The effect of poultry protein concentrate and phosphorus supplementation on growth, digestibility and nutrient retention efficiency in barramundi Lates calcarifer

Two experiments were conducted on juvenile barramundi testing the replacement of regular poultry by-product meal (PBM) with a premium grade poultry protein concentrate (PPC). In the first experiment, a series of five iso-nitrogenous and iso-calorific dietary treatments composed of incrementally increasing levels of PPC from 0% to 20%, substituting PBM were formulated (PPC0, PPC5, PPC10, PPC15, PPC20). The diets were restrictively pair-fed to 63g barramundi for 56 days. In the second experiment, a series of four iso-nitrogenous, iso-calorific and iso-phosphoric dietary treatments composed of incrementally increasing levels of PPC from 0% to 20% substituting PBM were formulated (PPC0+P, PPC6.7+P, PPC13.3+P, PPC20+P). An additional dietary treatment was formulated without any phosphorous supplementation (PPC20-P). The diets were restrictively pair-fed to 69g barramundi and the PPC20+P and PPC20-P were also tested when fed to satiation for 42 days. Experiment 1 demonstrated a clear linear trend towards poorer growth performance and feed utilisation when increasing the level of PPC in the diet. Significant differences were noted for final weight, weight gain, SGR and FCR (P<0.05) in the fish fed diets PPC15 and PPC20 compared to the PPC0. Protein retention efficiency and the apparent biological value of protein and energy was lower (P<0.05) in the PPC20 compared to the PPC0 fed fish. The apparent digestibility of crude protein, gross energy, phosphorous and several amino acids all increased with increasing level of PPC in the diet. In experiment 2, using iso-phosphoric diets, the reverse trend was demonstrated whereby increasing PPC improved the performance of barramundi. Significant differences were noted for the final weight, weight gain, SGR and FCR (P<0.05). When comparing the PPC20+P and the PPC20-P fed fish, there was a clear negative impact of not supplementing phosphorus, which was further compounded in fish fed to satiation (interaction, P<0.05). There were no significant differences in nutrient retention with the exception of calcium which increased with increasing PPC, and energy retention being higher in fish fed PCC20+P than PCC20-P treatments. The results of these two experiments demonstrate that the effective replacement of regular PBM with high quality PPC is achievable and when the diets are supplemented with phosphorus to an inclusion >1.5%, promotes faster growth, for fish fed restrictedly and to satiation. The results indicate a pressing need to redefine the phosphorus requirement of barramundi juveniles, notably to ensure the scope for growth is not affected when testing the nutritional value of novel protein sources.