The validity of using inert markers in digestibility studies must not metabolized, absorbed or disrupt in the digestive process or gut microbiota ; and should have the same kinetics through the gut, as the rest of nutrients of diet. The identification of P sources and their bioavailability in Litopenaeus vannamei diets assumes critical importance due to consequences of excessive phosphorus loads , but to assess the bioavailability of N or P, first it is necessary to establish the ideal marker for each species. Previous studies in fish (Fernandez et al. , 1999) reported that Ca or P differed according the chromic oxide dietary concentrations. Furthermore, dietary chromic oxide has been reported to pass through the gastro-intestinal tract at a different rate than nutrients. Thus in present study four inert markers were compared to select the optimal to evaluate the digestibility in shrimp feeds and then to validate to evaluate the digestibility with different P sources dicalcium (DCP), monocalcium (MCP), monoammonium (MAP), and monopotassium phosphates (MKP) in practical diets.
With this aim four trials were carried out: First and second trials were developed to establish the best inert marker (IM) for digestibility evaluation using four inert markers in a standard diet (Cr, Y, La and Ce) analyzing the diet digestibility and the inert marker retention in whole body, cephalothorax, and muscles (with and without intestine). Some accumulation of Cr was detected in the shrimp whole body, particularly in the cephalothorax. Furthermore, following a square Latin design it can observed an accumulation of Cr in the different tanks. Clear differences were observed in the accumulation of the inert marker in the shrimps with and without intestine. The third and fourth trial were carried out to assess the P use of three P sources: monoammonium phosphate (MAP, NH4H2PO4), monosodium/monocalcium phosphate (SCP-2%, NaH2PO4/Ca(H2PO4)2·H2O in proportion 12/88) and monosodium phosphate (MSP, NaH2PO4) usi ng a Control diet without P supplement.
Regarding the IM results, Y resulted the best option to establish the digestibility due to the high solubility after acid digestion and the lowest Y retention in the shrimp whole body when was compared with the other markers. In the digestibility trial (Table 1 ) Control diet obtained the lowest P digestibility, because raw materials had lower P digestibility than the P from inorganic phosphates. Likewise, diet SCP- 2% presented the best Ca digestibility values. The excretion of N or P did not present significant relevant differences, but w hen P and N excretion and retention was evaluated jointly with P and N digestibility the SCP-2% diet generated the lowest N waste in relation to the N intake . The MAP diet presented the highest amount of N residues due to N excretion , that was greater than in the rest of the diets, followed by the control diet.
Fernandez, F., Miquel, A.G., Martinez, R., Serra, E., Guinea, J.,Narbaiza , F.J., Caseras , A. & Baanante , I.V. (1999) Dietarychromic oxide does not affect the utilization of organic compo-nents but can alter the utilization of mineral salts in gilthead seabreamSparus aurata.J. Nutr.,129,1053–1059