Can enzymes be used to improve nitrogen utilisation and reduce nitrogenous wastes to meet the feed specifications specifically designed for RAS farming?
By M A Kabir Chowdhury
In recent years, recirculating aquaculture systems (RAS) have been gaining popularity worldwide. Efficiency of a RAS system largely depends on its nutritional wastes removal capacity. To improve the efficiency of the system, there is a need to reduce the nutritional wastes.
Some commercial feeds are already in the market specifically designed for RAS farming system. However, little is known about these feeds’ ability to improve system performance. It is a well-known fact that feed enzymes can improve nutrient utilisation by the animals, thereby, reducing nutritional waste output and organic load in the culture environment. In this article, we discuss some examples of using protease enzymes in aquafeed at various experimental set-ups to improve nitrogen utilisation and reduce nitrogenous wastes.
Improving protein efficiency ratio (PER)
The protein efficiency ratio or PER is calculated as body weight gain/protein intake. As we know, crude protein is usually calculated as 6.25 X N. Improving PER will therefore reduce the nitrogenous wastes to the rearing environment.
In Figure 1, data from various commercial trials conducted in laboratories (l) or cage farms (C) are presented. On average, there is a 14.3% increase in protein efficiency ratio, when a dietary protease is added to the diets.
The improvement in PER when a protease is added to the diets, largely depends on the type of raw materials used, crude protein and digestible protein levels, type and level of the dietary protease as well as the animal density. For example, in the case of the snakehead fish trial, better improvement in PER was reported in high density (HD) treatment compared to those reared in low density (LD) treatment.
Improving nitrogen digestibility
Among the aquafeed components, the most expensive raw materials are the protein sources. To optimise the cost of feed, formulators and nutritionists are forced to utilise more cost-effective raw materials. However, the use of these cost-effective sources in aquafeed has been limited due to imbalance or poor amino acid profiles, poor nutrient digestibility, and presence of some anti-nutrients.
It has been shown that using ex-vivo enzymatic treatment or in-feed enzymes can improve nutrient digestibility of these poor-quality raw materials. In Figure 2, apparent digestibility coefficient of nitrogen (ADC N) is presented. In all cases, the ADC N was increased in all diets containing protease irrespective of species type. However, the increase is more pronounced in poorly digestible diets. For example, in shrimp, the ADC N improved from 1.6% (18% fish meal diet) to 3.6% (10% fish meal diet) to 4.5% (0% fish meal diet) when a protease was added to the diets.
Reducing nitrogenous wastes (NH3-N)
It can be predicted that as utilisation of feed-nitrogen improves by the addition of dietary protease, there will be less wastes to the system.
In Figure 3, ammonia excretion by yellow catfish fed protease supplemented diets was measured. This trial was conducted in laboratory environment where graded levels (125, 150 and 175 mg/kg) of a protease were added to the control diet. All treatments were pair-fed (same amount of feed) for two weeks. During this trial, the level of excreted ammonia was measured daily. The data are presented here as the average hourly ammonia excretion (mg/kg/ hr).
A linear decrease (y = – 0.14x + 8.9, R2 = 0.93) in hourly ammonia excretion was observed with the increasing level of dietary protease. Despite the decrease, there were no differences in ammonia excretion between the control and the treatments with 125 and 150mg/kg dietary protease. However, significant differences (P<0.05) were observed between the control and the treatment fed 175mg/kg protease supplemented diets.
Similar results can be summarised for other enzymes such as phytase for phosphorus utilisation as well as carbohydrase such as xylanase, beta-mannanase and cellulase for better utilisation of dietary non-starch polysaccharides. However, the choice of enzymes must depend on the availability and type of the substrates. Arbitrary use of enzymes without proper understanding of the types and their substrate preference may well be a waste of valuable economic resources and must be avoided.