Effect of phosphorus intake on faecal and non-faecal phosphorus excretion in rainbow trout (Oncorhynchus mykiss) and the consequences for comparativephosphorus 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 fro m 1.0 to 6.2 g/kg dry matter, the upper level calculated to be slightly abo ve 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 triplicat e groups of trout initially weighing 51 g/fish for 43 days. P retention was determined by comparative whole body analysis. Faecal material was collect ed from a sedimentation unit attached to each tank and digestibility was de termined using acid-insoluble ash as a marker. Non-faecal P excretion was c alculated as the difference between intake of apparently absorbed P and ret ained P. Blood samples were taken at the end of the experiment. Responses o f trout to increasing P intake were described by non-linear functions. Dige stibility of lipid and carbohydrates and, consequently, energy were lower i n 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 no t 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 unaffe cted by P intake until the estimated P requirement was met. Above a P conce ntration of about 5 g/kg DM, non-faecal P excretion increased. Serum P-i co ncentration remained unchanged once this dietary P level was achieved. It i s concluded that trout adapt P absorption rate to increasing P intake alrea dy 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 ab sorption, 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 es timated P requirement and dropped towards 0.40. Similarly but more pronounc ed, marginal efficiency of utilisation, determined as Delta P retained/Delt a 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 evalua tion of P availability. However, for testing P availability, the concentrat ion of available P in the diet should not exceed 2.5 g/kg DM. As a conseque nce, the difference method is necessary for routine studies on P availabili ty 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 basa l diet and the mix, allowing for the calculation of availability for the te st source as the difference. (C) 2000 Elsevier Science B.V. All rights rese rved.