Chapter 5: Roots, tubers, bananas and plantains

Maize, high in energy due to its low fibre content and the high digestibility of its starch, is frequently used as the standard with which to compare other sources of energy for animals. However, roots, tubers, bananas and plantains are also rich sources of carbohydrate energy and, as such, constitute one of the basic components of the human diet in the tropics and subtropics. Because they can tolerate a wide range of climatic conditions and infertile or marginal lands, their production is relatively stable which means that they are particularly well suited to sustainable farming systems.

The energy in roots, tubers, bananas and plantains is in the form of starch, and the structure of the starch in tubers does not differ significantly from that in cereals. The main objective of this chapter is to show that the digestible energy content of these alternative feedstuffs approximates that of cereals, therefore, they might offer considerable potential to furnish a considerable large share of the nutrients currently provided by more conventional energy sources for pigs (FAO, 1992).

BANANAS AND PLANTAINS

Although bananas (Musa cavendishii) and plantains (Musa paradisiaca) are mainly used as human food, a considerable amount of reject fruit could be fed to livestock, particularly to pigs. The vegetative part of the plant, the pseudo-stems and leaves, contains more than 60% of the dry matter of the whole plant (Table 5.1) and has been used experimentally as meal for pigs in concentrate rations (Garcia et al., 1991a,b).

Green bananas contain 20 to 22% of dry matter, mainly in the form of starch. When bananas ripen, the starch changes into simple sugars, sucrose, glucose and fructose. Compared to the ripe pulp which contains only 0.5% lignin some 60% of the crude fibre in the whole green banana is lignin, and this affects its digestibility (Van Loesecke, 1950). The inorganic fraction is poor in calcium and phosphorus but rich in potassium. Both green and ripe bananas have a low crude protein content and are particularly deficient in lysine and in the sulphur-containing amino acids. Additionally, green bananas contain tannins which cause the unripe banana to taste bitter and which can affect palatability, and therefore, voluntary consumption.

Table 5.1. Average yield of different parts of banana plants.

Fresh Dry matter
Parts of the plant kg % kg %
Pseudo-stem 27.0 60.5 4.2 54.5
Leaves 6.4 14.3 0.5 6.0
Fruit 11.2 25.2 3.0 39.0
Total 44.6 100.0 7.7 100.0

Source: Foulkes et al. (1978)

Chemical composition and digestible nutrients

The chemical composition of bananas and plantains is shown in Table 5.2.

Table 5.2. Chemical composition of bananas and plantains (% DM).

Components DM CP CF EE Ash NFE
Banana pseudo-stems * 5.1 2.4 20.5 2.3 14.3 60.5
Banana pseudo-stems ** 6.3 4.9 25.1 - 17.7 -
Banana leaf meal * 94.1 9.9 24.0 11.8 8.8 45.5
Fresh banana leaves ** 19.5 11.4 28.3 - 10.9 -
Plantain pseudo-stems * - 2.8 13.8 1.2 15.6 66.6
Plantain leaves * - 9.5 23.1 5.6 13.3 48.5
Green bananas * 20.9 4.8 3.3 1.9 4.8 85.2
Ripe bananas * 31.0 5.4 2.2 0.9 3.3 88.2

Source: * FAO (1993); ** Garcia et al. (1991a)

The digestibility of raw or cooked, peeled or non-peeled, green or ripe and fresh or dried bananas for pigs is presented in Table 5.3. The high level of free active tannins in fresh green bananas and their residual presence in fresh ripe bananas is reflected in their negative protein digestibility. The ripe meal form produced the poorest results with regard to nutrient digestibility. This was no doubt due to the elevated temperature employed during processing which destroyed many of the nutrients.

Table 5.3. The digestibility of different forms of ripe and green bananas for pigs.

Type of banana % digestibility of: DE (MJ /kgDM) Source
DM OM N GE
Green bananas

Ripe bananas

Ripe banana meal

Green banana meal

76.9

84.3

50.5

83.6

-

-

-

-

-102.0*

-42.7

-126.6

3.4

-

-

-

-

13.39

13.05

7.13

13.42

Clavijo and Maner (1973)
Green bananas

Cooked green bananas

Peeled green bananas

Ripe bananas

83.5

87.9

88.6

89.5

84.2

88.6

89.1

90.1

-19.0

26.4

-1.6

38.4

79.5

84.3

85.5

85.5

13.31

14.39

14.39

13.92

Le Dividich and Canope (1975)

* Negative Protein Digestibility refers to the affect of this dietary component on total digestibility due mostly to the presence of lignin and tanins in green bananas

Using bananas and plantains

Bananas can be fed to pigs either fresh, ensiled (Le Dividich et al., 1976a; Le Dividich et al., 1976b), or in the form of a dry meal, even though the latter is extremely difficult to achieve. Ripe bananas are very palatable and their degree of ripeness affects performance. If fed non-peeled ripe bananas ad libitum, the pig will first eat the pulp leaving part of the peel; however, fed on a restricted basis, both the pulp and peel are eaten. If fed high levels of green bananas, palatability will affect voluntary intake and a lower consumption will affect the performance. Both bananas and plantains can, however, be sliced when green, dried in the sun, and in this way consumption will improve.

Although cooking green bananas improves consumption, growth performance does not equal that of ripe bananas. Table 5.4 shows that when growing/finishing pigs were fed relatively low amounts of ripe or green bananas, approximately one-third of total dietary dry matter, with a restricted amount of protein supplement, they had similar average daily gains and feed conversions.

Table 5.4. Performance of growing/finishing pigs fed bananas in different forms (30-90 kg).

% DM in diet Form ADG (g) DM feed conversion Source
0

45

59

71

cereal (control)

fresh green

cooked green

ripe

680

460

500

560

3.41

4.16

4.26

4.44

Hernández and Maner (1965), cited by Clavijo and Maner (1974)
34

36

green

ripe

450

460

3.55

3.55

Viteri, Oliva and Maner (1971), cited by Clavijo and Maner (1974)
59

66

green

ripe

510

570

4.29

4.63

Solis et al. (1985)

Calles et al. (1970) studied the performance of growing/finishing pigs fed free-choice ripe bananas with a restricted amount of either a 30 or 40% protein supplement. Growth performance, which significantly improved (660 vs. 770 g) when the 30% supplement was used, was assumed to be the effect of the additional intake of energy. It was suggested that the significant increase in the daily consumption of bananas during the first two to three weeks of the experimental period might have been associated, not only with the adaptation to a new feed, but also to the development of a larger stomach capacity.

Table 5.5 shows the performance of growing/finishing pigs fed different levels of green banana meal. When fed at increasingly higher levels, Celleri et al. (1971) found a reduction in average daily gain and a deterioration in feed conversion. Similar studies by Zamora et al. (1985) confirmed that pigs fed green banana meal at levels higher than 20-25% had lower performance.

Table 5.5. Performance of pigs fed different levels of green banana meal (30-90 kg).

% DM in diet ADG (g) DM feeed conversion Source
0 670 3.66 Celleri et al. (1971)
25 650 3.88
50 630 4.04
75 610 4.19
0 620 4.09 Zamora et al. (1985)
20 620 4.12
40 580 4.40

Fresh ripe bananas can be used as a basic feed for the gestating sow; the farrowing performance, i.e. the number and average piglet birth weight, did not differ from the control group fed cereals. In fact, one group of gestating sows fed fresh, ripe bananas showed an improved liveweight gain compared to a similar group fed cereals; upon farrowing there were no observed differences in the piglets. On the contrary, for the lactating sow, fresh, ripe bananas do not meet energy requirements, and fed ad libitum they may cause diarrhea which can affect performance (Clavijo and Maner, 1971).

Green banana meal can supply 50% of the ration for lactating sows with no significant difference in litter size at weaning but with a loss of sow body weight due to less digestible energy. This could be important because a loss of sow body weight can affect future reproductive performance. When used at the same level in the diet of 5-week old weaned piglets, the green meal controlled diarrhoea and produced a growth performance comparable to that obtained with the same level of cassava flour (Le Dividich and Canope, 1974).

Finally, banana leaf meal has been used to replace up to 15% of total dietary dry matter for growing pigs (Garcia et al., 1991b); performance was satisfactory both from the point of view of average daily gain and feed conversion (Table 5.6).

Table 5.6. Use of banana leaf meal for growing pigs (14-28 kg).

% DM in diet ADG (g) DM feed conversion
0 506 2.63
5 496 3.00
10 505 2.91
15 483 2.99

Source: García et al. (1991b)

CASSAVA

Cassava (Manihot esculenta Crantz) is one of the major sources of carbohydrates for humans in Latin American and Africa. Recently, there has been increased interest in the feeding value of cassava for pigs. Although in most developing countries, the average yield is 10 t/ha (FAO, 1991) recent data from Brazil have shown that it is possible to obtain 68 t/ha by using improved varieties, reasonably fertile soil and good management (Chandra, 1986).

Chemical composition

Cassava roots contain 30 to 40% dry matter, more than most roots and tubers. This depends on factors such as variety, soil type, moisture, climatic conditions and the age of the root at harvest. Starch and sugar are the predominant components of the dry matter, approximately 90%, with starch being the most important (Table 5.7).

Although the crude protein content of cassava root is 2 to 4% in dry matter, the true protein content is less than half this amount, due to the fact that 50% of the nitrogen in the roots is in the form of non-protein-nitrogen. Furthermore, the available true protein is deficient in the sulphur-containing amino acids (Table 5.8). The roots contain significant amounts of vitamins, particularly vitamin C, thiamine, riboflavin and niacin.

Table 5.7. Chemical composition of cassava roots and leaves (% DM).

Roots Leaves
Domínguez, 1985 Ravindran et al., 1982 Eggum, 1970 Ravindran et al., 1982
Crude protein 3.5 2.9 34.1 20.2
Ether extract 0.8 1.4 6.3 6.2
Crude fibre 4.2 5.0 10.7 29.0
Ash 4.1 2.3 6.2 7.8
Nitrogen free extract 87.4 88.4 42.7 36.8

Table 5.8. Amino acid composition of cassava roots and leaves (g/16g N).

Amino acid Roots Leaves
Threonine 2.08 3.52
Valine 2.78 5.30
Methionine 0.70 1.29
Isoleucine 2.08 4.06
Leucine 3.80 7.43
Phenylalanine 2.80 4.81
Lysine 0.35 4.81
Total 59.11 84.90

Source: Ravindran et al. (1982)

The yield of cassava leaves, depending on the variety and soil fertility, can vary from 2 to 8 tons of dry matter/ha/year (Oke, 1978). Cassava leaves have a relatively high crude fibre and crude protein content; values ranging from 17 to 34% crude protein in dry matter have been reported for cassava leaf meal (Ravindran et al., 1983). Unlike the roots, approximately 85% of the crude protein in the leaves is true protein, however, owing to their high crude fibre content, the digestibility is only 70 to 80% (Eggum, 1970). The amino acid pattern shows that cassava leaf protein is deficient in methionine but rich in lysine (Table 5.8). Since harvesting the leaves every two months does not affect root yield, and does increase leaf yield (Luteladio and Egumah, 1981), an interesting alternative might be to treat the leaves and roots as two distinct crops: the roots for carbohydrates and the foliage for protein, vitamins and minerals.

One of the major factors which has limited the more widespread use of cassava roots for pigs is its content of the cyanogenic glycosides, linamarin and lotaustralin. These glycosides, upon contact with the endogenous enzyme, linamerase, produce hydrocyanic acid. The reaction is initiated when the roots are crushed or the cellular structure is otherwise damaged. Presently, interest is centered on using varieties naturally low in glycosides and in the development of processing methods which reduce the danger of hydrocyanic acid toxicity.

Maner (1973) and Tewe (1992) outlined some of the processing methods used to reduce or eliminate the toxicity of cassava roots. Boiling destroys the enzyme, linamerase, removes the free cyanide and also the glycosides. Chopping or crushing, followed by sun drying, removes both the glycosides and the hydrocyanic acid; while drying with hot air removes the free hydrocyanic acid and destroys the enzyme, but leaves the glycosides largely intact. Ensiling can cause the destruction of the intact glycosides, thereby reducing the content of cyanide. In some varieties, the leaves contain even more cyanogenic glycosides than the roots; however, Rajaguru et al. (1979) pointed out that this should not be a serious problem since sun-drying can eliminate most of the cyanide in the leaves.

Digestibility

Several studies have emphasized the high digestibility of cassava root diets for pigs, and their comparison to cereals (Table 5.9). Sonaiya and Omole (1977) reported that digestibility was not effected by using up to 15% cassava in the diet, however, earlier studies had shown that with piglets (Aumaitre, 1969; Arambawela et al., 1975) and older pigs (Maust et al., 1972) the digestibility of cassava-based diets was superior to that of cereals. Likewise, no effect on the nutrient uptake was reported by Chicco et al. (1972) when cassava completely substituted maize. Tillon and Serres (1973), in support of this same observation, found that the digestibility of cassava root was not significantly modified by grinding, heating or drying and that the digestible energy of cassava root was comparable to that of maize and other cereals.

Table 5.9. Comparison of the digestibility of cassava root and cereal-based diets for pigs (%).

Cassava root a Cassava root b Cereal c
Dry matter 93.8 92.4 87.7
Crude protein 40.5 36.4 82.5
Ether extract 51.7 44.3 76.40
Crude fibre 48.8 54.5 58.2
Nitrogen free extract 98.5 95.6 83.7
Gross energy - 89.2 84.6
Digestible energy, MJ/kgDM 15.72 15.73 -

a Mesa and Maner (1970), cited by Pond and Maner ( 1974); b Totsuka et al. (1978); c Zamora and Veum (1979)

Eggum (1970) studied cassava leaf amino acid availability which ranged from 55% for valine and isoleucine to 84% for serine. He found that only 59% of the methionine was biologically available. The low values were attributed to the high crude fibre content and the presence of tannins in the leaves. Although cassava leaves are not a common source of energy, Allen (1984) has reported a value of 9.0 MJ/kg of dry matter.

Use for pigs

Fresh cassava roots

Raw cassava, as seen by the data in table 5.10, can supply the major source of energy for growing/finishing pigs. Fed ad libitum, on a ration of chopped raw cassava roots and a protein supplement, growing/finishing pigs gained weight less rapidly but as efficiently as those fed a maize-soya bean meal ration. The consumption of chopped fresh cassava roots by growing/finishing pigs varies according to the protein content of the supplement. The voluntary daily intake of cassava roots was reported to increase throughout the growing/finishing period when the amount of protein supplement increased (Job et al., 1975, cited by Maner et al., 1977). Trials have shown that fresh cassava, of low cyanide content and properly supplemented with a source of protein, minerals and vitamins, can be used as a major source of energy throughout the entire swine life cycle. However, if they are fed bitter roots, performance will suffer: consumption will decrease and they will exhibit a lower average daily gain, and in some cases even lose weight (Gómez et al., 1976).

Table 5.10. Performance of growing/finishing pigs fed fresh cassava roots.

% DM in diet Liveweight (kg) ADG (g) DM feed conversion Source
0 18-99 840 3.43 Maner et al. (1977)
68 " 790 2.80
0 21-86 750 2.81 Job et al. (1975), cited by Maner et al. (1977)
60 " 650 3.02
0 20-54 680 2.84 Buitrago et al. (1978)
51 " 630 2.76

Cassava root silage

In the humid tropics, sun drying is difficult, often resulting in a low quality product with severe Aspergillus mould and related aflatoxin contamination. Artificial drying of cassava roots is expensive, therefore ensiling, in addition to diminishing the content of hydrocyanic acid, is often a more viable means to store cassava for use as a feed resource (Gómez and Valdivieso, 1988).

Buitrago et al. (1978) compared cassava root silage to fresh chopped cassava roots for growing/finishing pigs and found that the average daily gain and dry matter feed efficiency were similar. However, when the silage material was prepared from the entire plant, roots and foliage, performance was affected. This was possibly the result of either the low palatability or the high fibre content of the silage material. Although there is limited data with respect to the use of chopped cassava root silage for sows, its use for growing/finishing pigs indicates that performance is comparable to that of chopped fresh cassava roots (Table 5.11).

Table 5.11. Performance of growing/finishing pigs fed * different forms of cassava: fresh, chopped, chopped root silage and root and foliage silage.

Treatment % in diet Liveweight (kg) ADG (g) DM feed conversion
Maize control diet 0 20-54 680 2.84
Fresh chopped cassava 51 " 630 2.76
Chopped cassava root silage 50 " 650 2.98
Fresh chopped cassava 61 18-98 750 3.43
Chopped cassava root silage 60 " 770 3.25
Root and foliage silage 53 " 640 3.52

Source: Buitrago et al. (1978); * plus a protein supplement

Cassava root meal

Comprehensive studies on cassava meal for pigs have been conducted by researchers from the Colombian Agricultural Institute and the International Center for Tropical Agriculture (CIAT). Cassava meal was prepared by chopping the whole root and drying it in a forced air oven at 82C. The dry cassava meal substituted maize in a 16% crude protein concentrate ration. There was a slight decrease in average daily gain as the level of cassava meal increased, however, feed intake was similar for all groups indicating that palatability was not a problem (Table 5.12). Adding 10% molasses increased consumption and improved average daily gain in all groups. A second experiment evaluated various protein supplements with high levels of cassava root meal. Performance was again satisfactory. Doubling the vitamin trace supplement improved performance, but in this instance, the addition of molasses produced no effect.

Table 5.12 Cassava root meal for growing/finishing pigs (% DM)

% DM in diet Liveweight (kg) ADG (g) DM feed conversion Source
0 19-105 770 3.47 Maner et al. (1977)
69 " 710 3.49
0 16-95 710 3.30 CIA T (1978)
68 " 650 3.40
0 21-96 720 2.67 Lougnon (1979)
50 " 680 2.93

Aumaitre (1969) compared iso-nitrogenous diets containing 50% cassava root meal to diets with the same amount of barley, oats, wheat or maize. The average daily gain of weaner pigs from 5-10 weeks of age was higher on the cassava diet; liveweight gains were reported of 416, 386, 380, 360 and 354 g/day, respectively. Feed per unit of gain was similar for all groups. More recently, data from studies at the University of Hawaii (Gómez, 1992) have shown that piglets from weaning to between 20-25 kg, fed diets containing 20 to 25% cassava meal, performed similarly or slightly better than those fed a maize-soya bean meal ration.

When cassava root meal was fed at levels of 22, 44 and 66% (Shimada, 1970) from 30 to 90 kg, performance was lowered, only, with the group fed the highest level. Cresswell (1978) found that diets based on cassava root meal may benefit from 0.2% methionine; its addition improved feed consumption and growth, and increased urinary thiocyanate excretion.

Researchers at CIAT have studied the effects of cassava/soya bean meal or maize/soya bean meal diets on swine reproduction. A consistent trend to produce smaller litters (8.4 vs. 10.0) when sows were fed on cassava meal (Maner, 1973; Gómez et al., 1976; Gómez, 1977) was attributed to a deficit of methionine. More recently, however, Gómez et al. (1984) showed that supplementation with 0.3% methionine did not improve reproductive performance and suggested that incorrect handling of the meal might have explained the difference (Table 5.13). Interestingly, piglet survival rate at 56 days on the methionine treatment was similar to that of maize.

Table 5.13. Cassava root meal* for pregnant-lactating gilts.

Performance parameters Maize Cassava meal Cassava meal plus 0.3% methionine
Gilts farrowed 14 10 10
Average number piglets born 8.5 9.1 9.4
Average birth weight, kg 1.09 1.06 1.07
Piglets weaned/litter at 56 days 7.1 8.2 8.0
Average piglet weaning weight, kg 16.9 16.2 16.5

Source: Gómez et al. (1984); * soya bean meal as protein supplement

Cassava leaves

Growth performance was lowered as the proportion of fresh cassava leaves was increased in the ration of growing/finishing pigs (Mahendranathan, 1971). This adverse effect was evidently due to the high level of hydrocyanic acid present in the fresh leaves, which affected the palatability. Additional attempts to use cassava leaf meal as a substitute for other protein supplements in swine diets have been less encouraging. CIAT (1978) and Alhassan and Odoi (1982) reported lower gains and poorer feed efficiency when cassava leaf meal was included at 20 and 40%, or at 20 and 30%, in diets for growing/finishing pigs, respectively (Table 5.14).

Ravindran (1990) substituted 10, 20 and 30% cassava leaf meal in a maize/soya bean meal diet for growing pigs and found that the average daily gains and feed efficiency decreased linearly with increasing levels of leaf meal. The performance of pigs fed a diet containing 10% cassava leaf meal was improved by the addition of methionine and additional energy supplementation. This same author emphasized that the relatively high crude protein level and lysine content of cassava leaves (See Tables 5.7 and 5.8) might justify the development of processing methods that would make this feed resource more competitive for swine production.

Table 5.14 Cassava leaf meal (CLM) as a supplement for swine rations

0% CLM 20% CLM 40% CLM
Initial liveweight, kg 15.1 15.1 15.3
Final liveweight, kg 101.1 98.3 96.8
ADG, g 720 570 550
DM feed conversion 3.20 4.30 4.70

Source: CIAT (1978)

SWEET POTATO

The productive potential of the sweet potato (Ipomoea batatas (L.) Lam.) varies from 24 to 36 t/ha of fresh roots (Morales, 1980) and from 4.3 to 6.0 tons of dry matter/ha of foliage. It is also possible to obtain up to three harvests, yearly (Ruiz et al., 1980). Although, the main nutritional importance of sweet potato is in the starch content of the root, it is also a source of important vitamins, such as; vitamin A, ascorbic acid, thiamine, riboflavin and niacin. Recently, it has been shown that the fresh vines can provide up to 27% of the dry matter and 40% of the total dietary protein for growing/finishing pigs (Table 5.20).

Chemical composition

The chemical composition of sweet potato roots and vines is shown in Table 5.15. The roots contain low amounts of crude protein, fat and fibre; carbohydrates make up between 80 to 90% of the dry matter in the roots. The uncooked starch is very resistant to hydrolysis by the enzyme amylase, however, when cooked, the hydrolizable starch fraction increases from 4% to 55% (Cerning-Beroard and Le Dividich 1976).

Table 5.16 shows the nutritional quality and deficiencies of sweet potato roots and vines in total sulphur-containing amino acids and lysine in terms of an ideal protein for pigs (Fuller and Chamberlain, 1982). The typsin inhibitors in raw sweet potato roots decrease protein digestibility, but they can be destroyed by cooking (Martínez et al., 1991).

Table 5.15 Chemical composition of sweet potato roots and vines (% DM)

Roots Vines
Noblet et al., 1990 Domínguez, 1990 Godoy and Elliot, 1981 Domínguez, 1990
Dry matter - 29.2 15.0 14.2
Crude protein 4.4 6.4 18.2 18.5
Ash 3.1 5.3 17.7 12.5
Acid detergent fibre 4.2 5.5 22.3 23.5
Neutral detergent fibre 6.9 - 26.2 -
Lignin 0.7 - 5.7 -
Ether extract 0.6 - - -
Gross energy MJ/kg DM 17.1 16.5 - 14.4

Table 5.16. Amino acid content of sweet potato roots and vines (g/100 g protein).

Roots Vines
Ideal * protein Purcell et al. 1972 Li 1982 Walter et al. 1978
Isoleucine 3.8 4.2-10.1 3.9-5.1 4.9
Leucine 7.0 7.8-9.2 6.2-7.9 9.6
Total sulphur 3.5 2.8-3.8 3.0-3.9 2.8
Phenylalanine + tyrosine 6.7 11.9-13.6 6.2-10.1 10.6
Threonine 4.2 5.5-6.3 5.1-6.1 5.3
Trytophan 1.0 0.8-1.2 - -
Valine 4.9 6.8-8.3 4.9-8.2 6.3
Lysine 7.0 4.2-7.2 4.3-4.9 6.2
Chemical score: total sulphur 100 80-109 85-110 80
lysine 100 60-103 61-70 88

* Fuller and Chamberlain (1982)

Digestible nutrients

Sweet potato roots, raw or cooked, and peeled or non-peeled, have been evaluated in digestibility trials. Peeling significantly increases crude protein digestibility but has no effect on digestible or metabolizable energy, or on the total digestibility of nutrients. Although cooking increases the digestibility of nutrients, it does not affect the utilization of energy (Table 5.17). Wu (1980) found that the net energy of the sweet potato, 8.5 MJ/kg dry matter, was only 79% that of maize, while Noblet et al. (1990) showed that net energy was the same, 12.3 MJ/kg of dry matter. Canope et al. (1977) reported that cooking improved the digestibility of all nutrients, especially nitrogen. Rose and White (1980) fed raw sweet potato roots to pigs and associated a low intake with a high digestible energy value, 15.8 MJ/kg dry matter.

Table 5.17. Digestibility of sweet potato root diets (%).

DM OM N GE DE (MJ/kgDM) Source
Raw 90.4 92.1 27.6 89.3 14.1 Canope et al. (1977)
Cooked 93.5 94.5 52.8 93.0 14.5
Raw 95.3 96.1 49.8 94.2 15.8 Rose and White (1980)
Cooked 85.5 - 76.0 89.2 14.7 Domínguez (1992)
Silage 90.1 91.0 32.0 89.0 16.3 Tomita et al. (1985)
Chips - 91.8 52.3 89.3 15.3 Noblet et al. (1990)

Tomita et al. (1985) evaluated silage made from sweet potato roots and found that the high digestible energy value was related to the high gross energy value of the silage. Lin et al. (1988) reported that poorer nitrogen digestibility was probably due to antitryptic factors, which although lowered, were not entirely eliminated by this method of conservation. Domínguez (1992) found that the inclusion, in dry matter, of 10% of fresh sweet potato vine to a diet of cooked sweet potatoes and soya bean meal lowered the digestibility of all nutrients. An increase in fibre was assumed to be the cause. However, the same author stated that the digestible energy value was acceptable, and even higher, compared to the value of 4.1 MJ/kg dry matter reported by Takahashi et al. (1968) for this foliage. Although in this diet the retention of nitrogen was low, it increased when 10% foliage was added, from 14.1 to 16.4 g/day, which suggests that sweet potato foliage is an acceptable protein source for pigs when included at moderate levels in the diet.

Use for pigs

Watt (1973) summarized the results of feeding sweet potato roots to pigs. He concluded that the use of cooked, as opposed to raw sweet potatoes, increased the average daily gain and that 500 g/day of a protein supplement supported optimal growth. Corring and Rettagliati (1969) also found that in rations for growing/finishing pigs cooked sweet potatoes were superior to the raw form. The data in the following table show, that as raw sweet potatoes progressively replaced maize, the daily feed intake and average daily gain decreased, however, there were no significant changes in feed conversion (Table 5.18).

Table 5.18. Raw sweet potato roots (% DM) as an energy source for growing/finishing pigs (32-90 kg).

Maize/raw sweet potatoes * 88.2/0.0 46.8/42.4 22.4/69.5 8.5/84.3
DM feed intake, kg/d 2.29 2.05 1.91 1.92
ADG, g 740 650 580 570
DM feed conversion 3.16 3.23 3.31 3.37

Source: Marrero (1975); * with a protein supplement

Sweet potatoes can also be chopped, sun-dried, and used as an energy source for pigs. The data in Table 5.19 show that the performance of pigs fed dried sweet potato chips, although inferior to pigs fed on maize, offers an additional and interesting option for feeding pigs in the tropics.

Table 5.19. Dried sweet potato chips as an alternative energy source for finishing pigs.

Sweet potato chips, % DM ADG (g) DM feed conversion Source
0 650 3.36 Tai and Lei (1970)
54-88 560 3.81
0 840 2.92 Cornelio et al. (1988)
46 720 3.38
0 640 3.79 Manfredini et al. (1990)
40 600 4.01

Table 5.20 shows the performance of pigs fed a basal diet of cooked sweet potato, with or without the addition of fresh sweet potato foliage, to replace 25 and 50% of the soya bean meal in the basal diet. A maize/soya bean meal diet was used as the control. The results suggest that, providing an adequate protein supplement is used, cooked sweet potato may totally replace maize for finishing pigs. The use of the fresh vines, at both levels, decreased total dry matter intake. This was probably due to their low dry matter content, 12 to 15 percent. With the lower level of substitution, which implied the use of fresh vines at a level of 13.6% of total dietary dry matter, the feed conversion was similar to that obtained with the sweet potato/soya bean meal basal diet. The higher level of fresh vines, 27% of dietary dry matter, resulted in a poorer gain and increased feed conversion. In an earlier experiment, when Kohn et al. (1976) substituted sweet potato vines for part of a maize/soya bean meal ration for pigs weighing 26-90 kg, it was also reported that the average daily gain decreased and the feed conversion increased.

The complete substitution of maize by cooked sweet potatoes for weaned piglets of 7 to 15 kg decreased the average daily gain from 329 to 284 g and increased the dry matter feed conversion from 1.95 to 2.48 (Mora et al., 1990). When the fresh vines were used to replace 10% of total dry matter, Mora et al. (1991) found that performance of 6 to 12 kg weaners tended to improve, both from the point of view of average daily gain (186 vs. 202 g/day) and feed conversion (2.80 vs. 2.50).

Table 5.20. Use of sweet potato roots and vines for growing/finishing * pigs.

Constituent:

          % DM

Ground maize 83.8 - - -
Soya bean meal 16.2 18.4 13.8 9.2
Cooked sweet potato - 81.6 72.6 63.8
Fresh sweet potato vines - - 13.6 27.0
Initial liveweight, kg 29.2 28.6 29.2 29.2
Final liveweight, kg 90.4 90.4 84.4 80.2
DM feed intake, kg/d 2.30 2.71 2.46 2.43
ADG, g 770 770 690 640
DM feed conversion 3.01 3.51 3.55 3.81

Source: Domínguez et al. 1991; * (30-90 kg)

OTHER TUBER CROPS

Yam (Dioscorea spp.) is a tropical or semitropical root crop that grows extensively in West Africa and to a lesser extent in other tropical areas. Although there are many types of yam, the most economically important are the white yam, (D. rotundata), the yellow yam (D. cayenensis) and the water yam (D. alata). The nutritional composition of yam (Table 5.21) varies among species and cultivars. The major carbohydrate component is amilopectin and only a small proportion of the total carbohydrate fraction consists of mono and disaccharides. Although the percentage of tryptophan in yam is rather high, 1.0%, it is deficient in lysine and the other sulfur-containing amino acids.

Taro or cocoyam (Colocasia esculenta Schott), which originated in India and South East Asia, is presently cultivated in many tropical and subtropical countries. The starch grains of the corms are very small which makes taro highly digestible. The level of crude protein, although slightly higher than that in yam, cassava or sweet potato, contains low amounts of the amino acids: histidine, lysine, isoleucine, tryptophan and methionine.

Table 5.21. Chemical composition of other tuber crops (% DM).

    Dioscorea sp

    Colocasia esculenta

tubers leaves tubers leaves
Dry matter 34.2 24.1 26.2 8.2
Crude protein 8.1 12.0 8.7 25.0
Crude fibre 2.6 25.3 1.7 12.1
Ash 5.2 7.9 4.0 12.4
Ether extract 0.8 2.3 0.4 10.7
Nitrogen free extract 83.3 52.5 85.2 39.8

Source: FAO (1993)

Table 5.22. Performance of growing pigs fed yam or taro.

Tuber Liveweight (kg) ADG (g) DM feed conversion Source
Cooked yam 20-56 580 3.25 Esnaola (1986)
Raw yam

Cooked yam

20-43

20-43

510

760

3.40

2.53

CIAT (1978)
Cooked taro 30-59 590 3.20 Anon (1986)

The taro leaf contains 25% crude protein in dry matter, in addition to calcium, phosphorus, iron, vitamin C, thiamine, riboflavin and niacin. Although, some results of feeding trials using these tuber crops are presented in Table 5.22, normally both yam and taro are too expensive to be used as livestock feed.

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