/kg of phytase supplemented in the diet was equivalent to 0.07% or 0.09% available phosphorus in starting period.(4) When serum calcium content was used as an evaluation indicator,500U/kg or 1000U/kg of phytase supplemented in diet was equivalent to 0.04% or 0.12% available phosphorus in growing period. When serum phosphorus content was used as an evaluation indicator,500U/kg or 1000U/kg of phytase supplemented in diet was equivalent to 0.03% or 0.01% available phosphorus was in growing period. The results indicated that phosphorus equivalency was not always increasd along with the supplemented levels of phytase.From the results all above, the conclusions are as follows:(1) True available phosphorus content could be predicted through total phosphorus and phytate phosphorus and true phosphorus availability in feeds. The prediction models were significant and the multiple correlation coefficient were high in corn, soybean meal, and wheat by-products.(2) Phosphorus equivalency of phytase was different when response indicator was different, and the phosphorus equivalency of phytase was also different when response indicator was same in different growth stage. The growth performance and tibia better as evaluation indicators were better than other indicators according to the coefficient of determination and P-value of regression equation.(3) By supplemented with phytase more than 900U/kg in low phosphorus diet, could avoid leg disease problems for phosphorus deficiency, when available phosphorus levels were 0.39%(0-14d) and 0.34%(15-35d), could meet the phosphorus requirements for ducks, growth performance had been reached normal levels, and apparent phosphorus availability was high, phosphorus emissions was reduced, to make effective use of phosphorus best.
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