Reducing phosphorus (P) levels in aqua feed without impacting growth, feed efficiency or health is key to the development of sustainable aquaculture. This is currently achieved using low-ash fishmeal and highly available P supplementation. However, phytase enzymes can efficiently release the P which is locked up as phytate in the feed raw materials. Phytase enzymes are found in high vegetable inclusion trout feeds and can help significantly reduce eutrophication.
The objective of the present study was to measure the efficiency of a 6-phytase (OptiPhos®) using three different concentrations: 250, 500 and 750 OTU/kg in feed for a growth performance trial in rainbow trout (Oncorhynchus mykiss). Feed was formulated with 48.4% crude protein, 22.3% crude fat and 22.3 MJ/kg gross energy. The enzymes were applied post-pelleting using a vacuum coater. The fish were separated into five treatments:
- Positive control (PC) with 1.14% P (1.7% mono calcium phosphate (MCP) in the feed formula); 0.48% phytate-P
- Negative control (NC) with 0.75% P, without any MCP addition
- NC diet plus 250 OTU/kg feed
- NC diet plus 500 OTU/kg feed
- NC diet plus 750 OTU/kg feed
The following growth performance indicators were recorded during the trial:
- Body weight (BW)
- Specific growth rate (SGR)
- Feed conversion ratio (FCR)
- Feed intake (FI)
An exogenous enzyme (OptiPhos®) added at 500 OTU/kg feed and 750 OTU/kg can significantly reduce the usage of inorganic P in trout feeds. It can be used as a tool to minimise the excess of P discharged into the environment or allow for higher biomass in compliance with the current local P discharge regulations.
Reducing the use of finite marine-harvested resources is a sustainability challenge threatening the future growth of the aquaculture industry. Plant-based ingredients and by-products are promising sources of protein and energy for aquaculture feeds. However, high dietary inclusion levels of plant proteins generally depress growth and feed efficiency. The poor growth performance commonly found in fish fed plant-protein-rich diets is generally related to the lower biological value (essential amino acid imbalance, impaired P availability, presence of anti-nutritional factors, higher carbohydrate fraction) of the plant-protein sources. The use of feed enzymes demonstrates an opportunity to enhance production economics resulting from the upgrade of the nutritional value of vegetable ingredients. In addition, enzyme inclusion offers a greater flexibility of ingredient use by feed formulators and more importantly, a reduced environmental impact minimising the excess of P discharged into the environment and a higher fish biomass. The present trial assessed the efficacy of graded supplementation doses (0, 250, 500, 750 OTU/kg feed) of phytase (OptiPhos® 8000 L, Huvepharma®) on the growth performance of rainbow trout.
Materials and methods
The product under evaluation was a phytase enzyme (OptiPhos® 8000 L) in a liquid form produced by Huvepharma®.
The trial comprised five experimental diets (Tables 1 and 2).
Pellets, with a size of 3.0 mm were manufactured by extrusion in a pilot-scale twin-screw extruder (CLEXTRAL BC45, France) with a screw diameter of 55.5 mm and a temperature ranging from 111 - 114oC. All batches of extruded feed were dried in a vibrating fluid bed dryer (model DR100, TGC Extrusion, France). The pellets were then allowed to cool at room temperature and, subsequently, the test enzyme at the various doses plus oil were applied by coating under vacuum in a DINNISEN Pegasus vacuum mixer (PG-10VCLAB).
Regarding the post-extrusion coating procedure of enzyme and oil: the target amount of the test enzyme (OptiPhos® 8000 L) was diluted in 2.5% demineralised water, emulsified with the oil on a high-shear mixer (Silverson L5T, United Kingdom) and sprayed onto the pellets under vacuum (760 mbar) for 3 minutes. The PC and NC diets (without enzyme supplementation) were also coated using the same procedure. 1.7% MCP was added to the PC diet.
Quadruplicate groups of 35 rainbow trout, with a mean initial body weight (IBW) of 11.7 ± 0.5 g were fed one of the five experimental diets for 90 days. Fish were grown in fiberglass tanks (volume: 300 L) supplied with flow-through freshwater, at a temperature of 13.9 ± 0.2oC and dissolved oxygen levels were maintained above 7.6 mg/L. Fish were hand fed 3 times per day (09:00, 14:00 and 17:00h) during the weekdays and twice a day during weekends (10:00 and 17:00) to visual satiety. The utmost care was taken to avoid feed wastage and to allow quantification of feed intake. Anesthetized fish (20 ml/L of AQUI-S, New Zealand) were individually weighed at the start of the trial and groups weighed on days 30, 61 and 90.
At the end of the trial (90 days of feeding), fish showed a 6-fold increase on their initial BW (Figure 1). The SGR ranged from 1.78 to 1.98%/day. In comparison to the NC group, the PC and all the phytase-supplemented diets led to a significant increase in final body weight (FBW) and SGR. Moreover, fish fed the PC diet and those supplemented with phytase at 500 and 750 OTU/kg showed a significantly higher FBW and SGR than those fed the diet with the phytase at 250 OTU/kg. FCR values were low (0.92 - 0.96) suggesting good feeding practices. FCR, FI and protein efficiency ratio (PER) were not affected by the dietary treatments (p > 0.05).
The overall growth performance can be considered as satisfactory for rainbow trout of this size. Fish fed a diet supplemented with MCP and OptiPhos® led to a significant increase in FBW and SGR compared with the NC.
Fish fed diets supplemented with OptiPhos® at 500 and 750 OTU/kg achieved the same performance in terms of FBW and SGR compared to the performance observed in fish fed the PC diet. Feed conversion ratio, FI and PER values were not affected by the dietary treatments.