Limitations to the use of plant derived ingredients in fish feed

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Acceptability

Plant ingredients, particularly those containing high levels of antinutrients have been found to have a bitter taste which could result in lack of acceptability of the feed. Soaking in water followed by drying has been shown to increase acceptance, and also rid the plant material of several toxic compounds. The disadvantage is that some soluble nutrients are also lost.

Nutrient inadequacy

Plant-derived proteins are conventionally deficient in amino acids such as lysine, methionine, cysteine and tryptophan. The amino acid composition of the various plants dealt with here, and some of their products are listed in Table  10. It can be seen that jatropha and moringa products have higher levels of one of the most commonly deficient amino acids (methionine) compared to soybean meal (among the most highly regarded plant protein sources). It should be mentioned here that soybeans have undergone considerable species betterment through breeding over the last several years and grow on highly enriched soil whereas the plants mentioned here have not and they grow on poor soils. There is thus scope for improving the genetic character and biological value of these plants through agronomic interventions. Synthetic amino acid supplementation have been effective in partially compensating for lower dietary levels in large stomached fish such as trout (Mambrini et al., 1999), whereas it is much less effective in the case of stomachless fish such as common carp (Becker, 1984; Murai, 1985). This group of fish happen to be the most important culture species, particularly in Asia. Plant products such as those obtained from jatropha and moringa that are rich in commonly deficient amino acids such as methionine could be used as a source of these amino acids in carp species.

The definitive amino acid requirements of fish are difficult to obtain. In most of the research papers, amino acid requirements are calculated from amino acid dose to growth response regressions. Many fish are known to use synthetic amino acids sub-optimally (see NRC, 1993 for references), and the requirement of individual amino acids depend on other factors such as digestibility of protein, and presence and availability of other amino acids (Lovell, 1998). The essential amino acid requirements presented in Table 11 are based on NRC recommendations (NRC, 1993). It can be seen that requirements for essential amino acids differ from species to species. There is therefore need to formulate diets keeping in mind the requirement of the fish species concerned. Since fishmeal has a superior quality as far as essential amino acid content is concerned, its complete elimination from feeds might adversely affect growth in most fish. A judicious mixture of different plant derived materials along with a minimal amount of fishmeal would seem to be the best choice in feed formulation to ensure nutrient adequacy and ready acceptance by the fish.

Antinutrients

As described in previous sections, most of the abovementioned plant-based nutrient sources contain high levels of various antinutrients. The most important among the antinutrients in plant-based material are glucosinolates, phytates, protease inhibitors, non-starch polysaccharides (NSP), saponins, tannins, lectins, and gossypols. Others such as phytoestrogens, alkaloids, cyanogens, mimosine, cyclopropenoid fatty acids, canavanine, antivitamins, and phorbol esters could also prove deleterious. From the fish feeding trials described above it is evident that common culture species do tolerate many of these antinutrients at inclusion levels of up to 15 per cent of plant-derived materials. The effects of antinutrients on finfish have been reviewed in Francis et al. (2001a). Hydrothermal treatment and soaking with water is efficient in removing high levels of antinutrients such as glucosinolates, protease inhibitors, lectins, tannins and saponins (see tables 7 and 9). Supplementing high phytate diets with the enzyme phytase have been found to increase availability of dietary phosphorous to various fish species (see Hardy, 2000). NSPs present in the diets could be neutralised to a certain extent by addition of enzymes such as glycanase (Hardy, 2000). Gamma-ray irradiation also holds some promise in neutralising the negative effects of certain antinutrients e. g. NSPs and saponins ( Siddhuraju et al., 2002).

Some of the secondary plant compounds may even have beneficial effects when present in diets of fish in small amounts. For example, saponins have been found to promote growth in common carp and tilapia when present in the diets at 150 mg kg^-1 (Francis et al., 2001b, 2002). Saponins might increase the digestibility of carbohydrate-rich foods because of their detergent-like activity, which reduces viscosity and thus prevents the normal obstructing action of such foods against movement of digesta in the intestine (the NSPs exert their antinutrient action by forming viscous clumps in the intestine, which obstruct the digestive process). Cyclical, short-term offers of trypsin inhibitors along with the diet have been shown to increase protein digestibility and growth performance in carp (Becker K., unpublished). Interactions among various antinutrients in a particular feed source may also have effects on their potency. Saponin-tannin, tannin-lectin and tannin-cyanogen interactions may reduce their individual toxic effects (see review by Francis et al., 2001a). These interactions may also result in effects that are more detrimental than those of individual antinutrients. More insights into the nutritional, physiological and ecological effects of antinutrients on fish need to be accumulated through studies using purified individual antinutrients and their mixtures in proportions similar to those in alternative nutritional sources in fish feeds. Such studies would provide data useful for designing optimum inclusion levels of plant-derived materials in aquaculture diets. Research should also be directed at: i) treatment methods that would neutralise the negative effects of the antinutritional factors and/or bring them down to harmless levels without affecting availability of other nutrients, ii) economic analysis in terms of cost:benefit ratio of incorporating the treated meal in fish diet, on which the use of these unconventional feed resources will be based, iii) exploitation of other lesser-known and lesser-researched seeds in fish diets, a collated information on some of these seeds is available in Makkar and Becker (1999a).

In conclusion, addition of any of the abovementioned plants/ plant products mentioned above beyond levels of 10-15 per cent replacement of fishmeal in the diet can be attempted only after adequate treatment of the material to reduce toxin levels and increase acceptability. More work toward techniques that would increase their nutritional value could prove to be profitable in many ways, although a cost-benefit analysis is called for in each instance.