Roots, tubers, plantains and bananas in animal feeding (2024)

INTRODUCTION

The use of cassava in livestock feeding has been limited. Reasonsinclude the presence of toxic cyanogenic glucosides, deficiency innutrients other than energy, dustiness of the dried products, mouldinessduring processing and the high fibre and ash content of the peel, whichlimits the selection of other ingredients which are high in these components.Nevertheless, the development of cassava products which meetminimum requirements for incorporation into commercial livestock feedproduction, in cassava producing areas, would certainly relieve thepressure on demand for available cereal grains. Additionally it wouldhelp guarantee the supply of energy for livestock feeding, in theseregions, that are perennially acutely short of animal feed ingredients anddue to unfavourable trade balances are unable to make up deficiencieswith imports.

As the presence of cyanogenic glucosides constitute a major limitationto the use of cassava in both human and animal foods there is the needto review current findings for the elimination of the toxic glucoside incassava products and also to examine the implications of feeding cassavaand its products on livestock production.

Nature of Cassava toxin

Cassava is fed to livestock in the fresh or processed form. In the wholeunbruised plant the cyanogenic glucoside remains intact in the form oflinamarin and lotaustralin. When the cellular structure is disrupted, theintracellular glucoside becomes exposed to the extracellular enzymelinamarase. Hydrocyanic acid (HCN) is then produced. The reaction hasbeen shown to proceed in two steps by Nartey, (1978) viz:

1. Cyanogenic glucoside is degraded to sugar and cyanohydrin (x - hydroxynitrile);

2. Cyanohydrin then dissociates to ketone and hydrocyanic acid. Thus,for linamarin the glucoside is first hydrolysed by linamarase toproduce B-D-glucopyranose and 2 - hydroxyisolentyronotrite oracetone - cyanohydrin, after which the latter is degraded to acetoneand HCN. Cyanohydrin produced as a result of linamarin activityis stable only under moderately acidic condition (pH 4.0); in neutralor alkaline condition it undergoes spontaneous hydrolysis to yieldHCN (Cooke et al. 1985).

In spite of the relative instability of cyanohydrin it coexists with intactglucoside and HCN in differently processed cassava products. It istherefore clear that the cyanide in cassava products exists in three forms:(i) the glucosides (linamarin and lotaustralin), (ii) the cyanohydrin and(iii) the free hydrocyanic acid (HCN).

However, the quantitative estimation of cyanide by various methods hasproduced incomparable results, and in many cases a grossunderestimation, emanating from quantification of free HCN alone in thereports of earlier investigators. The harmonization of current analyticaland presentation methods is therefore suggested.

EFFECT OF CASSAVA PROCESSING ON CYANIDE LEVEL

Cassava tubers are traditionally processed by a wide range of methods,which reduce their toxicity, improve palatability and convert theperishable fresh root into stable products. These methods consist ofdifferent combinations of peeling, chopping, grating, soaking, drying,boiling and fermenting. While all these methods reduce the cyanidelevel, the reported loss in cyanide content differs considerably due toanalytical methods, the combination of methods and extent to which theprocess(es) is(are) carried out.

The specific effects of various processing techniques on the cyanidecontent of cassava are discussed below:

Peeling

Many methods of processing cassava roots commence with the peelingof the tubers. Generally the cassava peel contains higher cyanide contentthan the pulp. Removal of the peels therefore reduces the cyanogenicglucoside content considerably. In studies carried out by the author, thepeel of the “bitter” cassava variety was shown to contain on average 650ppm and the pulp to contain 310 ppm total cyanide; the correspondingvalues for “sweet” varieties were 200 ppm and 38 ppm respectively. Theabove classification is conveniently based on the cyanide content; with thesweet varieties having most cyanide in the cortex and skin and little orno cyanide in the pulp, whereas the bitter varieties, more or less, havean even distribution of cyanide throughout the tuber. For these reasonsthe former can be eaten boiled while the latter has to be processed beforeit can be consumed.

Peeling, therefore, can be an effective way to reduce the cyanidecontent by at least 50% in cassava tubers. However, it should be notedthat while the peel contains a high glucoside content relative to the pulp,the glucosidase level is higher in the latter.

Grating

This process takes place after peeling and is sometimes applied to wholetubers. Grating of the whole tuber ensures the even distribution of thecyanide in the product, and will also make the nutrients contained in thepeel available for use. In the grated product, the concentration ofcyanide depends on the time during which the glucoside and theglucosidase interact in an aqueous medium.

Grating also, obviously, provides a greater surface area for fermentationto take place.

Soaking

Soaking of cassava roots normally precedes cooking or fermentation. Itprovides a suitably larger medium for fermentation and allows for greaterextraction of the soluble cyanide into the soaking water. The processremoves about 20% of the free cyanide in fresh root chips after 4 hours,although bound cyanide is only negligibly reduced. Bound cyanidebegins to decrease only after the onset of fermentaion (Cooke andMaduagwu, 1978). A very significant reduction in total cyanide isachieved if the soaking water is routinely changed over a period of 3–5days.

A variation to the soaking technique known as retting, was describedby Ayenor (1985). This process involves prolonged soaking of cassavaroots in water to effect the breakdown of tissue and extraction of thestarchy mass. A simulation of the technique, followed by sundryingshowed a reduction of cyanide of about 98.6% of the initial content in theroots.

Boiling/Cooking

As with soaking, the free cyanide of cassava chips is rapidly lost inboiling water. About 90% of free cyanide is removed within 15 minutesof boiling fresh cassava chips, compared to a 55% reduction in boundcyanide after 25 minutes (Cooke and Maduagwu, 1978). Cookingdestroys the enzyme linamarase at about 72°C thus leaving a considerableportion of the glucoside intact.

Fermentation

Microbial fermentations have traditionally played important roles in foodprocessing for thousands of years. Most marketed cassava products like“garri”, “fufu”, “pupuru”, “apu” etc., in Africa are obtained throughfermentation. The importance of fermentation in cassava processing isbased on its ability to reduce the cyanogenic glucosides to relativelyinsignificant levels. Unlike alcoholic fermentation, the biochemistry andmicrobiology is only superficially understood, but it is believed that somecyanidrophilic/cyanide tolerant microorganisms effect breakdown of thecyanogenic glucoside. It has been shown that the higher the retention ofstarch in grated cassava the better the detoxification process. This couldbe attributed to the fermentative substrate provided by the starch. Also,the longer the fermentaion process the lower the residual cyanide content.

In Nigeria, investigation of the effect of fermentation period on theresidual cassava toxins is currently being carried out. As a preliminarystage, the use of starter cultures recovered from fermentation effluents isbeing tested to increase the conversion of substrate to product and reducefermentation time.

However, Cooke and his co-workers using irradiated cassava found thatmircroorganisms are not necessarily involved in the breakdown ofcyanogenic glucosides. It is therefore clear that the effect of themicroorganisms on cyanide detoxification requires further investigation.

Generally, fermented cassava products store better and often are low inresidual cyanide content. Onabowale (1988) developed a combined acidhydrolysis and fermentation process at FIIRO (Federal Institute forIndustrial Research, Oshodi, Nigeria) and achieved a 98% (approx.)reduction in total cyanide after dehydration of the cassava flour for usein the feeding of chickens.

A process, which can be described as “dry fermentation”,is believedto occur in cassava peelings which are usually heaped for days, in manyparts of Africa, before feeding to ruminants. The process generates heatand mould growth is common. However, the measurement of HCNlosses during such a process has not been documented.

Ensiling

The ensiling process causes the disintegration of the intact glucoside viamarked cell disruption, drop in pH of ensiled medium and intense heatgeneration.

Ensiled cassava roots have been used for livestock feeding. Gomez andValdivieso (1988) reported that ensiling cassava chips reduced thecyanide content to 36% of the initial value after an ensiling period of 26weeks. We have also found that about 98% of the free cyanide was lostby ensiling cassava roots with poultry litter for 8 weeks.

Drying

Since cassava root contains about 61% water, coupled with the solubilityof its cyanogenic glucoside component, the dehydration (dewatering)process results in a substantial reduction in the content of this toxin in thepressed pulp. Drying is carried out using solar radiation (sundrying) orDriers (electric or fuel) depending on economic viability. The processis achieved at varying temperature.

Work by the author has shown that sundrying:

1. Results in a greater loss of total cyanide compared to laboratoryoven-drying at 60°C for 48 hours. Oven-drying apparentlyaffects the stability of linamarase which decomposes at 72°C.

2. Tends to produce greater loss of bound cyanide due to slowerdrying rate relative to oven drying.

3. Allows a longer contact period between the glucosidase and theglucoside in the aqueous medium. The effectiveness of enzyme/substrate interaction will, however, be dependent on the particlesize and environmental factors such as ambient temperature,insulation, relative humidity and wind velocity. Thus propersundrying is achieved in between 1–3 days in the dry season andin up to 8 days during the rainy season.

4. Facilitates the continuation of the fermentation process.

5. Is cost effective, but slow and often encourages the growth ofmould and other micro organisms including Aspergillus flavus(pathogenic), A. fumigatus; A. cherahen; A. teirenus; A. flaripes;A. japonicus; A. niger; A. ochracuss; and Penicillium rubrum(Clerk and Caurie 1968; Oke, 1978). This microbial growth canexpose the consuming animal to aflatoxicosis and/or mycotoxicinfection.

Because of the poor microbiological properties of sundried cassavaproducts, there is a need for quicker drying methods which will reduceor eliminate microbial proliferation and ensure optimal cyanidedetoxification.

An improvement in sundrying of cassava roots using inclined traydryinginstead of drying on concrete floors was reported by Gomez et al.(1984). The residual total cyanide content was 10–30% of the freshsample, with about 60–80% of the cyanide in the dried chips occurringas free cyanide. The comparative advantage of this method could be dueto good conductivity of the tray. Gomez et al. (1984) indicated thatmore than 86% of HCN present in cassava was lost during sundrying.Bound cyanide which is less volatile can be a greater contributor tocyanide toxicity in sundried products than free HCN which vaporizes atabout 28°C. yet the former is frequently unestimated though potentiallytoxic.

Table 1 shows the hydrocyanic acid content of cassava and its productsused for livestock feeding.

Source: Tewe and Iyayi (1989)

EFFECTS OF RESIDUAL TOXINS

Cassava toxicity

The cyanogenic glucosides were initially thought to be of little consequenceto mammals as long as the cassava hydrolytic enzyme had beeninactivated. However, the ingestion of high concentrations of cyanogenicglucosides from fresh cassava roots and leaves have been reported to belethal in numerous species of animals. This was because the possibilityof hydrolysis during digestion was not adequately understood, despiteearly reports that oral doses of pure linamarin produced physiological andbiochemical changes in rats and chick embryos even in the absence oflinamarase activity (Philbrick et al. 1977; Maduagwu and Umoh 1988).

The subject is now better understood. On excess consumption ofunprocessed cassava there is the enzymatic breakdown of the glucosidereleasing HCN and thereby causing poisoning.

Cassava toxicity may be acute and/or chronic. Acute toxicity resultsfrom ingestion of a lethal dose and death is caused by the inhibition ofcytochrome oxidase of the respiratory chain by cyanide. This has beenreported in goats ingesting cassava leaves (Obioha, 1972), and also innon-ruminants, like pigs, when fed fresh uncooked tubers.

The level of total HCN varies widely in cassava tubers, and death hasbeen more common with the “bitter” varieties containing levels of HCNhigher than 500ppm (Tewe and Iyayi, 1989). Where sub-lethal doses ofcyanide are consumed, the inhibition of cellular respiration can bereversed by the removal of HCN by respiratory exchange or thedetoxification process. The latter proceeds via many pathways, thoughprobably the most important is the reaction of cyanide with thiosulphateto form thiocyanate and sulphite. The cyanide is initially trapped in theerythrocyte fraction of the blood and later converted to the less toxicthiocyanate.

Chronic cyanide toxicity on animals can affect both the growth andreproductive phases of development, each of which will be consideredlater.

It should be pointed out that, while the lethal dose has been estimatedat between 0.5 and 3.5 mg/kg body weight or 30 and 210 mg for 60 kgadult human, the lethal dosage for various animal species has not beenestablished. Bolhuis (1954) classified the toxicity of cassava cultivars asfollows:

1. Innocuous: less than 50ppm fresh peeled tuber;

   See Also

   A review of cassava in Africawith country case studies on Nigeria, Ghana,the United Republic of Tanzania, Uganda and Benin

2. Moderately poisonous: 50–100ppm fresh peeled tuber;

3. Dangerously poisonous: more than 100ppm fresh peeled tuber.

A reclassification should take into consideration the potentiallyreleasable, bound cyanide, and so correct the deficiency of that ofBolhuis, which assumed that all cyanide was available as free HCN.

Effect of chronic Cassava toxicity on the growth phase

The ingestion of fresh or processed cassava based diets causes reducedgrowth rates in rats, pigs, African giant rats, sheep and goats (Tewe etal., 1977; Tewe and Maner, 1981; Tewe, 1983). The animals also haveincreased serum and urinary levels of thiocyanate, which is a continuouscause of depletion of sulphur containing amino acids (Tables 2 and 3).The thiocyanate also inhibits the intra-thyroidal uptake of iodine, causesan increase in secretion of thyroid stimulating hormone (TSH) and causesa reduction in thyroxine level which is necessary for growth. It is thusa goitrogenic factor, which was demonstrated by Tewe et al. (1984), whor*ported a significant reduction in serum thyroxine levels in growing pigsfed cassava peel diets containing 96 ppm total cyanide (Table 4).

In rats and pigs consuming inadequate amounts of protein and sulphuramino acids, the serum thiocyanate concentration becomes lower as theanimals become unable to adequately detoxify cyanide. Additionally, thiscondition can also aggravate deficiencies in selenium, zinc, copper andvitamin A. Even with sufficient protein intake, consumption of cassavaflour based rations can result in parakeratosis in pigs, attributable to zincdeficiency, aggravated by the cyanide in cassava diets. Other featuresinclude paralysis of the hind limbs and muscular weakness.

a,b,c: means with differentsuperscripts in horizontal rows are significantly differentP<0.01).

Source: Tewe and Kasali, (1986).

a,b,c Meanswithout common superscript in horizontal rows are significantlydifferent (P<0.05)

^{a, b} means without common superscripts in horizontal rows are significantly different (P<0.05).

Source: Tewe et al., 1984

In poultry, there are scant reports of toxicity due to cassava cyanide.However, depression in growth rates of broilers consuming cassava dietsis common, and especially when a significant amount of the grain isreplaced without proper protein supplementation. This observation isascribed to a lower protein content in cassava and the extra need forsulphur amino acids. The author has shown, however, that the performanceof poultry on cassava diets is satisfactory as long as the total HCNcontent in the final ration does not exceed 100 ppm. Such rations musthowever be nutritionally balanced, and in particular contain sufficientsulphur containing amino acids.

Effect of cassava chronic toxicity in the reproductive phase

Chronic cyanide toxicity appears to pose more problems with breedingstock as they remain on farms longer than growing animals. However,very few studies have been conducted in this area.

Studies carried out with gestating pigs (Tewe and Maner, 1981),showed that, when fed fresh cassava containing 0, 250 and 500 ppmcyanide, maternal and foetal serum thiocyanate levels only increased inthose receiving the 500 ppm CN diet (Table 5). In this study a slightincrease in the thyroid weight, with increasing levels of cyanide, wasonly observed, in pigs fed the two lower levels of CN, with definitepathological changes noted in the thyroids of those fed the 500 ppm CNdiet.

Although the consumption of the cassava diet during gestation did notaffect performance during lactation, milk thiocyanate and colostrumiodine concentrations were significantly higher (P>0.05) in the animalsfed diets containing the highest level of cyanide. Otherwise, the size oflitters and weights of the young produced from pregrant rats and pigs fedon the various cassava diets were essentially normal.

Maner (1972) reported that a fresh cassava based diet had an identicalnutritional value to a corn based diet fed gestating pigs. However, in thisstudy the cassava fed sows, also maintained on pasture, had an increasedstill-birth rate and slightly inferior weight gains in post-lactation.

Source: Tewe, 1983

Means followed by different superscripts, in horizontal rows, are significantly different(P>0.05)

Studies have also been carried out at Obafemi Awolowo University, IleIfe,Nigeria on the reproductive performance of rabbits fed cassava baseddiets. These were carried out over three breeding periods and showedthat the performance of pregnant and lactating does, were insignificantlydifferent, from those receiving non cassava diets, in terms of litter sizeand birth and weaning weight of offspring (Omole and Onwudike 1982).

SUPPLEMENTAL VALUE OF NUTRIENTS

Protein and amino acids

The quantity and quality of protein supplementation in cassava based dietsis critical, and especially with regard to the content of sulphur containingamino acids. Elemental sulphur as well as methionine supplementationhave been reported to significantly improve protein utilization in pigs(Job, 1975). The requirement for sulphur-containing amino acid is foruse in the rhodanase detoxification pathway.

Iodine and other dietary minerals

There are little or no reports of specific extra-requirements for otherminerals in the diets of animals consuming cassava products. However,as already discussed, since thiocyanate resulting from cyanidedetoxification competitively inhibits iodine uptake, there is a need foriodine supplementation to avoid the thyroid malfunctioning. Cyanideaggravation of selenium, zinc, and copper deficiencies also calls for thesupplementation of cassava diets with these minerals.

Palm Oil

The use of palm oil has been shown to be of benefit when feedingcassava based diets. Omole and Onwudike (1982) found that when rabbitsfed diets containing up to 50% of cassava peel meal, were supplementedwith palm oil, their serum thiocyanate levels remained unaltered. Theimproved performance with feeding the palm oil was attributed to theincreased calorie intake of the animals. Formunyan et al. (1981) alsoreported that the rate of hydrolisis of cyanogenic glucosides in cassava,to produce the toxic hydrogen cyanide, is greatly reduced in the presenceof palm oil. They suggested that this occurs because the supplemental oildelays the decomposition and therefore prevents the absorption of thecyanogenic glucosides.

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