Non Nutritive Feed Additives

Smit Ramesh Lende

College of Fisheries - Veraval

Introduction

Feed formulation is essentially applied nutrition. A number of terms and expressions are introduced that will be put to practical use as information is presented on the nature and qualities of various feedstuffs and the information presented on the nutrient requirements of fish. Precise understanding of these terms is essential to their correct application. One must recognize that some of these terms have a built-in error that cannot be escaped. This does not eliminate their usefulness in feed formulation. However, one must appreciate the fact that some are useful approximations of the values and not true values.

The terms that one needs to understand to formulate practical fish diets are: crude protein level; energy level, either expressed as metabolizable energy (ME) or as digestible energy (DE); specific amino acid levels; crude fibre level; and ash level. Since most complete practical fish diets are supplemented with a vitamin premix at levels in excess of the dietary requirement, this category of nutrients will be ignored temporarily. The potential problems occur when one fails to recognize that all of the above mentioned terms, except ME and DE, represent the quantity or level of a nutrient in the feed as determined by chemical tests on a specific sample of a feedstuff. These chemical tests generally correlate well enough with biological methods of feed evaluation (growth studies, tissue, levels) to be very useful to feed formulators, but they are still chemical tests that are subject to experimental error during nutrient level determination. For example, the proximate composition of fish meals changes during the spawning season. Generally, the lipid levels increase before spawning and decrease after spawning. This will alter the percent of protein, ash, and carbohydrates in fish meal as the seasons change. Similarly, many plant feedstuffs vary in proximate composition with their stage of maturity at harvest, location grown, and other environmental conditions, such as the weather. Tabled values represent an average value that is usually close enough to the actual value to allow accurate feed formulation. However, one must be aware that assumptions are being made in order to recognize the potential sources of error that may exist.

Metabolizable energy and digestible energy values are obtained biologically and, thus, should accurately represent the true energy value of feedstuffs to fish. However, ME values may be obtained in different ways (faeces collection methods) and thus may be subject to experimental error. It has recently been reported that the digestibility of feed by rainbow trout was lower at 7°C than at 11°C or 15°C. At 11°C and 15°C body size (18.6 g, 207.1 g or 585.7 g) did not affect feed digestibility. The digestibility of carbohydrate and energy was slightly reduced by meal size in rainbow trout fed at 1.6 percent body weight. Protein and lipid digestibility was not reduced by meal size. Obvious differences exist between fish species in nutrient digestibility, especially in the carbohydrate fraction of feed. Herbivorous and, to a lesser extent, omnivorous fish have longer digestive tracts than do carnivorous fish and are able to obtain more digestible energy from carbohydrates. An awareness of these facts will prevent misuse of ME and DE values.

Each feedstuff in any diet formulation should be present for a specific reason; i.e., it is a good energy source, it is rich in a limiting amino acid, etc. In addition, each feedstuff in a particular diet formulation should be the least costly ingredient available for its particular function in the diet. This leads to another assumption in feed formulation; that is, any nutrient in a particular feedstuff, such as an amino acid, is just as valuable as the same nutrient in any other feedstuff. This allows feed formulators to interchange one feedstuff with another as cost and availability change. Thus, it is assumed that there is no "ideal formulation", but rather an almost infinite number of possible feed formulations that met the nutritional needs of the fish equally well. While this assumption may not be entirely valid and some nutritional judgement must be employed in any feed formulation, it does seem to be valid in most cases. As with the previously mentioned assumption, an awareness of the potential pitfalls involved is necessary for the fish feed formulation so that allowances can be made in diet formulation and problems can be anticipated and avoided.

Nonnutritive Feed Additives

Non nutritive feed ingredients are additives that are included in diets for reasons other than to provide nutrients. For the most part, these compounds have little or no nutritional value, yet they are important constituents of fish feeds, increasing pellet stability, diet safety, diet flavor, and animal and fish performance and health status and influencing the quality of the final Table product.

Nonnutritive feed ingredients include

1. Feed binders

2. Carotenoid supplements

3.Therapeutants and Nonspecific Immune Stimulants

4. Probiotics

5. Enzyme Supplements

6. Hormones

7. Antioxidants

8. Fiber

9. Water

10. Flavorings and Palatability Enhancers

1. Feed Binders

Fish feeds must be formed in to particles or pellets that are strong enough to withstand normal handling and shipping without disintegrating. More-over, fish feeds must be somewhat water-stable. These requirements make it necessary for feeds to contain binders. There are numerous materials that act as binders in fish feed, including regular feed ingredients and ingredients added solely for their binding properties. Some binders are by-products of cereal grains or plants and provide nutrients to the diet. For example, 20% pregelatinized potato starch is added to eel diets to increase the water stability of the dough and to provide energy. Other commonly used binders include bentonite, lignin sulfonate, and hemi cellulose extract, none of which provides nutrients to the diet.

Bentonite is a naturally occurring clay consisting mainly of trilayered Aluminium silicate. It is available as either sodium bentonite or calcium bentonite. Sodium bentonite has, by definition, more than 1% and less than 2% available ion content, or sodium exchange. It swells when added to water, while calcium bentonite does not. Both sodium and calcium bentonite may be added to dry, compressed fish feeds at no more than 2% to act as a binding agent and also as a lubricant, increasing pellet mill production rates and pellet mill die life (Reinitz 1983). Some bentonites also bind aflatoxin, carrying it through the gut without harming the fish. Lignin sulfonate is a product of the wood pulping industry. It aids in pellet binding, reduces fines, and permits the addition of more steam during the manufacture of compressed pellets. Lignin sulfonate is added at up to 4% as a pelleting aid in dry, compressed (steam-pelleted) feeds. Hemi-cellulose extract is a product made by spray-drying the concentrated, soluble byproduct of pressed wood manufacture. It is less commonly used than lignin sulfonate. Moist and semi moist fish food production requires the use of both nutritive and non nutritive binder materials.

Nutritive binders include oat groats, vital wheat gluten, finely milled wheat bran, cottonseed meal, gelatin, fish hydrolyzates, and pre gelatinized starches. Nonnutritive binders include tapioca, carboxymethylcellose, alginates, agar, and various gums. Chitosan, carageenan, and collagen have been evaluated as binders but are not commonly used. Semi moist feeds, containing 25—35% moisture, can often be made into satisfactory pellets by careful selection of feed ingredients that possess binding properties. However, when feed formulations contain ingredients that do not possess suitable binding properties, it is necessary to add ingredients specifically to bind the diet. Moist feeds, having moisture contents of 35 to 70%, always require the addition of a binder. For example, semi purified test diets, such as H440, the Oregon Test Diet, and the Guelph semipurified diet, include gelatin and carboxymethyl cellulose as binders. Moist diets, which are combinations of wet fish ingredients and dry meal, may contain 0.5—2.0% alginates as binders. Heinen found that alginates were better binders than gum, carageenan, chitosan, collagen, carboxymethyl cellulose, and corn starch in a 41% moisture diet. Agar was an effective binder, but expensive. Calcium ions and a sequestrant, such as sodium hexametaphosphate, must be present in diets containing alginatesas binders to control alginate activation.

2. Carotenoid Supplements

A great deal has been written about the addition of carotenoid pigments to fish diets to colour flesh and/or eggs. Over 300 pigments are found in various plants and animals, with xanthophylls and carotenoids being the most important classes of carotenoid pigments that add color to fish. For the most part, xanthophylls are found in plants, such as corn, and carotenoid pigments in crustaceans and fish. Some finfish and shellfish possess the ability to convert certain xanthophyll pigments to carotenoid pigments. Gold fish and common carp can convert the yellow xanthophyll pigment, zeaxanthin, to the red carotenoid pigment, as taxanthin. Similarly, Penaeus japonicus, a shrimp, can convert both β —carotene and zeaxanth into astaxanthin. Salmon, trout, and red sea bream, which normally have pigmented flesh and skin, do not convert xanthophylls pigments to the carotenoids, can thaxanthin, and astaxanthin. In nature, they receive these pigments in their diet. Fish raised in hatcheries and farms must receive canthaxanthin and/or as taxanthin in their diets to become pigmented; in addition, carotenoid supplementation is necessary for salmonid offspring to produce viable offspring. In nature, carotenoid pigments are synthesized by algae and bio concentrated in the food chain, ultimately ending up in fish.

Carotenoid supplementation of fish diets is accomplished by adding natural materials containing the desired carotenoid pigments, carotenoid extracts of natural products, or chemically synthesized pigments. Natural materials that pigment fish include herring gull eggs, salmon eggs, paprika, zooplankton, krill products, Haematococcus algae, and processing waste from shrimp, crab, and crayfish processing. Dietary levels of 20% or more of wet crustacean processing waste are required to get the desired pigment in trout and salmon. Concentrated carotenoid extracts of red crab and cray fish are effective dietary supplements for salmonids The amount added to the diet depends on the concentration of carotenoid pigments in the extract, but dietary levels normally range from 3 to 7%, replacing added fats and oils. Synthetic canthaxanthin is a commercial product containing a minimum of 10% canthaxanthin and is added to commercial feeds at 0.05% to produce a dietary canthaxanthin level of about 50 mg/kg feed. Astaxanthin is themost widely used, manufactured carotenoid pigment. It contains 8% asta-xanthin, by weight, encapsulated in gelatin, and is added to fish feeds at approximately 0.065% to produce a dietary astaxanthin level of 45mg/kg feed. Astaxanthin is produced by several microorganisms, including Phyaffia yeast and Haematococcus algae meal, and products are being produced from these microorganisms specifically for use in fish feeds. Because they are produced naturally, they are desirable for use in salmon production for markets demanding a natural food product. Krill products fill the same market niche and are also effective feed palatability enhancers.

3. Therapeutants and Nonspecific Immune Stimulants

Therapeutants are added to fish feeds to treat, cure, mitigate, or prevent disease. A number of drugs are effective against fish diseases, although in the United States, the only ones approved for use with fish feed are sulfamethazine, terramycin and furox. Erythromycin and azithromycin have been used to treat bacterial kidney disease in captive brood stock of endangered salmon stocks, but they are not allowed in normal production.

In Europe, oxalinic acid is used in feeds as an antimicrobial drug. As with livestock feeds, medicated fish feeds have specific labeling requirements, including a warning to withdraw for a proscribed length of time before the fish are marketed. Antibiotics have been supplemented at sub therapeutic levels for decades in poultry and swine feeds to stimulate growth. Their benefit is derived through control of intestinal microflora, preventing toxin-producing microorganisms, such as Clostridium perfringens, from becoming established in large numbers and lowering the growth rate of the animal. This practice has never been used in aquaculture, in part because it is not effective, due to differences between aquatic and terrestrial animals with respect to intestinal microflora. Given the serious concerns about antibiotic resistance and human health, it is likely that antibiotic supplementation in terrestrial animal production for growth promotion will be limited or possibly eliminated in the future. Non specific immune stimulants, sometimes referred to as neutriceuticals, are another story. They are unregulated feed additives that are intended to enhance the health and well-being of farm and companion animals. In fish, the focus on neutriceuticals lies in making the fish less susceptible to infectious disease. The most common supplements are β-glucans, which are fragments of the cell walls of yeast and mycelial fungi.

The rationale behind their use is that β-glucans supposedly come into contact with leukocytes in the intestinal mucosa. Glucans supposedly possess the same chemical signals as infectious disease agents and, therefore, activate the leukocytes. Glucans are also hypothesized to physically attach to pathogens and thus render them inactive. In fact, although glucans have been shown to reduce fish disease, and also to stimulate the non specific immune response of fish, exactly how they work is not known. Other theories of their mode of action have been presented, but none, as yet, has been proven. Glucans are sometimes effective, and other times not. Questions remain concerning the effective dose, route of administration, and chemical form. There are many forms of β-glucans, and other materials that stimulate the immune system of fish.

4. Probiotics

Probiotics are live, microbial feed supplements that are thought to stimulate animal, and possibly fish growth, by affecting the microbial flora population in the gut of the animal. Probiotics may be a single species of microorganisms or a mixture of species. The concept behind their use is that the species of microorganisms present in the supplement colonizes the gut and outcompetes detrimental species of microorganisms, thus limiting their numbers and allowing the animal (fish) to avoid wasting metabolic energy fighting the effects of detrimental microorganisms. Obviously, probiotics must be added to feeds after pelleting.

5. Enzyme Supplements

Enzyme supplements are either single, purified enzymes or crude enzyme preparations containing multiple enzymes that are added to feeds to enhance the digestion of feed components that the fish either cannot digest or cannot digest efficiently. Phytase is an example of a single enzyme supplement used in poultry and swine feeds and likely to be used in fish feeds in the near future. Phytase hydrolyzes phytate, the storage form of phosphorus in seeds, i.e., grains and oilseeds. Phytase liberates phosphorus from phytate, thus making it available to the animal or fish. Enzyme supplements are available to assist in the digestion of complex carbohydrates, collagen in skin and bones, and other feed constituents. Enzymes are typically denatured at temperatures above 65°C, so adding them to feed mixtures before compression or extrusion pelleting destroys their activity. Thus, enzyme supplements are typically sprayed on feeds after pelleting.

6. Hormones

The use of anabolic steroids in domestic animal feeds is no longer permitted in many parts of the world due to concern about hormone residues in food products. The same concerns exist for fish products, and the addition of steroids and other hormones to the diets of fish raised for market will almost certainly never be approved. However, there are some aquaculture situations in which the addition of hormones to fish diets for a short period may pose no human health risk and may prove useful to fish culturists.

Hormones fall into three categories:

(1) those that affect growth and feed conversion,

(2) those that affect sexual development,

(3) those that affect osmoregulation.

In public salmon culture in the Pacific Northwest, salmon fingerlings are reared in freshwater hatcheries until the optimum time of release. After release, the fingerlings migrate to the ocean, spend 2—4 years growing, and return as adults to the near-shore are as where they enter the fishery. For some species and stocks, the size at hatchery release is positively correlated with the percentage of returning adults. However, many hatcheries cannot rear fish to the optimum size for high ocean survival by the required time of release. Fish growth rates can be accelerated by supplementing diets with anabolic steroids and thyroid hormones, thereby increasing feed intake and metabolic efficiency. An alternative is to add compounds or feed components that stimulate hormone production or that overcome it.

7. Antioxidants

Antioxidants are chemical compounds that are added to feed ingredients to control oxidation of lipids. Other food components, such as carotenoid pigments and tocopherols, can also undergo oxidation. The mechanism of highest concern in feed manufacturing is autoxidation, also known as atmospheric oxidation, which is the oxidation of moderately unsaturated fatty acids, resulting in products that produce off flavors and off odors. The rate of autoxidation of lipids can be accelerated by an increased radiation level, divalent cation concentration, temperature, and oxygen concentration. Autoxidation of lipids is a process involving three steps. The first step involves the formation of free radicals and is called initiation. Initiation is enhanced by a number of factors, including light, heat, UV radiation, and the presence of divalent cations, such as copper and iron, known as prooxidants. The second step in autoxidation is called propagation and involves the reaction of free radicals formed in the initiation step with more free double bonds on fatty acids, forming a number of secondary product sand radicals. The final step is termination, in which free radical production slows, and finally stops; various secondary products of fatty acid oxidation react in various ways to form stable end products. Because the propagation step itself forms more free radicals than it uses, autoxidation reactions are autocatalytic, meaning that once oxidation starts, it continues at an accelerating rate until substrates (double bonds) are used up. The number of free radicals formed from oxidation of individual fatty acids is related to the number of its double bonds, making oxidation of the fatty acids in fishoils (very unsaturated) a much more rapid process than oxidation of less unsaturated lipids.

Antioxidants work by chelating pro-oxidant divalent cations, by acting as free radical acceptors, or by donating hydrogen. The latter two functions are considered sacrificial because once an antioxidant molecule reacts, it no longer possesses antioxidant properties and is therefore "destroyed" in the process. Thus, antioxidant concentrations fall during the initiation phase, and once they are used up, oxidation reactions proceed very rapidly. Antioxidants added to lipids and feeds to prevent oxidation by reacting with free radicals are phenolics, such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), and amines, such as ethoxyquin (Thorisson et al. 1992). BHA and BHT are added to feeds at a level of 0.1%, while ethoxyquin is added at 0.015%. Other antioxidants in use include dilaury l thiodi propionate, propyl gallate, and thiodipropionate. Antioxidants that prevent oxidation by chelating metallic pro-oxidantsinclude ascorbic acid, phytic acid, tartaric acid, oxalic acid, and ethylenediaminetetraacetic acid (EDTA). There is a synergistic effect when phenolicor amine antioxidants are combined with an antioxidant that chelates pro-oxidants.Many lipid sources contain naturally occurring antioxidants, mainly to-copherols. These compounds inhibit autoxidation of lipids until they areused up, at which time the rate of oxidation reactions increases very rapidly.The period of time during which antioxidants prevent oxidation is called the induction time. Chemical tests to detect lipid oxidation, such as peroxide values and TBARS, cannot measure induction time, and low values from these tests can give a false sense of security to a feed company. By testing a lipid source or feed before and after an accelerated oxidation test, such as the Shall oven test, the induction time can be estimated, and appropriate precautions taken to avoid oxidation.

8. Fiber

Fiber is the non nutritive portion of feed ingredients that is measured as crude fiber in proximate analysis. It is indigestible by salmonids and other carnivorous fish, but channel catfish have intestinal microflora capable of digesting a small portion of dietary fiber. Some herbivorous fish, such as grass carp, derive nutrients from fiber but some, such as Tilapia aurea, do not. Fiber is added to semipurified diets to facilitate binding as well as to increase digestion efficiency. Generally, fiber is not added to practical diets; rather it is avoided because it passes through the fish and adds fecal solids to rearing water and farm effluents. This point is critical in aquaculture systems semploying water recirculation and in rainbow trout farming, where high volumes of water are discharged into rivers and lakes. Upper limits for fiber in feed formulations are generally specified, thus eliminating many potential fish feed ingredients and restricting the levels of others. In diets for fish that do not possess the ability to digest fiber, levels of fiber above 3—5% are not recommended. Fiber levels as high as 8—12% are tolerated by most fish, but such levels often result in growth depression. Fish fed diets high in indigestible fiber increase their feed intake and gastric evacuation time, but the extent to which fish can compensate in this manner is limited .

9. Water

The water content of feeds ranges from 6—10% for dry-compressed or extruded pellets to 65—70% for high-moisture, wet pellets. The moisture content of feeds is important because of the potential for microbial growthin high-moisture feeds, and the moisture content is critical in the pelleting process, where it is added to the mixture as live steam just prior to pelleting. Steam pelleting and cooking extrusion increase the moisture content of the feed mixture to approximately 18 and 23%, respectively, but the pellets are dried to < 11% immediately after pelleting. Some fish species accept moist feeds more readily than dry feeds, particularly Pacific salmon fry and large-mouth bass. However, brown trout and turbot grow equally well on moist or dry diets. In the past, researchers reported that chinook salmon reared in marine net-pens grew more rapidly when fed diets containing 15—30% water than when fed dry diets, but improvements in feed formulation and manufacture have eliminated this effect.

10. Flavorings and Palatability Enhancers

Fish are very sensitive to certain tastes in their feed, a trait that can be both harmful and beneficial in diet formulation and manufacture. For example, chinook salmon fry are extremely sensitive to the presence of low levels of dietary soybean meal and respond to its presence by reducing their intake. Trout are less sensitive to dietary soybean meal, although in semi purified diets, the addition of a "fishy" component to the diet to mask the taste of soybean meal must sometimes be made to induce trout to consume feed. Flavorings are common feed additives in the pet food industry but their use in aquaculture diets is only beginning to be investigated. Generally, feed acceptance is not a major problem among cultured species of fish, with the exception of fry and certain species of cold-water fish. Extracts of crustaceans, such as krill, and certain amino acids may increase appetite in fry and crustaceans, respectively

Refrances

John E. Halver and Ronald W. Hardy - Fish Nutrition 3^rd ediition

Aquaculture development and coordination programme. Fish feed technology FAO