Feed additives are substances added in aqua feeds to protect the liable nutrients, to improve nutrient availability and utilization. Among the various feed additives like antioxidants, immunostimulants, coloring and anabolic agents, the one gaining focus in the recent years id the enzyme supplements. The growing demand in aqua feed industry stress the importance of replacing fish mean and fish oil with plant proteins and carbohydrate sources sustainability of the aquaculture sector. However, most plant based feed stuffs have a wide variety of anti-nutritional factors such as phytin, non-starch polysaccharides (NSP) and protease inhibitors, which may impair nutrient utilization, fish performance and health. Moreover, the endogenous enzymes produced by the microbes in the digestive tract are insufficient to degrade these compounds. Therefore studying on improving digestibility of the feed with exogenous enzymes is the need of the hour. This article highlights the importance of enzyme supplements and the issues to be addressed.
Enzymes and its types
Enzymes are one of the many type’s o proteins in the biological systems. The primary function of the enzyme is to catalyze the rate of a reaction without itself undergoing any alteration. Enzymes are involve in anabolic an catabolic pathways of digestion, body metabolism and provide a natural way to transform complex feed components into absorbable nutrients.
Enzymes tend to be very specific catalysts that act on one or at most a limited group of compounds known as substrates. Most of the enzyme supplements in fish feed belong to the hydrolases group.
ROLE OF ENZYMES AS FISH FEED ADDITIVES:
To enhance digestibility of the feed
Fish meal (FM) had traditionally been used as the best protein source in aqua feeds due to its high contents of essential amino acids (EAAs), fatty acids, low carbohydrate and low levels of anti-nutritional factors. In recent years one prominent area of research in aquaculture nutrition is the replacement of fishmeal with plant based protein ingredients s to support the globally expanding aquaculture industry and to ensure its sustainability. However, bean meal, rapeseed meal, and corn gluten meal or wheat gluten, have a wide variety of anti-polysaccharides (NSP) and protease inhibitors, which may impair nutrient utilization, fish performance and digestive enzyme activities of digestive tract. For instance, the NSP in Chlorella increases influences the digestion and absorption of nutrients. Hence, in order to digest phytin,phytase in supplemented in the feed.
To supplement the activity endogenous enzymes
The herbivorous fish synthesize the cellulose enzyme not by the animals, themselves but by certain microorganism. However, the enzyme sufficiently, therefore exogenous cellulose is needed to supplement in the fish diets especially when using plant ingredients.
To promote secretion of endogenous enzymes
Supplementation of exogenous enzymes (Protease, Xylanase) on diets of juvenile hybrid tilapia (Orechromis miloticus*Orechromis auresus) fed with plant based feedstuffs (Soyabean, rapeseed and cottonseed meals) at a dose of 1.0 and 1.5g Kg- improved the specific growth rate, feed efficiency ratio and apparent protein retention with increasing dietary enzyme levels. Moreover, it also significantly increased both protease and amylase activities in the intestine and the hepatopancreas of juvenile hybrid tilapia. These results suggest that exogenous enzyme supplementation can promote the secretion of endogenous enzymes by the fish. In addition, the cellulose supplementation in duckweed incorporated diets of grass carp increased various digestive enzyme activities, such as cellulose, amylase and protease but not the lipase activity.
To increase the utilization of nutrients
Carbohydrates supplementation increases digestibility of energy yielding nutrients, such as starch and fat, because NSP reduces the capacity for nutrient absorption by reducing enzyme accessibility to substrated, Carbohydrases act to improve nitrogen and amino acid utilization as well, by increasing the access to protein for digestive proteases. The improvement in fat digestibility is especially noteworthy because NSP are known to increase hydrolysis of bile salts and hence reduce fat utilization.
To reduce the feed cost
Carbohydrate is indispensable in fish feed, it constitutes an inexpensive source of energy. In many fish species a dietary carbohydrate supply appears to be necessary, as it improves growth. When sufficient non-protein energy is not available, protein is first used as an energy source rather than for growth purposes. This phenomenon is known as the ‘protein sparing effect’. Carbohydrase enzymes allow incorporation of “low-processed” plant ingredients in diets of carnivorous or omnivorous fish, which could reduce the feed cost as well as increased ingredient nutrient flexibility.
To modulate the gut microbiota
Cellulose had been repotted to change the intestinal microbiota of grass carp in terms of bacterial species and density. Previous studies in intestinal microbiota and their association with nutrients intake. Thus it is speculated that the change of microbiota may be due to intestinal nutrition, which is altered because of the digestive enzymes improved by the supplemental cellulose. In addition, gastrointestinal microbiota was confirmed to influence immune status, disease resistance, survival, and feed utilization. Carbohydrase reduces the viscosity of the digestive juices, encouraging higher diffusion rates, preventing accumulation of particulate matter for microbial adhesion, and lowers flow of solids rather than liquid. These factors retard the shedding of microbes and prevent proliferation of harmful bacteria. Carbohydrases also promote and support growth of the beneficial bacteria by increasing the proportion of lactic and organic acids, reduces ammonia production, and increase volatile fatty acid concentration, indicative of hydrolytic fragmentation of NSP.
Besides, the above mentioned factors, it should be borne in mind that other abiotic components in the rearing environment also plays important role in the activity of the enzyme present in a feed. For instance the efficacy of any enzyme in marine fish feed depends on the salinity and water temperature of the rearing system. Food digestion in marine finfish species must proceed under high saline ( chloride) concentrations. Water temperature also has been found to influence the enzyme efficiency, in seriola quinqueradiata (Yellowtail amberjack). This fish exhibited seasonal changes with higher trypsin and chymotrypsina activities in the intestinal contents and lower pepsin activity in the stomach contents at low water temperatures.
Conclusion
Enzymes are catalysts in the realization of chemical reactions in the body. Among the various enzyme supplements used phytase has been extensively studied as it has a major role in phosphorus utilization and thus helps in the development of low pollution feeds. These enzymes besides improving the Food Conversion Ratio (FCR) and Protein Efficiency Ratio(PER), it also regulate the blood glucose level, leukocyte activity and the immune status of the organisms. Further research need to be done to address the above said problems.
Source: Aquaculture Spectrum