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1. INTRODUCTION

Anti-nutritional substance are defined as those generated in feed stuffs by the normal metabolism of the species from which the material originates and by different mechanism exerting effects contrary to optimum nutrition. These anti-nutritive substances are often referred to as toxic factors because of the deleterious effects they produce when eaten by animals. The term toxic factor is misleading because there is an implication that the substances are lethal beyond the certain level .

Anti-nutrients(Anti-nutritional factors)

Definition:

Substances which either by themselves or through their metabolic products, interfere with food utilisation and affect the health and production of animals

General characteristics

• Products of secondary metabolism

• Found in virtually all plants to some degree

• Common in tropical forages

• Defensive role:    Bitter, poisonous, bad odour, anti nutritive / immune suppresive

                        There are many anti nutritional factors present in feeds and fodders which affect the utilization of nutrients .Some of these are toxic if they are consumed in large quantities. Few of them are dealt here.

 

2. CLASSIFICATION OF ANTINUTRITIONAL FACTORS

On the basis of the types of nutrients affected and the biological response produced in the animal the toxic factors can be classified into 5 major groups as follows.

1. Substance depressing digestion or metabolic utilization of proteins

1. Protease inhibitors

2. Lectins of ricins (Haemagglutinins)

3. Saponins

4. Polyphenolic compounds (tannins)

2. Substaces reducing the solubility or interfering with the utilization of mineral elements

1. Phytic acid

2. Oxalic acid

3. Glucosinolates

4. Gossypol

3. Substance inactivating or increasing the requirements of certain vitamins and hormones

1. Antivitamins A, D, E, K and antipyridoxins

2. Mimosine antihormone

4. Cyanogens

5. Nitrate and nitrite

6. Moulds and mycotoxins in animal feedstuffs

 

1. Substance depressing digestion or metabolic utilization of proteins:

1. Protease inhibitor

Substances that inhibit proteolytic enzymes and thereby growth of nonruminants are disturbed. In case of soybean two types of protease inhibitors are found

1. Kunitz inhibitors have few disulphide bonds and inhibit trypsin

2. Bowmanbrik inhibitors have a high proportion of disulphide bonds and inhibit trypsin and chymotrypsin Feeding raw soybeans to pigs chicks and rats have resulted in reduced growth rate , pancreatic hyperplasia and low production although ruminants are capable of utilizing raw soybeans without suffering any deleterious effects, a better response in milk production and growth rate is obtained on diets containing treated soybean.

The inhibitory substances are mostly heat labile and thus before feeding any  leguminous grains to non ruminants, they must be treated by heat . Among the anti-nutritional factors present in soybean seed, the main ones are protease inhibitors – Kunitz trypsin inhibitor (KTI) and Bowman-Birk inhibitor, and lectins. Protease inhibitors represent 6% of the protein present in soybean seed. Approximately, 80% of the trypsin inhibition is caused by KTI, which strongly inhibits trypsin and therefore reduces food intake by diminishing their digestion and absorption. Another effect of KTI is the induction of pancreatic enzyme, hyper secretion and the fast stimulation of pancreas growth, hypertrophy and hyperplasia. Due to this, raw soybean cannot be used for feeding monogastric animals. Heat treatment doesn’t completely eliminate these factors and may decrease protein solubility. Despite the efficiency of thermal treatment to reduce protease inhibitors, residual inhibition (10-20%) is maintained (Carvalho et al., 1998). By this reason, a part of the breeding program of the Maize Research Institute Zemun Polje is aimed at developing soybean cultivars with reduced trypsin inhibitors content.

 

2. Lectins of ricins

It is found in both plant and animal tissue. In castor bean cakes a toxic fraction capable of agglutinating human red blood cells  was noted as ricins. Lectins are protein in nature digestion resistant by pancreatic juice. Although very resistant to destruction by dry heat, lectins are destroyed by the same conditions as those used to inactivate protease inhibitors.

C.Saponins

The important common forages which have caused saponin poisoning in livestock are lucerne, white clover, red clover and soybean. 

Chemical structure and general properties

Saponins are plant glycosides which yield on hydrolysis sugars (pentose, hexose and uronic acid) and aglycones derived from polycyclic ring systems and are known as sapogenins. Saponins are divided into two main groups from the point of view of chemical nature of sapogenins: steroids or triterpenoides with the triterpenoid sapogenins further divided into three classes based on the compounds ursane and oleanane/lupine. They have three significant characteristics

1. A bitter taste

2. Foaming in aqueous solution

3. Haemolysis of red blood cells

Due to bitter taste of saponins it has been suggested that the reduced feed intake is due to unpalatability.

Biological activity of saponins can be related to the following characteristic properties

1. Structurally saponins have a lipid soluble aglycone and a water soluble glycine part which jointly confers lower surface tension and thus forms stable foam when dissolved in water/

2. Most of the saponins readily combine with cholesterol resulting minimisation of the activity of original saponins.

3. They ossess the ability to haemolyse red blood cells

4. They tend to alter the permeability of the cell wall and thereby exert a general toxicity on many organized tissues

Adverse action on excessive eating

1. It increases the respiratory rate which later becomes irregular.

2. It inhibits action of certain enzymes for e.g. α-chymotrypsin.

3. In general the effects of injestion of saponins include excessive salvation, increase respiratory tract secretion, gastroenteritis, vomiting, diarrhoea, haemolysis, haematuria and damage to livers and kidney tissues, cystitis, bloating, reduction in gastric motility, reduction of cholesterol absorption from the gut, lowering of blood and liver cholesterol levels, reduction of food intake and growth rate.

4. Polyphenolic compounds (tannins)

It is present in number of vegetable extracts which are responsible for converting putrescible animal skin into the stable product leather by the tanning process. It is also known as tannic acid, gallotannin, gallotannic acid. These substances are also termed as group of phenolic non-nitrogenous plant toxins that are frequently glycosides with astringent properties.

Types of tannin

Chemically tannins may be grouped into two main categories

1. Hydrolysable tannins

2. Condensed tannins

Most tannin abstracts appear to contain mixture of both types of tannins but generally one or the other predominates at least in any given part of the plant

 

1. Hydrolysable tannins

It can be readily hydrolysed by water, acids, bases or enzymes. Hydrolysable tannins are those in which gallic acid and its related compounds like hexahydro-xydiphenic acid are linked in sufficient proportion to a sugar by glycosidic linkages to provide polyphenolic compounds of relatively high molecular weight. Such compounds are also know as gallotannins because of the presence of gallic acid. On hydrolysis such compounds yield glucose or some other polyhydric alcohol together with gallic acid or other phenolic acids related to it.

 

2. Condensed tannin

Compounds contain only phenolic nuclei although polysaccharides or proteins may be irreversibly linked to them in vivo or during the course of isolation. On treatment with hydrolytic reagents tannins of this class  give no significant yields of lower molecular weight compounds but instead tend to polymerise especially in acid solution to yield insoluble amorphous red colured products known as phlobaphene. Most tannis of this type are formed by the condensation of two or more molecules of flavan-3-ons such as catechin or flavin-3,4-diols such as leucocyanidin or the mixture of the two. Tannins of this type are usually termed non-hydrolysable or more commonly condensed.since the majority of tannins appear to be formed by the polymerization of flavans, it is convenient to refer to this group specifically as flavolans and to use the term condensed tannin in this wider sense.

 

Distribution of tannins in livestock feeds and fodders

Name of the feed stuff	Approx. %
Sorghum	0.004-10.50
Milo	2.00-3.00
Salseed meal	9.00-12.00
Mango seed kernel	5.00-7.00
Mustard oil cake	2.80-3.20
Rape seed cake	3.00-3.50
Lucerne meal	0.10-3.50

 Methods of detannification

1. Physical treatments:

• Soaking and cooking of tannin containing feed stuffs have been found very effective in decreasing the tannin content. However, these treatments cause a substantial loss of dry matter between 20-70 %.

• Anaerobic storage of moist sorghum grains for two and nine days at 25⁰C resulted in 40% and 90% reduction in tannins respectively. The nutritive value of the treated grains was found to be higher.

2.Chemical treatments:

            a) Addition of tannin complexing agents like polyethylene glycol (PEG) and polyvinyl pyroldone(PVP) prevent formation of                 complexes between tannin and protein as well as break the already formed complexes, thus liberating protein.

b) Treatment with alkalies like NaOH, Ca(OH)₂ and lime water of sal seed meal was found to be very effective in removing tannins(74-100%), but treatments with Na­₂CO₃ and NaHCO₃ were comparatively less effective( about 50%) removal.

c) Formaldehyde treatment

d) Treatment with methanol and acetone

e)Treatment with 0.1% HCl

 

2.Substances reducing the solubility or interfering with the utilization of mineral elements

Gossypol

            It is a toxic phenolic compound present in cotton seeds. Heat treatment destroys the gossypol. In ruminants ,it is less toxic.

Glucosinolates:

            These are found in almost all the species of crucifera family. The plants , seeds and oil cakes of different mustard and rape varieties are rich sources of this glucosinolates .These compounds reduce the incorporation of iodine into the precursor of thyroxine resulting in iodine deficiency and development of goiter. Prolonged water soaking or cooking of feeds inactivates the effects of goiterogens

Phytic acid:

            Phytic acid (Inositol hexaphosphoric acid) forms insoluble salts with essential minerals like calcium, iron, magnesium and zinc in food rendering them unavailable for absorption into the blood stream (Bingham, 1978). About half of the phytic acid phosphorus content taken by man is excreted unchanged. The remaining unavailable for utilization, the knowledge of phytate content can be made use of in calculating the quantity of phosphorus available out of the total phosphorus in a diet. Phytic acid and its  hydrolysis products are associated with inhibition of calcification in rats (Robert andYudkin, 1999).

Oxalate:

            The name oxalate is attributed to its occurrence in plant oxalis (Wood sorel). The earliest interest in the toxicity of oxalate arose because of instances of severe or fatal human poisoning following the eating of large quantity of the leaves of certain plants i.e., rhubarb, known to contain relatively large amounts of oxalates, (Osagie, 1998).

 

3.Substance inactivating or increasing the requirements of certain vitamins:

1. Anti-Vitamin A:

Raw soyabeans contain an enzyme Lipoxygenase, which catalyses oxidation of carotene, the precursor of vitamin-A. It has been noted that 30% of ground, raw soyabeans in the diet of dairy cattle produces a sharp lowering of vitamin A and carotene in blood plasma. The enzyme can be destroyed by heating soyabeans for 15 minutes with steam at atmospheric pressure.

2. Anti-Vitamin D:

Rachitogenic activity of isolated soya protein(unheated) has been found with chicks and pigs. The effect could be partially eliminated by increasing the vitamin D in diet by 8-10 folds. Autoclaving eliminates the rachitogenic activity.

3.  Anti-Vitamin E:

containing raw kidney beans (phaselous vulgaris) produce muscular dystrophy in lambs by reducing plasma vitamin E. Alcohol extraction of the beans reveals 2 factors with anti-vitamin E activity, one being alcohol soluble heat stable, the other being heat liable and alcohol insoluble. By autoclaving beans the anti vitamin activity is eliminated.

4. Anti-Vitamin K:

“Sweet clover disease” is characterised by a fatal haemorrhagic condition in cattle and has been known over 20 years. The active principle responsible for this disease is dicoumarol, which reduces the prothrombin level of the blood, thus interfering with blood clotting mechanisms. The effect is due to reducing vitamin K utilisation in the production of thrombin by the liver

5.  Anti-Pyridoxine:

It has been demonstrated that the nutritive value of linseed meal for chicks can be considerably improved after extracting the meal with water and autoclaving. An antagonist of pyridoxine, (a member of B vitamins) from linseed which has been identified as 1- amino-D- proline, and occurs naturally in combination with glutamic acid as a peptide is known as linatine.

f) Mimosine (Leucaena leucocephala):

The plant subabul and other legumes contains a toxic substance known as Mimosine which is free amino acid, present at about 3 to 5% of dry matter. The young leaves are relatively richer in mimosine content.

            Mimosine , is also present in leucocephala forage and  its  seeds. The poor nutritive value of leucocephala arises principally from its toxic amino acid, mimosine and its immediate degradation product, i.e., 3-hydroooxy-4(1H) pyridine(DHP). The toxicity depends upon different classes of ruminants  and  on the different geographical regions of the same class of ruminants. Alopecia, loss of appetite , excessive salivation, in coordination of gait, enlarged thyroid gland and  poor breeding performance were reported in cattle and buffaloes grazing/fed on lucaena. Poor wool growth and haemorrhagic cystitis were observed in sheep fed on leucaenia diet .Thyroid enlargement was also observed. In goats also alopecia and enlarged thyroid were observed. In some geographical regions leucaena feeding didnot show any toxicity symptoms in ruminants like in Hawaii.

            The Mimosine acts as an analog of tyrosine and blocks its iodination for the synthesis of hormone thyroxine by inhibiting the enzymes tyrosinase and tyrosine carboxylase. The mimosine occurs in almost all parts of the plants and its level in young plants is about 12 percent of dry matter and 3 to 5 percent in the seeds.

Mimosine is degraded in the rumen by the enzymes mimosinase produced by bacteria to 3,4 DPH which is further broken down to pyruvic acid and ammonia. These bacteria are present in ruminants

4.Cyanogens:

In plants the glucoside is non-toxic in the intact issues and as stated earlier, when the plants are damaged or begin to decay, hydrolytic enzymes from the same plant is released, liberating HCN. This reaction can take place in the rumen by microbial activity. The HCN is rapidly absorbed and some is eliminated through the lungs, but the greater part is rapidly detoxified in the liver by conversion to thiocyanate. Excess cyanide ion can quickly produce anoxia of the central nervous system through inactivating the cytochrome oxidase system, and death can result within a few seconds. Based on the intensity animal shows nervousness, abnormal breathing, trembling or jerking muscles, blue coloration of the lining of the mouth, spasms or convulsions and respiratory failure.

5.Nitrates and Nitrites:

Animal forages and drinking water when contaminated with inorganic nitrates and nitrites causes an acute toxicosis in cattle resulting from the formation of methmoglobin(a true oxidation product of haemoglobin) which is unable to transport oxygen because the iron is in the ferric form rather than the usual ferrous state. Symptoms seen in acute toxicity include laboured breathing (dyspena), grinding of teeth, uneasiness and excessive salivation.

6.Aflatoxins:

Aflatoxins are common term used for a group of toxins used for a group of toxins which differ in their chemical structure and in intensity of producing toxic effects. As the various compounds of this group are produced by the fungus Aspergillus  flavus and A. Parasiticus they resemble very much to each other and are commonly termed aflatoxins.

The production of toxins by the above two fungi depends on the following conditions:

1. The strain of the fungus

2. The nature of the substrate

3. The temperature and humidity of the environment immediately surrounding the area of mould growth.

4. The optimum temperature of the aflatoxins is around 27⁰C.

 

Toxicosis in farm animals by aflatoxins:                             

Species	Age	Aflatoxin content (mg/kg;ppm)	effects
Calf	Weaning	0.2-2.2	Stunting, death, liver damage
Steer	2 yrs	0.2-0.7	Liver damage
Cow	2 yrs	2.4	Liver damage
Pig	Newborn	0.23	Stunting
Pig	2 weeks	0.17	Anorexia, Stunting
Pig	4-6 weeks	0.4-0.7	Stunting,  Liver damage
Chicken	1+ weeks	0.8	Stunting,  Liver damage
Duck		0.3	Liver damage, death

 

3. CONCLUSION:

Anti-nutritional factors are present in different food substances in varying amounts, depending on the kind of food, mode of its propagation, chemicals used in growing the crop as well as those chemicals used in storage and preservation of the food substances It is found in most food substances which are poisonous to animals or in some ways limit the nutrient availability to the body.

 Plants evolved these substances to protect themselves and to prevent them from being eaten.  However, if the diet is not varied, some of these toxins build up in the body to harmful levels (Norman and Potter, 1987). Some vitamins in food may be destroyed by anti-nutritional substances. Aflatoxin in groundnut has been found to cause severe liver damage if eaten, they  are heat stable and can be degraded by strong acid and alkaline treatment (Harold and Pattee, 1985). These anti-nutritional factors must be inactivated or removed, if values of food substances are to be fully maintained.

 

 

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