Modelling digestible phosphorus content of salmonid fish feeds

Accurate estimation of digestible phosphorus (P) content of fish feeds is essential to formulating feeds that meet nutritional requirements of fish but minimize P waste output from fish culture operations. Phosphorus is a component of different chemical compounds in feeds. Differences in chemical characteristics of these compounds may result in different P digestion dynamics. A model to estimate digestible P content of salmonid fish feed based on levels of different P types was constructed by integrating data from 22 studies. Phosphorus types present in feed ingredients were classified into broad chemical categories: bone-P, phytate-P, organic P, Ca monobasic/Na/K Pi supplement, and Ca dibasic Pi supplement. The relationship between digestible P content of feeds and various P chemical compound contents was modelled through a multiple regression approach.Multiple regression analysis yielded the following model: digestible P = 0.68 bone-P + 0 phytate-P + 0.84 organic P + 0.89 Ca monobasic/Na/K Pi supplement + 0.64 Ca dibasic Pi supplement + 0.51 phytase/phytate − 0.02 (phytase/phytate)2 − 0.03 (bone-P)2 − 0.14 bone-P * Ca monobasic/Na/K Pi supplement (P < 0.0001, R2 = 0.96). The results suggest that the digestibility of different P types differ significantly and the digestibility of bone-P is not additive. A subsequent digestibility trial validated the model and suggested that the model accurately predicted digestible P content of diets formulated with a wide variety of ingredients commonly used in fish feeds.