Response of ruminants fed on cassava and its by-products
Nutrient content of cassava
Utilization of cassava products and by-products by ruminants
By-products of cassava tuberous root processing
Constraints to widespread utilization of cassava products
Future research needs
References
O.B. Smith
Cassava is a primary, secondary, or supplementary staple food for over 200 million people in Africa. To a limited extent, it is used as a livestock feed, particularly in nonruminant diets. Ruminants can be fed on cassava tuberous mot, foliage, peel and residue obtained after processing cassava for fufu, gari, lafun (flour), and starch. Data presented show: tubers and peel are good energy sources, which when well fortified with nitrogen, minerals, vitamins, and roughage, promoted positive and high performance levels in dairy and beef cattle, sheep, and goats: the foliage, which is rich in nitrogen. fiber and ash, is a source of nitrogen and roughage for ruminants; residue from cassava processing degrades very well in the rumen, and has a high potential as energy feed. Constraints to increased utilization of cassava products as ruminant feeds include the difficulty of obtaining sufficient amounts and the cyanide content.
World production of cassava over the last two decades has steadily increased, mainly because of increases in the areas under cultivation. According to FAO (1985), total world production in 1968 was 85.6 million tonnes grown on 9.8 million ha. By 1979, the cultivated area was 13.5 million ha, with a total production of 123.3 million tonnes. The area cultivated in 1985 increased to 14.2 million ha, with a total production of 136.5 million tonnes. The yields in tonnes/ha for the 3 periods were respectively, 8.7, 9.2 and 9.2 for 1968,1979 and 1985.
These figures are probably lower than the actual production figures, because cassava is still grown largely by subsistence farmers who cultivate small scattered plots which are often missed out during compilation for agricultural statistics. Nevertheless, based on the available figures, the annual world production of cassava is only exceeded by six other crops, mainly wheat, rice, maize, barley, potatoes, and sugar beets.
The most important use of cassava is as human food, serving as a primary, secondary or supplementary staple for over 200 million people in Africa. Cassava is also used as livestock feed, and regularly fed to sheep and goats on small-scale subsistence farms in Africa. In a recent survey of smallholder sheep and goat farmers in southwest Nigeria, a majority of the farmers indicated that cassava products and by-products were regularly fed to their animals as supplementary feed to grass and hay (Anon. 1988). The potential of cassava as a grain substitute in livestock feed is yet to be fully exploited, as only a small proportion of total world production is currently being used, mainly in compounded nonruminant diets. Ruminants can be fed not only cassava tuberous roots, but also the stem, leaves, and peel, and the various by-products of tuber processing such as residues from starch, gari, and fufu manufacture.
This paper reviews available information on the response of ruminants to a diet of cassava and its by-products. Constraints to increased utilization of these materials for ruminant feeding are identified, and solutions suggested to stimulate their increased use in ruminant feeding.
The principal products of the mature cassava plant (12 months), expressed as a percentage of the whole part, were estimated as: leaves 60%,, stem 44%, and tubers 50%. By-products of tuber processing are: peel 8% and pomace 17% (Devendra 1977). Other by-products include residues from the manufacture of gari, fufu and cassava flour (lafun). The nutritive value of any feed, and the livestock response to such feed, depend on a number of factors, including nutrient contents and availability; animal age, physiological state, the species, and associative effects of other feeds.
The proximate, mineral and vitamin contents of cassava products are shown in tables 1, 2, and 3. The variability in the values shown in the tables is due to strain and varietal differences, location, soil type, other environmental conditions, and the method of chemical analysis (Seerley 1972). Processing is perhaps the most important factor responsible for the variation. Tubers may be peeled or unpeeled, washed (ash content), sun-dried or oven-dried while leaves may be analyzed fresh, or after writing, dehydration or fermentation. Many authors often ignore these details in reporting nutrient contents, which renders comparisons inaccurate and therefore appraisal of nutritive values difficult and unreliable.
Table 1. Proximate content of cassava
|
Constituents (%) |
Leaves (foliage) |
Peel |
Tuber |
||||
|
Range |
Mean |
Range |
Mean |
Range |
Mean |
||
|
Dry matter |
19.5-33.0 |
(25.3) |
27.3-33.5 |
(29.6) |
13.0-43.3 |
(30.8) |
|
|
|
Crude protein |
14.7-36.4 |
(25.1) |
2.8-6.5 |
(4.9) |
1.5-3.5 |
(2.3) |
|
|
Crude fiber |
4.8-15.4 |
(11.4) |
10.0-22.0 |
(16.6) |
1.3-7.7 |
(3.4) |
|
|
Ether extract |
4.0-15.2 |
(12.7) |
0.5-2.2 |
(1.3) |
0.8-3.2 |
(1.4) |
|
|
NFEa |
37.3-51.9 |
(46.1) |
62.5-72.9 |
(68.5) |
88.0-94.1 |
(88.9) |
|
|
Ash |
5.5-16.1 |
(9.0) |
3.5-10.4 |
(5.9) |
1.6-4.1 |
(2.5) |
Sources: Rogers and Milners 1963, Oyenuga 1968, Seerley 1972, Devendra 1977,
Khajaren et al. 1977, Montaldo 1977, and Asaolu 1988
Note: a NFE = nitrogen free extracts
Table 2. Mineral content of cassava
|
Mineral (mg/kg) |
Leaves |
Peel |
Tubers |
|
Calcium |
1.1-1.4 |
0.31 |
0.02-0.35 |
|
Phosphorus |
0.25-0.30 |
0.13 |
0.07-0.46 |
|
Magnesium |
nd |
0.22 |
1.10 |
|
Copper |
8.0 |
nd |
nd |
|
Iron |
450 |
904 |
8-65 |
|
Manganese |
46.0 |
nd |
18.0 |
|
Zinc |
28.0 |
nd |
nd |
Source: Chadha 1961, Barrios and Bressani 1967, Devendra 1977, and Hutagalung 1977
Note: nd = not determined
Table 3. Vitamin content of cassava
|
Vitamin |
Leaf content |
Tuber content |
|
Vitamin A (I.U.) |
100 000-300 000 |
550 |
|
Riboflavin (mg) |
2.5-4.3 |
0.3-0.8 |
|
Thiamine (mg) |
0.3-2.7 |
0.4- 1.6 |
|
Niacin (mg) |
8.5-35.3 |
0.6-1.6 |
|
Vitamin C (I.U.) |
520-1800 |
5-360 |
Source: De Brochard et al. 1957, Jones 1959, Chadha 1961, Müller et al. 1975,
Hutagalung 1977, and Montaldo 1977
The tables also show that cassava tuberous root is low in protein, fat, trace minerals, and vitamins, and therefore mainly a source of energy. The bulk of the tuber (90 percent) consists of carbohydrates (Seerley 1972), made up of 34.5% fiber, and 96% nitrogen free extracts (NFE) (Hutagalung et al. 1973, Müller et al. 1975). According to Vogt (1966), the NFE is made up of 80% starch, the main soluble carbohydrate, and 20% sugar. A peculiarity of cassava tuberous root starch is the high amylopectin content (70%) making it a particularly suitable energy source for ruminants, particularly when combined with nonprotein nitrogen in feeds (Mailer 1977). Although the peel contains a higher level of crude protein than the tuber, the total protein weight in the peel is low. Like the tuber flesh, the peel is deficient in fat, minerals, and vitamins, and would be useful mainly as an energy source. The energy value varies with the amount of flesh retained during the peeling process. In contrast to the tuber, cassava leaf is rich in protein (25%), ash (9%), fat (12.7%), and fiber (11%). The protein is of good quality, and the amino acid profile apparently compares favorably with that of soybean meal (Khajaren et al. 1977). It is high in lysine but low in sulfur-containing amino acids. The high level of crude fiber of cassava leaf or foliage (leaves and stems) makes it a particularly useful source of roughage for ruminants.
Cassava foliage (leaves and stems): Leng and Preston (1976) suggested that ruminant feeding systems based on poor quality tropical foliages, crop residues or agroindustrial by-products, in which protein is one of the first limiting factors, may require additional protein and roughage to maintain an efficient rumen ecosystem that will stimulate nutrient intake and improve animal performance.
Several authors subsequently showed that cassava foliage could efficiently serve as a protein and roughage supplement to such diets. Moore (1976) demonstrated the feed value of cassava foliage for ruminants in a trial in which steers weighing 250 kg were fed Pennisetum purpureum with varying levels of cassava foliage (table 4). Feed intake, growth rate, and feed efficiency were improved in diets containing cassava foliage supplements. Another set of steers fed on a basal diet of chopped sugarcane supplemented with either cottonseed cake, cassava foliage or Desmondium distrotium foliage showed respectively, similar growth rates of 0.66, 0.62 and 0.58 kg/day.
Table 4. The value of cassava foliage as supplement to elephant grass
|
Parameter |
Diet |
||
|
100% grass |
75% grass + 25% cassava foliage |
50% grass +50% cassava foliage |
|
|
Daily gain (kg) |
0.31 |
0.46 |
0.45 |
|
Dry matter intake (kg/day) |
5.4 |
6.3 |
6.1 |
|
Feed efficiency (kg day/kg gain) |
17.6 |
13.7 |
13.7 |
Source: Moore 1976
The results of other studies designed to evaluate animal responses to cassava foliage feed as a supplementary source of protein and roughage are summarized in table 5. In general, animal responses to the utilization of cassava foliage as a protein roughage supplement to sugarcane-based diets was positive but low (Meyrelles et al. 1977b). Higher and better responses were obtained when cassava foliage was used as a supplement to molasses-urea based diets (Fernandez et al. 1977, Ffoulkes and Preston 1978). Meyrelles et al. (1977a) attributed the poor animal responses on sugarcane-based diets to:
1. Low sugar content of the sugarcane diet, a- situation aggravated by fermenting the sugarcane for 24 hours before feeding,
2. High solubility of the cassava foliage, and
3. Hydrogen cyanide (HCN) toxicity.
Indeed, evidence in subsequent trials (Meyrelles et al. 1977b, 1977c) showed that the combination of low sugar content of the basal diet and highly soluble cassava foliage protein could have contributed to the poor animal performance. No evidence was provided to implicate HCN toxicity.
Table 5. The value of cassava foliage as nitrogen-roughage source for growing cattle
|
Basal diet a |
Cassava foliage (%) |
Response |
Reference |
|
Sugarcane |
0, 15, 30, 45 |
Low response and not related to level of supplementation |
Meyrelles et al. 1977a |
|
Molasses |
3 (fresh) |
Live-daily gain of 0.58 to 0.66 kg |
Fernandez et al. 1977 |
|
Sugarcane |
0, 15, |
Low growth rate, better on cassava foliage |
Meyrelles et al. 1977b |
|
Sugarcane |
20, 40 |
Low growth rate (0.14-0.24 kg/day), better with cassava foliage +urea |
Meyrelles et al. 1977b |
|
Sugarcane |
20 |
No effect of sulfur, improved performance with cassava chips |
Meyrelles et al. 1977c |
|
Molasses |
2, 3, |
Linear increase in growth rate, 0.37, 0.47, 0.91 kg/day |
Fernandez and Preston 1978 |
|
Molasses |
4.5 (fresh) |
High intake (6.1 kg/day), high growth rate (0.9 kg/ day) on cassava foliage |
Ffloukes and Preston 1978 |
Note: a Sugarcane is chopped, cassava is in chips
An experiment with dairy cattle in Costa Rica by Murillo (1952) which compared the value of cassava leaf meal with that of alfalfa meal showed that cassava leaf meal was a valuable feed for dairy cattle. Production of cows fed on cassava leaf meal was 90-96 percent of those fed on alfalfa meal. It was therefore concluded that cassava leaf meal was an economic replacement of alfalfa leaf meal. According to Ffloukes et al. (1978), cattle fed on chopped cassava foliage only, consumed up to 2% of their body weight (dry matter) and digested the material fairly well (66.5% dry matter digestibility). More recently, Smith et al. (1988) compared the rumen degradability of some foliages in cattle and goats. In all these ruminants a similarly high 48-hour degradability of 84.3% (mean) for cassava leaves was obtained which was higher than the degradability for Leucaena leucocephala, Gliricidia sepium, bamboo and oil palm leaves. Cassava foliage is thus a valuable feed material for ruminants. The feed value apparently decreases with age, and Müller (1977) suggested the foliage should be harvested at 3-4 months to ensure high nutrient content and to avoid reduction in tuber yield.
Cassava peel: This is an important source of energy in ruminant feeding systems, serving either as the main basal diet or as a supplement. Cassava is rarely fed fresh because of the high level of cyanogenic glycoside in the material. Sun drying, ensiling and fermentation are used to reduce the concentration of the glycosides to tolerable levels. Recent studies at the Obafemi Awolowo University, Nigeria, showed that cassava peel is rapidly and well degraded in the rumen. Asaolu ( 1988) reported dry matter losses of 70% (dried peel) and 73% (ensiled peel) in 24 hours in the rumen of sheep. Odunlami (1988) obtained a t1/2 value of 26 hours for dried peel in goats. This value was similar to that obtained from cassava tuberous root meal (26.7 hours), yam peel (28.9 hours), and plantain peel (27.8 hours). Cassava peel dry matter losses in this study were 78% and 88% in 24 and 48 hours, respectively.
In another study comparing the rumen degradability of several crop residues in cattle, sheep and goats, Smith et al. (1988) reported high dry matter losses for cassava peel in the three ruminant species, with a mean value of 83% in 48 hours. These high rumen degradability values suggest that cassava peel could serve as a useful energy feed in ruminant diets.
Larsen and Amaning-Kwarteng (1976) fed grazing cross-bred cattle a supplement made up of molasses and dried or ensiled cassava peel at 0.7 percent of body weight, for about 6 months. Weight gains recorded were 0.07 kg/day for control (cattle grazed with no supplement), 0.29 kg/day for test (cattle grazed and supplemented with dried peel), and 0.33 kg/day for cattle grazed and supplemented with ensiled peel. In another study, Fomunyan and Meffeja (1987) fed sheep on three levels of dried cassava peel (O. 35, 70 percents of diet) in combination with Pennisetum purpureum at 70,35, and 0 percents of diet, respectively. Cottonseed cake was supplied as a protein supplement. Dry matter intake, digestibility and growth rate increased linearly with increasing dietary levels of cassava peel (table 6). It was concluded that cassava peel-based diets have great potential as dry season feedstuff for sheep.
Table 6. Feed value of sun-dried cassava peel for sheep
|
Parameter |
Diet |
||
|
70% grass |
35% grass + 35% peel |
70% peel |
|
|
Dry matter intake (kg/day) |
0.87 |
1.36 |
1.06 |
|
Weight gain (g/day) |
45.2 |
106.7 |
227.1 |
|
Dry matter digestibility (%) |
50.7 |
79.0 |
88.1 |
Source: Fomunyan and Meffeja 1987
Dried cassava peel fortified with urea was compared with rice-straw fortified with urea as a dry season supplement for grazing sheep in Ghana (Otchere et al. 1977). The results showed that while sheep supplemented with rice straw and cassava peel gained weight, sheep used as control lost about 15 percent of their body weight during the dry season. The sheep supplemented with cassava peel gained more weight and maintained this weight gain advantage during the following rainy season when the control animals exhibited a high degree of compensatory growth.
Cassava could also be fed dried or as silage. Optimum conditions for making good quality cassava peel silage have been worked out by Asaolu (1988), who indicated that good quality silage could be obtained when peel is chopped to equal lengths of about 2 cm for easy compaction. Moisture contents should be reduced from 70-75 % to about 40% by wilting (air drying) for 2 days before ensiling. A reduction of moisture content will ensure good fermentation even if the peels are not chopped to uniform lengths Asaolu 1988). Under these conditions, cassava peel silage after 21 days was light brown in colour, firm in texture and had a pleasant odor. The pH was 4.4, and no fungal growth was observed.
Such good quality cassava peel silage and dried peel were fed to West African Dwarf sheep by Asaolu (1988). Two groups of sheep were fed diets made up of 80 percent dried (group 1) or ensiled (group 2) peel, supplemented in each case with 20 percent Gliricidia leaves. The sheep were compared to a control group fed solely on Gliricidia The performance of the sheep is summarized in table 7 which shows that sheep fed mainly on cassava peel with a small supplement of protein-rich Gliricidia efficiently put on weight during this period.
Table 7. Performance of sheep fed ensiled or dried cassava peel
|
Parameter |
Control |
Test |
|
|
100% Gliricidia |
20% Gliricidia + 80% ensiled cassava peel |
20% Gliricidia + 80% dried cassava peel |
|
|
Dry matter intake (kg/day) |
1.0a |
0.7b |
0.6b |
|
Daily gain (g) |
106.0a |
81.0b |
59.0b |
|
Feed efficiency |
9.9 |
9.8 |
10.8 |
|
Dry matter digestibility (%) |
81.6a |
76.0b |
72.0b |
Source: Asaolu 1988
Note: Within a row, values with the same letter are not significantly different (P< 0.05)
The data confirms a previous suggestion by Larsen and Amaning-Kwarteng (1976) that sheep may utilize cassava ensiled peel better than sun-dried peel. Hydrogen cyanide (HCN) intakes on the three diets were 48.8 (control) 93.3 (ensiled peel) and 166.1 (dried peel), all values in mg/day/head (Asaolu 1988). The differences observed in the performance of the sheep were attributed in part, to the different levels of HCN in the diets. Differences in protein intake also probably had an effect.
Another dry season feed formulation based on grass-legume foliage, cassava peel and poultry excrete was evaluated by Okeke and Oji (1988). The three feed materials were ensiled in the ratio of 60:20:20 on wet basis. and fed to West African Dwarf goats. Control goats were fed a maize silage diet, both diets being isonitrogenous ( 14 percent cassava peel). On the basis of favorable consumption and digestibility of the cassava peel diet, as well as normal rumen and blood metabolites, Okeke and Oji 11988) recommended that in anticipation of dry-season feeding, cassava peel could be used as an energy supplement in an ensiled mixture of grass-legume foliage and poultry excreta.
Cassava peel has also been fed to ruminants after fermentation. Adebowale (1981) substituted fermented cassava peel for maize to feed West African Dwarf sheep at graded levels of 0, 20, 40, and 60 percents over a 6 month period. It was concluded that fermented cassava peel should not constitute more than 20 percent of a concentrate diet in order to avoid a reduction in performance.
From the foregoing, it is clear that cassava peel could be fed to ruminants either as the main energy source, or as an energy supplement to foliages or poor quality feed. Because of rapid rate of degradability of cassava peel in the rumen, maximum animal response under these two feeding strategies can only be achieved with the provision of protein feed whose rate of rumen degradability is comparable to that of cassava peel for proper synchronization of energy and protein utilization.
Apart from proximate contents there is very limited published information on the feed value of the variety of by-products obtained when cassava is processed for human consumption. In Nigeria, as in many other West African countries, cassava is processed into gari, fufu, lafun (cassava flour) and starch. The residues or by-products obtained from cassava processing are rich in fiber and soluble carbohydrates and are potential energy sources for ruminants. An indication of their potential feed value is given by the rumen degradability values obtained by Smith et al. ( 1988) for fufu residue (78.5% in 48 hours) and gari residue (88.5% in 12 hours).
Utilization of tubers
There is sufficient experimental data to suggest that ruminants respond favourably when fed on cassava tuberous roots. For example, the results summarized in table 8 show that: (a) all ruminant species can benefit from cassava feeding, and (b) associative effects of other feeds on the nature of response are important.
Ahmed (1977) fed fresh cassava tuberous roots to Friesian steers as an energy supplement to artificially dried grass. No effects were observed of the supplementation on any of the parameters, including dry and organic matter, and metabolizable energy. Cassava supplementation did not improve nutrient digestibility, probably because of inadequate dietary protein causing poor utilization of the extra energy supplied by the cassava. Rumen microbes broke down the more easily degradable soluble carbohydrates supplied by the cassava, rather than the cellulose in the basal grass diet. This led to a significant reduction in crude fiber digestibility in the cassava supplemented diet.
Table 8. Ruminant response to cassava tuberous root feeding
|
Ruminant species |
Basal diet |
Role and level of cassava tuber in diet |
Response |
References |
|
Dairy cattle | ||||
|
Cow |
Maize diet |
Substitute for maize |
Lower milk yield but cost of production reduced |
Peixoto et al. 1955 |
|
Holstein |
Grazing |
Energy supplement |
Milk yield increased by 19.5% |
Assis 1962 |
|
Holstein and Zebu |
Fresh and ensiled cane |
Energy supplement |
No response to supplement |
Estima 1967 |
|
Holstein and Zebu |
Maize diet |
Substitute up to 41.5% |
Decreased production but lower cost |
Cardoso et al. 1968 |
|
White Fulani |
Hay, maize and protein sources |
Replaced maize |
Increased milk and fat yield |
Olaloku et al 1971 |
|
Beef cattle | ||||
|
Calf |
Commercial concentrate |
Energy supplement |
Increased live weight gain |
Johnson et al. 1968 |
|
Bull |
Maize-based concentrate plus hay |
Replaced maize, but enriched with 2% urea |
Growth rate of 0.81 g/day, efficiency of 10.1; performance similar |
Lhoste 1974 |
|
Steer |
Artificially dried grass |
Energy supplement at 21% and 42% of diet |
Intake similar, fiber digestibility depressed |
Ahmed 1977 |
|
Steer |
Sorghum- based diets |
Replacement at 68%, 78%; plus urea |
Reduced organic matter intake and growth rate; digestibility and feed conversion similar |
Tudor 1985 et al. |
|
Goat |
Maize diet |
Replacement at 0, 20, 40, 60% |
Reduced performance at levels 40% and 60% |
|
|
Goat |
Gliricidia sepium |
Supplement at 30 DM/kg |
Improved digestibility and digestible dry matter intake, but reduced growth rate |
Anon. 1986 |
|
Goat |
75% Gliricidia + 25% Leucaena |
Supplement at 15 and 30 g dry matter/kg |
Improved digestibility and similar growth rate as control |
Anon. 1986 |
|
Sheep |
Pangola grass hay, corn cob and bran |
Substitution of corn-cob with cassava (20%) |
Improved digestibility, body weight gain, and rumen function |
Chicco et al. 1971 |
|
Sheep |
Rice-straw based diet |
Supplementation at 20-80% |
Increased digestibility |
Devendra 1977 |
|
Sheep |
Molasses- urea diet |
Supplementation at 20-80% |
Decreased digestibility |
Devendra 1977 |
By contrast, Lhoste (1974) obtained a much better response when maize was replaced with cassava in cattle diet. The cassava was enriched with 2% urea, and cattle weight gains obtained were 0.81 kg/day for both maize-based and cassava-based diets. On both diets, cattle required 10 kg feed dry matter per kg weighs gain. It was therefore concluded that it was feasible to substitute cassava for maize, provided urea was added to offset a nitrogen shortage.
A series of experiments was carried out by the Goat Research Group of the Obafemi Awolowo University in Nigeria during 1986 to evaluate the response of goats fed cassava tuberous roots as an energy supplement to grass-legume foliage basal diets. A summary of the results is shown in table 9. In the first trial, goats were fed a basal diet of Gliricidia ad libitum, with or without cassava supplement at the rate of 30g dry matter/kg metabolic weight. The cassava was fed to the goats three hours before the basal Gliricidia offered. The results showed an improvement in dry matter digestibility, which was however not reflected in the growth rates.
This discrepancy was attributed to a poor energy-nitrogen synchronization in the rumen. Most of the energy supplied by the cassava supplement was already dissipated before the nitrogen from the basal diet was made available.
Table 9. Response of goats fed grass-legume forages to cassava supplement
|
Basal diet |
Method and level of cassava feeding |
Digestibility (%) |
Growth rate (g/day) |
||
|
basal |
basal + cassava |
basal |
basal + cassava |
||
|
Gliricidia |
30g dry matter/kg weight, fed once a day 3 hrs before basal diet |
nd |
nd |
23.3 |
14.3 |
|
Leucaena (25%)+ Gliricidia (75%) |
30g dry matter/kg weighs, fed once a day 2 hrs before basal diet |
59.3 |
70.8 |
37.4 |
43.4 |
|
Guinea-grass hay (50%) |
30g dry matter/kg weight, split-fed three times a day |
60.5 |
72.7 |
18.6 |
53.6 |
|
+ brewers grain (50%) |
|
|
|
|
|
Sources: Obafemi Awolowo University Coat Research Group 1986; Odunlami 1988
Note: nd = not determined
This was confirmed by the t 1/2 values (in hours) of the feeds used: cassava 26.7, gliricidia 53.7, leucaena 88.9, guinea-grass hay 210.0, and brewers grain 46.2 hours. Taking these into consideration, the feeding pattern was changed in subsequent experiments. In experiment 2, cassava was fed 2 hrs after the basal diet. The results showed that the growth rates reflected the digestibility patterns. In the third experiment, cassava supplement fed once a day a few hours after the basal feed was compared with split-feeding three times a day, simulating a steady energy release that matched the slow N release from the basal diet. The results indicated that supplementation improved digestibility; growth rate figures suggested an improved energy nitrogen utilization. It could be concluded that in order to obtain maximum response from ruminants fed on cassava tuberous roots, two peculiarities of the material need to be recognized-the low nitrogen content and the rapid degradability in the rumen. Appropriate measures such as fortification with nitrogen, preferably nonprotein nitrogen, and split-feeding several times a day will ensure efficient utilization of nitrogen-poor basal feed by ruminants.
One major constraint to the use of cassava products and by-products for ruminant feeding is the problem of obtaining sufficient amounts especially for beef and dairy cattle. By-products such as fufu and gari residues are not produced in sufficiently large quantities at accessible spots for easy collection and feeding to ruminants on a large scale. At present, they can only be used by small-scale farmers to feed sheep and goats.
Although the prospects of cassava leaves in ruminant diets appear promising, little is known about the effect of frequent foliage harvest on root yields. In order to fully exploit the potential feed value of cassava foliage for ruminants, agronomic studies are required to provide information on appropriate varieties for high foliage yield, plant population, cutting intervals and fertilizer practices.
Perhaps the greatest constraint to feeding cassava to ruminants is the problem of cyanide toxicity. Cassava plants possess two cyanogenic glycosides, linamarin and lotaustralin, which are broken down by hydrolytic enzymes present in the plant to hydrocyanic acid (HCN), a toxic compound, and acetone and glucose. The HCN content of cassava varies with variety, growing conditions, age of plant, and the part of plant. Typical values reflecting this variability are 568-950 mg/kg in root bark and 2200 mg/kg in fresh root pulp.
The level of HCN that causes toxicity in animals is not known precisely, although it has been suggested that levels below 50 mg/kg are harmless. Acute HCN toxicity in ruminants is not common. Devendra (1977) reported a 40 percent mortality in weaned kids fed fresh leaves of a bitter variety containing 180-240 mg/kg of cyanide. Poor performance of ruminants fed on cassava products has sometimes been attributed to chronic HCN toxicity, although other reasons such as low dietary nitrogen could also be responsible. There is evidence that ruminants can use a variety of sulfur donors including elemental sulfur to detoxify cyanide of dietary origin. Thus, Blakley and Coop (1949) have indicated that HCN was rapidly detoxified in the rumen and liver by reactions using sulfide ions or cystine, concluding that approximately 1.2g of sulfur was required to detoxify 1.0g of HCN. Observations by other workers (Wheeler et al. 1975) showed that the supply of sulfur licks to ruminants effectively protected them against chronic cyanide toxicity. Therefore, HCN toxicity should not constitute a constraint to the feeding of cassava products and by-products to ruminants, as simple processing techniques such as sun-drying, ensiling and fermentation combined with the provision of adequate dietary sulfur effectively protect the animals from HCN toxicity.
Some research needs that have been identified by this review are as follows
· Evaluation of the feeding value of the by-products of cassava processing and the development of feeding systems or packages based on these products.· Agronomic studies on the production of cassava foliage specifying suitable varieties, plant populations, fertilizer requirements and cutting intervals, among other things.
· Development of appropriate feeding systems for cassava products to ensure optimal response.
· Detailed studies on chronic cyanide toxicity as it affects performance and the role sulfur supplementation could play in alleviating the condition.
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