Effect of feeding broilers diets differing in susceptible phytate content

Measurements of total phytate phosphorus content of diets may be deceptive as they do not indicate substrate availability for phytase; it may be that measurements of phytate susceptible to phytase effects are a more accurate measure of phosphorus (P) availability to the bird. To verify this hypothesis, an experiment was conducted to compare diets formulated to contain either high or low susceptible phytate, supplemented with either 0 or 500 FTU/kg phytase. Susceptible phytate was determined by exposing the feed samples to conditions that mimicked the average pH of the proximal gastrointestinal tract (pH 4.5) and the optimum temperature for phytase activity (37 °C) and then measuring phytate dissolved. Ross 308 birds (n = 240) were fed one of 4 dietary treatments in a 2 × 2 factorial design; 2 diets with high (8.54 g/kg, 57.90% of total phytate) or low (5.77 g/kg, 46.33% of total phytate) susceptible phytate, containing 0 or 500 FTU/kg phytase. Diets were fed to broilers (12 replicate pens of 5 birds per pen) from d 0 to 28 post hatch. Birds fed diets high in susceptible phytate had greater phytate hydrolysis in the gizzard (P < 0.001), jejunum (P < 0.001) and ileum (P < 0.001) and resulting greater body weight gain (BWG) (P = 0.015) and lower FCR (P = 0.003) than birds fed the low susceptible phytate diets, irrespective of phytase presence. Birds fed the high susceptible diets also had greater P solubility in the gizzard and Ca and P solubility in the jejunum and ileum (P < 0.05) and resulting greater tibia and femur Ca and P (P < 0.05) content than those fed the low susceptible diets. All the susceptible phytate was fully degraded in the tract in the absence of added phytase, suggesting the assay used in this study was able to successfully estimate the amount of total dietary phytate that was susceptible to the effects of phytase when used at standard levels. No interactions were observed between susceptible phytate and phytase on phytate hydrolysis. Hydrolysis of phytate was greater (P < 0.05) in the gizzard of birds fed the diets supplemented with phytase, regardless of the concentration of susceptible phytate in the diet. Phytase supplementation resulted in improved BWG (P < 0.001) and FCR (P = 0.001), increased P solubility (P < 0.001) in the gizzard, Ca and P solubility (P < 0.001) in the jejunum and ileum and Ca and P concentration (P < 0.001) and strength (P < 0.001) in the tibia and femur. Pepsin activity was higher in birds fed the diets supplemented with phytase (P < 0.001) and was greater (P = 0.031) in birds fed the high susceptible phytate diets compared with the low susceptible phytate diets. Findings from this study suggest that there may be a measure more meaningful to animal nutritionists than measurements of total phytate.