Effect of phosphorus intake on faecal and non-faecal phosphorus excretion in rainbow trout (Oncorhynchus mykiss) and the consequences for comparative phosphorus availability studies

The effect of dietary phosphorus (P) concentration on faecal and non-faecal P excretion of rainbow trout was studied with P concentrations ranging from 1.0 to 6.2 g/kg dry matter, the upper level calculated to be slightly above the P requirement. Eight semi-purified diets were prepared with varying proportions of dibasic sodium phosphate and an inorganic binder to achieve intended P concentrations. Diets were fed at a restricted rate to triplicate groups of trout initially weighing 51 g/fish for 43 days. P retention was determined by comparative whole body analysis. Faecal material was collected from a sedimentation unit attached to each tank and digestibility was determined using acid-insoluble ash as a marker. Non-faecal P excretion was calculated as the difference between intake of apparently absorbed P and retained P. Blood samples were taken at the end of the experiment. Responses of trout to increasing P intake were described by non-linear functions. Digestibility of lipid and carbohydrates and, consequently, energy were lower in fish fed the basal P level than in fish fed any supplementary level of P. The partial efficiency of digestible energy for retention, however, was not affected by the dietary P level. With increasing P intake, faecal P excretion increased non-linearly. Basal non-faecal P excretion was estimated at 3.7 mg/kg BW per day and was unaffected by P intake until the estimated P requirement was met. Above a P concentration of about 5 g/kg DM, non-faecal P excretion increased. Serum P-i concentration remained unchanged once this dietary P level was achieved. It is concluded that trout adapt P absorption rate to increasing P intake already before the P requirement is met, whereas non-faecal P excretion happens when the supply of available P is sufficient for P retention. Marginal P absorption, determined as Delta P apparently absorbed/Delta P intake, showed a maximum of 0.73 at an intake level, which was approximately 33% of the estimated P requirement and dropped towards 0.40. Similarly but more pronounced, marginal efficiency of utilisation, determined as Delta P retained/Delta P intake, showed a maximum of 0.88 and tended towards 0.10 at high intake. Apparent absorption of P proved a suitable measure for comparative evaluation of P availability. However, for testing P availability, the concentration of available P in the diet should not exceed 2.5 g/kg DM. As a consequence, the difference method is necessary for routine studies on P availability in trout. This method implies that individual P sources are mixed with a low-P basal diet and that the availability is determined for both the basal diet and the mix, allowing for the calculation of availability for the test source as the difference. (C) 2000 Elsevier Science B.V. All rights reserved.