Ant-Nutritional Factors Limiting their Optimum Use
The anti-nutritional factors (ANFs) are non-fibrous natural substances which occur as natural constituents of plants and animal feeds, as artificial factors added during processing or as contaminants to the eco-system. Ruminants and mono-gastric species frequently differ in susceptibility to the same toxicant, mainly due to the action of rumen microorganism. For example, trypsin inhibitors which are ANFs for mono- gastric animals, do not exert adverse effects in ruminants because they are degraded in the rumen. Chicken are less sensitive to ANFs than the piglets. ANFs can be broadly classified as proteins (lectins, proteinase inhibitors), non-protein amino acids (neurolathyrogens, canavanine, mimosine), carbohydrates (galactomannan gums), polyphenolic compounds (tannins), glyc osides (cyanogens, saponins), alkaloids (Erythrina alkaloids), metal-binding agents (phytic acid) and indospicine in the creeping indigo (Indigofera spicatu).
Commonly encountered toxic / anti-nutritional factors in several of the non- conventional feedstuffs is summarized and presented in Table 9. Soybean meal contains toxic and inhibitory substance such as allergenic, goitrogenic and anti-coagulant factors. Protease inhibitors are responsible for the growth retarding property when raw soybeans are fed. The inhibitors are inactivated by heating and hence the preference in the use of roasted meal in place of raw soybean meal. Cottonseed contains gossypol which is a polyphenols aldehyde and acts as anti-oxidant and polymerization inhibitor, toxic to mono-gastric animals. The use of rapeseed meals (produced from Brassica napus) had some restrictions for their use in swine and poultry rations due to the presence of glucosinolates accompanied by thioglucosidase known as myrosinase. These exhibits a variety of toxic effects manifested as goiters and liver and kidney poisoning. Canadian meals are produced from canola varieties which have less than one-eighth of the glucosinolate content of traditional rapeseed meals. Evidence on permissible levels of inclusion of rapeseed meal in the diet is conflicting and those for high and low glucosinolates differ markedly. Extensive research studies have been carried out to find out the effective levels of incorporation in the rations of livestock based on the nature of anti-nutritional factors present.
Oilseed meals such as groundnut cake are highly susceptible to mould infestation leading to ailatoxicosis when stored in sub-optimal conditions. There are several reports of deaths in calves less than 6 months of age when fed on contaminated groundnut meal.Inclusion of 150-200 kg/ tonne of toxic groundnut in dairy cow diets have shown significant fall in milk yield. Exposure to sunlight and use of hydrated sodium calcium alumino-silicates and use of cultures of yeast such as Saccharomyces cerevisiae1026 have proved to be beneficial in controlling incidence of aflatoxicosis. Tolerance levels of aflatoxicosis were reported by CLFMA to range from 150 ppb in purebred chicks to 200 - 400 ppb in crossbred broiler chicks.
Tannins: These are polyphenolic compounds of high molecular weight and are, found in two forms namely, hydrolysable tannins and condensed tannins. The tannins form complexes with proteins, cellulose, starch etc and interfere in their digestion. Numbers of chemical treatments were developed to remove tannins but none has been commercially exploited. Salseed meal and tree leaves are also rich sources of tannins.Hydrolysable tannins are potentially toxic and cause poisoning in animals if sufficiently large amounts of tannin containing plant material are ingested. However, pyrogalol, the decarboxylated product of gallic acid, is produced in high concentration in the rumen of sheep and causes mithaemoglobinaemia. In mongastric animals, or in ruminants if the rumen is by-passed, hydrolysable tannin such as tannic acid can be absorbed through intact or injured gastro-intestinal tract and ultimately causes kidney and liver necrosis. Tannins also inhibit the activity of the enzymes of rumen microbes.
Condensed tannins, unlike hydrolysable tannins are not toxic to the ruminants, and when their concentration is below 4 per cent of dry matter they improve the nutritive value of herbage by binding to plant proteins and protecting them from excessive degradation in the rumen, besides preventing the establishment of parasitic nematodes. Incorporating CT into forage diets at a concentration of 1-4 % of dry matter is likely to improve protein and energy absorption and production.The two types differ in their nutritional and toxic effects. The condensed tannins have a more profound digestibility reducing effect than hydrolysable tannins, whereas the latter may cause varied toxic manifestations due to hydrolysis in the rumen. Formation of complexes of tannins with nutrients, especially proteins, has both negative and positive effects on their utilization. In small quantities (2-3%), condensed tannins are useful as they prevent bloat and protect proteins but when present in large quantities, reduce forage quality.
Saponins: The saponins are widely distributed in plants like lucerne, clovers and in mahua seed cake: The ruminants can breakdown saponins in the rumen while mono- gastric animals cannot.Saponins are characterized by bitter taste and foaming properties thereby affecting the palatability and feed intake. The aglycones in certain saponins increase the permeability of the membranes of red blood cells. Feeding of saponin rich feedstuffs can cause substantial reduction in the protozoal numbers, reduce rumen degradation of feed protein, decrease the amount of microbial protein flowing to the small intestine and decrease the apparent digestibility of the diet.
In ruminants, some reports of toxicity due to dietary saponins have also appeared, symptoms include listlessness, anorexia, weight loss and gastro-enteritis. Mowrin, a saponin in Brassica latifolia seed cake is not toxic to calves when consumed orally at as high a level as 94 g/d. This indicates that saponins from different plant species have varied biological effects probably due to structural differences in their sapogenin fractions. The adverse effects of saponins can be overcome by repeated washing with water which makes the feed more palatable by reducing the bitterness associated with saponins.
Protease Inhibitors: These are small protein molecules and inhibit the activity of proteolytic enzymes trypsin and chymotiypsin. The pancreas responds to these inhibitors by enhancing the synthesis and secretion of proteolytic enzymes which are regulated by feed-back mechanisms. This results in pancreatic hypertrophy and diverts the supply of sulphur amino acids from the synthesis of body tissues to that of the proteolytic enzymes. A relationship between pancreatic size and the response of species to protease inhibitors has been observed. Amylase inhibitors also occur in beans which reduce the digestibility of starch in non-ruminants.The trypsin inhibiting activity of beans differs little among varieties and from year to year and any variation is possibly due to differences in maturity. Haemagglutinin activity is very high in lupin seeds, low or absent in peas and fairly low in beans.
Lectins (Phyto-hemagglutinins): Lectins are proteins in chemical nature and agglutinate red blood cells. The highest concentrations of lectins are found in seeds but, in the leaves, their concentration is low due to translocation. The injurious effect of lectins is due to their affinity to carbohydrate moieties on cell membranes of the intestinal wall and causes a non-specific interference with nutrient absorption.Ricin occurs in castor beans (Ricinus communis) which has been reported to cause poisoning in all classes of livestock. Due to ricin, de-oiled castor seed cake (CP 35%) is seldom used as livestock feed. However, castor bean meal can be detoxified by autoclaving at 20 psi for 60 min for incorporation in sheep diets. The lectin in a toxic and non-toxic variety, of Jatropha curcas was inactivated by heating and moist heat was more effective than dry heat.
Cyanogens: Cyanogens are glucosides of a sugar and a cyanide containing aglycoside. Intact cyanogens are innocuous but can be hydrolyzed by enzymes to release HCN which is toxic. Ruminants are more susceptible to HCN toxicity than the non-ruminants. The presence of two cyanogenic glycosides, linamarin and lotaustralin, in cassava is a major factor limiting its use as a feed. The HCN in cassava can be detoxified by fermentation, acid hydrolysis or both. Acid hydrolysis is most effective in removing 98% of the total cyanide content of cassava roots while fermentation combined with acid hydrolysis and fermentation alone can reduce cyanide content by 95 and 88 %, respectively.
Glucosinolates (Thio-glucosides): Glucosinolates are thio-esters containing an organic aglycone which are also called goitrogenic glycosides. A number of approaches have been studied for removal of glucosinolates and hydrolyzed aglycone to improve the quality of rapeseed by-products. These include chemical and physical treatments,ammoniation, aqueous ethanol extraction, methanol ammonia water extraction, heat treatment followed by water extraction, preparation of protein concentrates and aqueous enzymatic processing. Bacteria ‘Bacillus cereus’ and yeast ‘Trichosporon cutaneum’ with the ability to degrade glucosinolates was isolated during the degradation of thi - oglucosides in the defatted rapeseed meal. Rapeseed contains glucosinolates and erucic acid which are injurious to livestock. Canadian plant breeders have evolved cultivars in which both these substances are in very low quantities the important variety developed being canola which is currently cultivated in the northern India in place of conventional rape seed.
Gossypol: Gossypol is a polyphenol present in cotton seeds leading to toxicity if not processed scientifically. It binds iron as well as proteins and amino acids essentially the same as do tannins. Gossypol is known to affect liver enzymes as well. Gossypol impairs energy metabolism, respira tion of spermatozoa and inhibits testicular steroidogenesis. These might be the biochemical reasons for gossypol induced male infertility. There is a positive correlation between free gossypol and available lysine; as free gossypol decreases, so does available lysine. Cotton varieties with low gossypol content have also been evolved.