Availability of phosphorus from feedstuffs fed to rainbow trout, Oncorhynchus mykiss

In this study, we determined both apparent and true availability of phosphorus (P) from feedstuffs fed to rainbow trout. Feedstuffs evaluated were anchovy meal, sardine meal, menhaden meal, herring meal, spray dried blood meal, flame dried blood meal, poultry by-product meal, canola meal, solvent-extracted soybean meal, full-fat soybeans, heat processed soybean meal, peanut meal, corn gluten meal and cottonseed meal. Barium carbonate was used as the indicator and fecal samples were collected by dissection. One of the diets containing canola meal and the diet containing cottonseed meal were poorly accepted and precluded calculation of a realistic P availability value. Both diets containing blood meal contained insufficient P concentrations for distinguishing between dietary and endogenous P. Apparent phosphorus availability (APA) values ranged from 19.5 to 50.5% for fish meals and as high as 30.7% for the plant protein sources. Endogenous P excretion averaged 225 mg kg(-1) body weight and was used to correct APA values to true phosphorus availability (TPA). True P availability ranged from 21.5 to 55.4% for fish meals and 9.7 to 48.4% for the plant feedstuffs. Supplementation of plant protein sources with phytase significantly increased APA values to 46.2 to 75.6%. Endogenous secretions of P decreased over time, but clinical signs of P inadequacy did not occur until between 6 and 10 days of feeding a P-deficient diet.