64 SEPTEMBER 2012 Fishmeal is used as a major protein source in aquafeeds because it has a high protein content, excellent amino acid profile, high protein and amino acid digestibility, and excellent palatability. However, inconsistent supply, greater demand, and rising prices have made prompted the search for a substitute. There are possibilities for replacing fishmeal with other ingredients, but there has been no breakthrough because alternative feedstuffs are not available in sufficient quantity where they are needed. Full fat soybean meal is the most popular alternative to fishmeal, providing most of the protein needed to produce commercial aquafeeds at cheaper cost. However, use of soybean meal in aquafeeds is restricted by anti-nutritional substances. Meticulous processing methods must be used to eliminate antinutritional factors. The protein content of wheat, oats and barley is 11-14 percent. However, there are grain varieties that contain up to 25 percent protein, but cultivated area is limited because yield is low and there are marketing problems. Inclusion of such high-protein grains in feeds for omnivorous fishes can greatly reduce the need for fishmeal. High dietary levels of plant proteins usually result in reduced culture performance caused by reduced feed consumption, essential amino acid deficiency, and the presence of anti-nutritional factors and indigestible components. Supplementation of deficient feeds with crystalline amino acids, exogenous enzymes (e.g., phytase), and palatability enhancers increases feed consumption and improves the nutritional value of plant protein sources. Anti-nutrients in Plant Feedstuffs Anti-nutrients are defined as substances that, by themselves or through their metabolic products in living systems, interfere with food utilization and affect the health and production of animals (Makkar 1993). Anti-nutrients can be broadly divided into four groups (Francis et al. 2001, Agbo 2008): I. Factors affecting protein utilization and digestion (e.g., protease (trypsin) inhibitors, tannins and lectins) II. Factors affecting mineral utilization (e.g., phytates, gossypol pigments, oxalates and glucosinolates) III. Antivitamins IV. Miscellaneous (e.g., mycotoxins, mimosine, cyanogens, nitrate, alkaloids, photosensitizing agents, phytoestrogens and saponins). Anti-nutritional Factors The major anti-nutritional factors commonly present in plantderived feedstuffs used in aquafeeds are summarized in Table 1. Phytic acid is one of the most powerful anti-nutritional factors in plant feedstuffs. The antinutritional activity of phytic acid can be eliminated by the addition of enzymes such as phytase. Phytic acid or phytate in cereals, legume grains and oil seeds is bound with phosphorus, calcium and magnesium, trace elements such as iron and zinc, and protein and amino acids. Most fishes do not have endogenous enzymes to break down phytate and release nutrients; therefore, they pass through the gut undigested. This is why greater proportions of valuable nutrients from plant sources are not utilized by aquatic animals and are wasted as excreta. Phytase enzyme releases phosphorus and bound minerals and amino acids from phytate, paving the way for maximum utilization of nutrients. The advantages of phytase are: • Phosphorus bound in phytate becomes more available as a nutrient. Therefore, inclusion levels of phosphorus can be reduced. • Performance of aquaculture operations is under scrutiny because nutrients are discharged into surrounding ecosystems. Addition of phytase reduces the release of nutrients into the environment by making more bound phosphorus available to fish for growth. • Phytase added to diets improves protein and amino acid digestibility in fishes. • Phytase can improve the metabolic energy of feeds by breaking down the phytate-lipid complex. • Plant protein sources can be substituted for animal protein sources (e.g., fishmeal), reducing feed costs. Non-starch polysaccharides reduce the growth performance of aquatic animals. The anti-nutritional effects mainly result from increased viscosity of digested material in the intestine and enclosure of nutrients, making them unavailable to digestion. Because aquatic animals lack intestinal enzymes for degradation of non-starch polysaccharides, supplementation of degrading enzymes in the diet will result in better feed utilization. Anti-nutritional Factors in Plant Feedstuffs Used in Aquafeeds Vikas Kumar 1, Debtanu Barman 2*, Kundan Kumar 3, Vikash Kumar 3, Sagar C. Mandal 4 and Eef De Clercq 1 Full fat soybean meal is the most popular alternative to fishmeal, providing most of the protein needed to produce commercial aquafeeds at cheaper cost. However, use of soybean meal in aquafeeds is restricted by antinutritional substances. Meticulous processing methods must be used to eliminate anti-nutritional factors.
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