MAGNESIUM HYDROGEN PHOSPHATE (MHP, MgHPO4) RECOVERED FROM SWINE MANURE AS A NEW PHOSPHORUS ADDITIVE FOR FRESHWATER FISH
Phosphorus (P) sources like monocalcium phosphate (MCP) and dicalcium phosphate (DCP) are being supplemented to the diet to meet the requirement of P for maximum growth. The present study summarizes the supplemental effect of various P sources and magnesium hydrogen phosphate (MgHPO4, MHP) as a new P source on growth, utilization of P and physiological responses in carp (Cyprinus carpio) and far eastern catfish (Silurus asotus). Either MCP, DCP, TCP (tricalcium phosphate) or MHP was added to the diets at the level of 2%, while control diet did not contain P source, which was replaced by cellulose. The major ingredients were constituted with fish meal (25%), soybean meal (40%), wheat flour (27%), fish oil (2%), and soy oil (2%) as a basal diet containing 42.5% protein and 6.5% lipid. The MHP was recovered from swine manure using a pilot scale reactor with the effective volume of 0.4 m3 and manufactured in Kangwon National University.
All experimental diets were manufactured to sinking pellets using a twin-extruder. Available P in diets was determined through digestibility trial. Following a 24 h fasting, respective 5 groups (three replicates/group) of 450 fish of a mean body weight of 6.5 g (carp) and 11.3 g (catfish) were randomly allotted to each of 15 tanks (0.4 × 0.6 × 0.36 cm, effective volume of 66 L). The feeding experiment lasted for 9 (carp) and 8 (catfish) weeks during which each diet was hand-fed to apparent satiety twice a day (08:30 and 16:30) during 6 days per week. A recirculation freshwater system where dissolved oxygen was maintained at 5.5~6.4 mg O2 L-1 was employed. Water temperature maintained under natural condition.
Carp fed the MHP showed the highest weight gain (WG) of 278%, which was not significantly different (p > 0.05) from that (270%) of fish fed the MCP, while fish fed the control and TCP showed lowest WG among the treatment groups (p < 0.05). Feed conversion ratio (FCR) ranged from 0.99 (MHP) to 1.29 (control). The highest protein efficiency ratio (PER) and specific growth rate (SGR) were found in fish fed the MCP and MHP, while fish fed the control and TCP showed the lowest. Relative weight gain (RWG) was significantly higher in DCP, MCP, and MHP among treatments. Catfish fed the MCP showed the highest WG (484%), which was not significantly different (P>0.05) from those of fish fed the DCP and MHP. The same trends were also found in FCR, PER and SGR. P availability of MHP was 97.8 and 90.9% for carp and catfish, respectively. Inorganic P in plasma maintained equally at 6.7 mg/dL in carp fed MCP, DCP and MHP, while that was 15.4, 13.2 and 15.8 mg/dL in catfish fed the same diets, respectively.
From the studies, the potential use of MHP recovered from swine manure was proven sufficiently to replace MCP as an alternative P source with respect to WG and FCR as well as P availability.