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Latest revision as of 15:54, 5 July 2015

Plant Resources of Tropical Africa Introduction

List of species

Agave sisalana Perrine

Protologue: Tropical Plants - US House of Representatives Rep. no. 564: 87 (8–9, 16, 47, 60, 86) (1838).

Family: Agavaceae (APG: Asparagaceae)

Chromosome number: 2n = 150 (pentaploid)

Synonyms

• Agave rigida Mill. var. sisalana Engelm. (1875).

Vernacular names

• Sisal, sisal hemp, sisal agave, hemp plant (En).
• Sisal, langue de bœuf, pite sisal (Fr).
• Sisal, agave, linho sisal (Po).
• Mkatani, mkatani mkonge (Sw).

Origin and geographic distribution

Agave sisalana is probably of southern Mexican origin, but wild forms are not known. It has been widely introduced in the tropics and subtropics, in India between 1885 and 1892, in Tanzania in 1893, in Brazil at the end of 19th Century, and in Kenya between 1903 and 1908. Agave sisalana is now present in many tropical and subtropical countries. Until the 1960s Tanzania was the leading producer of sisal, but since then Brazil has become by far the most important producer, followed by Tanzania, Kenya, Madagascar and China. Other countries in Africa that commercially produce sisal include Guinea, the Central African Republic, Ethiopia, Malawi, Mozambique, Angola, South Africa and Morocco. Sisal has also been grown in Uganda, Zimbabwe and Mauritius, but its exact distribution in tropical Africa is unclear

Uses

The main sisal product is the long fibre (‘line fibre’) from the leaves, which constitute the major part of the ‘hard fibres’ of commerce. This fibre is mainly used to make twine, rope, string, fishing nets, buffing cloth, dart boards and hammocks. It has been widely used to make binder and baler twines, but this use has declined steadily over the past decades due to the increasing application of synthetic twines. Sisal fibre is also woven into material for carpet-backing, sacks, industrial fabrics and matting and is used as padding in cars and upholstery. Shorter fibres (‘tow’) are used for the production of upholstery material, mats, carpets, building panels and cellulose. In rural areas in tropical Africa sisal fibre is used for making rope, string, mats and sandals. In parts of southern Africa it seems to have replaced Sansevieria spp. as the main source of fibre for these purposes.

Sisal fibre is used for making specialty papers, such as cigarette paper, newsprint, bag paper, carbon paper, safety and banknote paper, filter paper and tea bags. However, sisal pulp is more often used in the production of common grade paper, in blends with wood pulp to add porosity or to reinforce weaker pulps, such as those from recycled paper. In recent years possibilities for the use of pulped sisal fibre as reinforcement in composite board manufacturing has been investigated. Whole leaves and sisal waste material may also be used as a source of pulp for papermaking.

The long and straight inflorescence stalks are used for house construction, fencing and thatching. Waste material after fibre extraction and boles remaining at the end of the life cycle of a crop may serve as animal feed, either directly or after ensilage. A very hard wax is obtained from the leaf-cuticle and pectin can be obtained from sisal leaves. The pulp from the leaves after fibre extraction may be returned to the field as manure. Waste material is sometimes fermented to produce an alcoholic beverage or for biofuel production. Sisal is planted as a fence plant, to mark field boundaries, and to stabilize soil. Several edible mushrooms, notably Coprinus cinereus, grow in large quantities on the waste heaps of sisal factories in Tanzania and provide a year-round source of food.

Hecogenin from sisal leaves has been used as a precursor in the partial synthesis of corticosteroids such as cortisone, hydrocortisone and prednisone. In traditional medicine in East Africa a root decoction is drunk as a diaphoretic and a leaf decoction as a diuretic, while sap from the leaves is taken to treat constipation and stomach-ache and externally applied to cuts. In Morocco sap from the leaves is used as a wash for skin diseases. It is also used against syphilis, pulmonary tuberculosis, and jaundice. It can be used as a laxative. In India the plant is considered emmenagogue and abortive.

Production and international trade

In terms of production, sisal occupies 6^th place among fibre plants, representing 2% of the world’s production of plant fibres, and it accounts for about 70% of the world’s hard fibres. World sisal production reached its peak of over 600,000 t per year in the 1960s, but has declined gradually since then due to the introduction of synthetic fibres and stagnating prices. The annual sisal production in Tanzania peaked in 1964 at about 230,000 t from about 230,000 ha, whereas in Brazil a peak of about 300,000 t was reached in the 1970s. While Brazil has nearly regained the level of its peak production, production has dwindled in most other countries. Tanzania has become the second largest producer, but at a production of about 10% of its peak. Kenya now holds 3^th place, while Madagascar, which has kept its annual production much more stable at about 15,000 t, takes place 4, before China. According to FAO estimates the average annual world sisal fibre production (line fibre and tow) in 2004–2008 was about 350,000 t, the major producers being Brazil (230,000 t), Tanzania (29,000 t), Kenya (25,000 t), Madagascar (18,000 t) and China (17,000 t).

The international trade in sisal has fluctuated strongly over time. Total world exports averaged about 90,000 t per year in 2004–2008, the principal exporters being Brazil (50,000 t), Kenya (19,000 t), Tanzania (16,000 t) and Madagascar (6000 t). The main importers were North Africa, Mexico, China, Portugal and Spain. The price of sisal on the world market has declined from US$ 713/t in 1979 to US$ 519/t in 1987, it remained stable at about US$ 550/t until the year 2000, but has risen again in nominal terms in recent years to about US$ 592/t. Sisal from East Africa, where the crop is mainly grown on estates, is of better quality than that from Brazil, where it is mainly produced by smallholders, and East African sisal fetches higher prices on the world market.

Properties

Each sisal leaf contains (700–)1000–1200(–1400) fibre bundles which, as in other Agave species, can be distinguished into 2 main types:

– ‘mechanical fibres’, constituting 75% of the fibres in the leaf and mainly concentrated in 3–4 rows in the peripheral zone below the epidermis; these fibre bundles, nearly round or horseshoe-shaped in cross-section, keep the leaf rigid and rarely split during processing, thus being the main determinant of the fineness of the resulting fibre.

– ‘ribbon fibres’, constituting 25% of the leaf fibres, are found in a line in the centre of the leaf; they coalesce and become lignified towards the end of the leaf to form the terminal spine. They are also present in other parts of the leaf and serve to protect the vascular bundles; those covering the phloem are large and crescent-shaped and tend to split longitudinally during processing, whereas those covering the xylem are weak, thin-walled and are mostly lost during decortication.

The average fibre content of the leaves is about 3.5–4%, but it increases during the life cycle from about 2% to 4.5–5%. The fibre bundles are composed of fusiform cells, strongly bonded together. The ultimate fibre cells are (0.3–)1.5–4(–15) mm long and (8–)15–30(–50) μm in diameter, circular, polygonal or oval-polygonal in cross-section with cell walls (2–)4–6(–9) μm thick and a small, roundish lumen. The fibre cells may taper to a blunt point or be rounded. The fibre number, length and strength do not change with the age of the leaf after it has unfurled. However, the average strength of the fibres increases and the fineness decreases slightly with the age of the plant at which the leaf was formed. Sisal fibre is hard, coarse, long (1–1.5 m), very strong and nearly white, cream or pale yellowish. Quantitative information on the physical properties of the fibre shows wide variation in, for example, tensile strength (80–840 N/mm²) and elongation at break (2–29%). Typical values of the properties of sisal fibre are: tensile strength 510–635 N/mm², elongation at break 2.0–3.0%, and Young’s modulus 9400–22,000 N/mm². Sisal can be used for making sacks, but cannot be spun as fine as jute. On a dry weight basis sisal fibre contains approximately 54–66% α-cellulose, 12–17% hemicelluloses, 7–14% lignin, 1% pectin and 1–7% ash.

Sisal pulp has an exceptionally high tear strength, good porosity, high bulk, high absorbency and high folding endurance, making it suitable for specialty papers and for reinforcing other pulps. Sisal is best pulped using chemical methods. Normally the cold soda process is used, because it is relatively cheap and produces no harmful chemicals. The yield with this process is 50–55%. Sisal fibre is also suitable for making high-quality fibreboard. Pulped fibre is suitable as reinforcement in composite boards, e.g. in boards made from cardenol, a polyphenol from the shells of cashew nuts, a major product of East Africa and Brazil.

Sisal contains several steroidal saponins including hecogenin, its content increasing with the age of the plant. Hecogenin from sisal can be used in the partial synthesis of corticosteroids, but only if it is not overly contaminated by tigogenin, another sapogenin, because this results in unacceptable losses in product quality and yield. East African sisal yields relatively clean hecogenin with only 5–10% tigogenin, but sisal from other parts of the world often contains more tigogenin, e.g. 20–30% in Brazil. Pulp from sisal leaves has shown strong molluscicidal activity against the schistosomiasis vector Biomphalaria glabrata.

The value of waste material from sisal leaves after fibre extraction as animal feed has been analysed in Ethiopia. Dry matter content of the fresh waste was 82 g/kg; per 100 g dry matter sisal waste contained: crude protein 6.4 g, ash 17.7 g, ether extract 1.6 g, neutral detergent fibre (NDF) 33.3 g, acid detergent fibre (ADF) 26.9 g, lignin 14.2 g, total phenolics 1.0 g, total tannins 0.2 g, condensed tannins 0.01 g, phytate 2.0 g, saponins 0.08 g. Other analyses indicate a protein content of sisal waste of about 5–6% and that of the bole of only about 2%. The bole is rich in inulin, which makes it difficult to dry for industrial processing e.g. into fuel-alcohol. Experiments in East Africa indicate that the waste material from the leaves after fibre extraction is a suitable source material for fermentation and distillation into fuel-ethanol.

Adulterations and substitutes

Abaca (Musa textilis Née) and henequen (Agave fourcroydes Lem.) may substitute for sisal or mixed with it. Compared to abaca, sisal ropes are less strong, harsher on the hands, swell more and quicker in water without regaining their shape on drying, and tend to break without warning, whereas abaca will show signs when it starts to break. For these reasons and because sisal ropes become stiff when wet and do not float, sisal is less suitable for marine purposes than abaca. Cantala (Agave cantala Roxb.) fibre is less strong than sisal fibre, but finer, more supple and whiter. Fibres from New Zealand hemp (Phormium tenax J.R.Forster & G.Forster), Mauritius hemp (Furcraea foetida (L.) Haw.) and Sansevieria spp. are softer than sisal fibre and may be used instead of sisal for specific purposes. Sisal and other hard fibres face strong competition from synthetic products, such as polypropylene and nylon.

Sisal is more difficult to pulp than abaca, as it requires a higher pressure, longer cooking time and/or more chemicals. Furthermore the pulping yield of sisal is lower. Because sisal fibre cells are shorter than those of abaca, paper made from sisal has a higher porosity but lower tensile and bursting strength than paper made from abaca. Therefore, sisal is inferior to abaca for the production of specialty papers, except for filtration media.

Description

Robust, monocarpic, perennial herb, 3–9 m tall when flowering, with numerous leaves crowded in a dense rosette; roots fibrous, originating from the base of the leaf scars at the bottom of the stem, spreading horizontally up to 3(–5) m, and vertically up to 150 cm, but concentrated in the upper 30–40 cm of the soil; stem short and thick, 120 cm × 20 cm, with an apical meristem; rhizomes arising from buds in the axils of leaves below ground level, numbering 5–10 at one time and about 20 in the total life span, 1.5–3 cm in diameter, growing to about 2 m in length before surfacing and producing suckers. Leaves succulent, arranged in an ascending spiral; stipules absent; petiole absent; blade linear-lanceolate, 75–185 cm × 10–15 cm × 2–4.5 cm, base fleshy, bulbous, triangular in cross section, blade gradually broadening to the middle and narrowing into a terminal sharp, lignified, dark brown spine up to 3 cm long, concave above and convex below, margin usually spineless, surface dark green but covered with a white waxy layer. Inflorescence a panicle on a long peduncle, 2–8 m tall, branches widely spreading, 30–100 cm × 2 cm, apically 5–6 times branched trichotomously, bearing about 40 flowers per branch. Flowers erect, protandrous; pedicel short; perianth tubular, 6-lobed, 5–6 cm long, pale green, tube 1–2 cm long, lobes oblong, on inner side of the top with a tuft of hairs; stamens 6, attached above the middle of the perianth tube, accrescent during anthesis, finally 6–8 cm long; ovary inferior, 3-celled, style much accrescent during anthesis and finally 6–8 cm long, stigma 3-lobed. Fruit (rarely produced) an ellipsoid capsule, tapering at base, green and fleshy when young and black and dry when ripe, with about 150 seeds. Seeds rounded-triangular, thin, flat, papery, black. Bulbils copiously produced on the inflorescence branches, consisting of a meristem, 6–8 reduced leaves and a rudimentary stem with rudimentary adventitious roots.

Other botanical information

Agave comprises about 275 species distributed mainly in arid and semi-arid regions from the south-western United States southward to western Panama, the Caribbean and Venezuela. The taxonomy of the genus is very complicated and the genus has been subdivided into several subgenera and sections. However, different views exist on delimitation and classification of Agave. The long-fibre cultivated species are all classified in subgenus Euagave section Rigidae, except Agave sisalana which is classified in section Sisalanae. The basic chromosome number of the genus is x = 30, and it contains diploids, triploids, tetraploids and pentaploids. In Central America, where the genus probably originated, Agave has been used by humans as a source of food, drink and fibre for at least 9000 years.

Agave sisalana, the original and long the most important of the long-fibre Agave crops on a world scale, is probably of hybrid origin, with Agave angustifolia Haw. and Agave kewensis Jacobi as possible parents. In the 1960s in Kenya a chance selection was made from Agave sisalana and developed into cv. ‘Hildana’. It is characterized by a large habit, many but shorter leaves and delayed flowering. It is grown in East Africa. Most other long-fibre Agave cultivars also derive at least partly from Agave angustifolia, a species classified in the section ‘Rigidae’. Together they account for about 85% of the hard fibres of commerce. Hybrid 11648, released in Tanzania in 1960, was obtained by backcrossing a hybrid of Agave amaniensis Trel. & W.Nowell and Agave angustifolia with Agave amaniensis. Because of its high yield, it has largely replaced Agave sisalana in Tanzania in spite of its susceptibility to diseases and waterlogging. Commercially no distinction is made as its fibre is similar to that of Agave sisalana. Also in the literature on the agronomy, processing and properties of sisal, the distinction is rarely made. However, reports in Kenya indicate that its pulp yield is somewhat lower and the pulp slightly less strong. Little information exists on the extent to which this hybrid has replaced Agave sisalana elsewhere, but it is widely planted in Kenya and Brazil, and has played an important role in the expansion of sisal production in China. Agave amaniensis is a diploid, with sword-shaped leaves c. 165 cm long, very glaucous blue, thick at the base and tipped with a reddish brown spine. It is originally described from plants introduced to and cultivated in Amani (Tanzania).

Kaptura is the name given to short sisal plants occurring on many estates. They are characterized by short, slender, light bluish-green leaves, an expanded growth habit with a short central spike, a low fibre yield (30–50 m of leaf is required to produce one tonne of fibre, compared with the usual 13–15 m) and high sucker production (4 times that of Hybrid 11648). Kaptura plants have been identified as Hybrid ML 487, which was bred at Mlingano but later identified as being inferior to other hybrids, and was therefore never officially released.

Other Agave species grown for their long fibres, include henequen (Agave fourcroydes), the second most important species. It is only grown in Mexico and some Central American and Caribbean countries, with Mexico and Cuba as main producers. It has been suggested that it is an old selection from Agave angustifolia. The fibres of henequen are coarser than sisal. They are used for production of ropes, nets, floor-covering and harvest binding strings. The plants are cultivated also as hedges and living fences, e.g. in Cuba and are used for medicinal purposes. Agave angustifolia var. letonae (Taylor ex Trel.) Gentry (synonym: Agave vivipara var. letonae (Taylor ex Trel.) P.I.Forst.) produces the Salvadorian henequen hemp of El Salvador. Agave cantala, yielding cantala or Maguey sisal, is also classified in the section Rigidae, but not directly related to Agave angustifolia. It is of importance as a fibre crop in the Philippines. Agave cantala has been grown in Tanzania, but plants flowered too early and suffered from diseases.

Growth and development

Sisal plants have a short stem or bole on which the leaves and the central bud (often called ‘spike’) are borne. In the central bud immature white leaves are packed tightly around the meristem until they are pushed outwards by the growth of succeeding leaves and unfurl. The angle between unfurling leaves and stem gradually widens until the lower leaves are almost horizontal. After having produced (180–)200–250(–300) leaves, which may be up to (3–) 6–9(–20) years after planting depending on climate and soil conditions, a long flowering shoot (‘pole’) is produced. Its initial growth rate is 10–12 cm per day. In Java poles are formed throughout the year, but in more seasonal climates there is usually a flush of pole formation after the rainy season. After the pole has reached its final length, flowering branches are produced. Flowering starts on the lowest branch and proceeds upwards, taking several weeks until all branches have flowered. The stamens dehisce 2–3 days before the style is fully elongated and the stigma is sticky and receptive. When female flowering takes place on a branch, the stamens on the branch above it shed pollen. Pollination is mostly by insects, mainly bees, but wind-pollination can also occur. Although the pollen of Agave sisalana is viable, the flowers usually abscise. Where it has produced seed, this is probably due to contact with pollen from Agave angustifolia or related species. It has also been suggested that fruiting depends on external conditions. Seeds have been obtained in the Kenyan highlands, Indonesia and Brazil by cutting back the inflorescence in an early stage of its development, but this technique was not successful at lower altitudes in East Africa. Bulbils are normally formed on the panicle after the flowers are shed and usually appear in the bottom branches of the pole before the upper flowers have finished flowering. The bulbils grow to a length of 6–10 cm in about 3 months, after which they are shed. One plant can produce up to 4000 bulbils. After the production of flowers and bulbils the entire plant dies. However, a sisal plant may produce 20 or more suckers during its life cycle. Suckers are formed at the end of rhizomes. They normally start to form when plants are about 1 year old, are most prolific in the 2^nd and 3^rd year and become fewer as the plants age.

During the initial vegetative phase of the sisal plant each new leaf is 0.6–0.8 cm longer than the preceding leaf. When leaves are regularly harvested the leaf length increase is less, but the rate of leaf unfurling remains almost the same. When sisal becomes reproductive, new leaves progressively become shorter. Based on these characteristics the life cycle can be divided into 4 phases: 1, the immature period from planting to first cut; 2, the period during which relatively short leaves are cut (usually the 1st and 2nd cut); 3, the period during which long leaves (about 120 cm) are harvested; 4, the period just before poling, when leaves are becoming shorter.

Sisal follows the Crassulacean Acid Metabolism (CAM) pathway. CAM plants are able to fix CO₂ at night and photosynthesize with closed stomata during the day, thus minimizing water loss. A mature sisal plant, excluding roots, weighs about 60 kg. On a dry weight basis the leaves constitute 70%, the roots 22% and the bole 8% of an unharvested plant. An average leaf weighs about 0.7 kg.

Ecology

Sisal is a hardy tropical plant needing full sunlight and moderate relative humidity. It grows best in regions with an average annual rainfall of 1000–1250(–1800) mm, but is often grown with less. The maximum temperature should be 27–32°C, with minimum temperatures of 16°C or higher and daily fluctuations not exceeding 7–10°C. Sisal is damaged by frost and hail. Under dry conditions or at low average temperatures it forms fewer leaves per year and has a longer life cycle. In tropical Africa it is grown up to 1800 m altitude.

Sisal prefers sandy-loam soils but can be grown on a range of soils, provided they are rich in bases, especially Ca, and well drained, as sisal does not tolerate waterlogging. The pH should be between 5.5 and 7.5, though sisal has been grown on soils with pH 4–5. Hybrid 11648 does not tolerate waterlogging.

Propagation and planting

Sisal is propagated vegetatively with bulbils or suckers. Though suckers are directly available from the field, bulbils are often preferred, because they are produced in greater numbers, making selection possible and thus giving a more uniform crop. Bulbils may be collected from the ground after they have fallen, or the pole may be cut and the bulbils shaken into sacks. In very dry years bulbils may be in short supply. Bulbils at least 10 cm long are planted in nurseries at a spacing of about 50 cm × 25 cm and a depth of 1.3 cm. Application of sisal waste in the nursery is beneficial to plant growth. After 12–18 months the plants are ready to be planted out into the field. At transplanting the fibrous roots around the base of the plantlets are usually cut off and the lower leaves may be pulled off.

Before planting in the field, the soil is cleared mechanically or by hand and it may be ploughed shallowly. The optimum density is 4000–6000 plants/ha, depending on climate and soil. Most sisal estates in East Africa have a density of 5000 plants/ha, obtained by a spacing of 2.5 m × 0.8 m or by double rows 3.5–4 m apart, with 1 m between the rows of each pair and (0.75–)0.8(–1) m between plants within each row. A density of 7000 plants/ha has been mentioned for sisal grown for pulping in Brazil. The planting depth is 5–8 cm.

In-vitro propagation of sisal is possible, as complete plants have been regenerated from rhizome and stem explants on various growth media, supplemented with various concentrations of plant hormones, e.g. benzyladenine (BA), kinetin, naphtalene acetic acid (NAA), indolacetic acid (IAA), indolylbutyric acid (IBA) and 2,4-D, either alone or in combination. Shoot regeneration may occur either directly or from callus, and regenerated shoots root readily.

Sowing of legume cover crops such as Calopogonium mucunoides Desv., Centrosema pubescens Benth. and Pueraria phaseoloides (Roxb.) Benth. is recommended. On fertile soils or with proper fertilization, young sisal may be intercropped with maize, beans or cotton without adverse effects on the sisal crop unless the other crops are planted very close to the sisal rows. Rotation is not necessary if sisal processing waste is returned to the field and is not commonly practised in East Africa.

Management

Weeds should be controlled in the first 2–3 years after transplanting, by hand, or by mechanical or chemical means. Important weeds in sisal plantations include: couch grass (Cynodon dactylon (L.) Pers.), nut grass (Cyperus spp.), African couch (Digitaria abyssinica (Hochst. ex A.Rich.) Stapf), lalang (Imperata cylindrica (L.) P.Beauv.), cow-itch (Mucuna pruriens (L.) DC.) and Guinea grass (Panicum maximum Jacq.). After 2–3 years, weeds may be allowed to grow during the rains and cut down at the beginning of the dry season to conserve moisture and provide mulch. The nutrient removal per t of fibre is about 27–33 kg N, 5–7 kg P, 59–80 kg K, 42–70 kg Ca and 34–40 kg Mg, but the majority of the removed nutrients can be returned to the field with the waste material after fibre extraction. Fertilizer recommendations depend on soil characteristics and cropping history. Lime application is recommended in highly acidic soils. ‘Purple leaf tip’, in which the leaf tip becomes reddish-purple and the leaf margins curve upwards, is associated with exhausted acid soils and a shortage of calcium, but other factors may also be involved. Potassium deficiency causes ‘banding disease’, characterized by 10–15 cm wide horizontal bands of purplish-brown necrotic tissue, especially at the transition between the leaf base and the leaf blade, resulting in wilting and bending over of the leaf blade. Nitrogen application tends to shorten the crop cycle, but the total number of leaves is not affected. Care should be taken in using ammonium sulphate which may increase soil acidity. Suckers should be removed and may be used for propagation. Old sisal fields are sometimes kept in production by maintaining selected suckers, but this method is not recommended as it is better to replant the field.

Diseases and pests

The most serious disease of sisal is bole rot caused by the fungus Aspergillus niger entering through the leaf bases after leaves are cut. It causes a wet rot which becomes yellowish-brown and soft, with a pinkish margin, and it may lead to plant collapse and death. The incidence can be reduced through removal of infested material and harvesting under dry conditions. The fungus also causes a basal dry rot when it enters the base of the bole through an injury. Zebra disease causes striped lesions on the leaves and may also cause bole rot. It is caused by Phytophthora nicotianae and mainly occurs on poorly drained soils. Hybrid 11648 is especially vulnerable. Korogwe leaf spot is a virus disease occurring in Tanzania which gives very poor grade fibre and often renders the crop unusable. Hybrid 11648 is very susceptible to this virus disease, Agave sisalana only slightly.

The only serious insect pest of sisal is the agave weevil or Mexican sisal weevil (Scyphophorus acupunctatus; synonym: Scyphophorus interstitialis), first recorded in Tanzania in 1914. The larvae damage the subterranean parts of young plants and may cause substantial losses. They also feed on leaves in the central bud, giving a shothole effect, whereas the adult weevil damages the crop by feeding on the youngest leaves before and shortly after unfurling. Planting before or in the early rains and the application of insecticides in the soil around young plants can control the pest.

Harvesting

Sisal leaves are harvested at regular intervals during the life cycle of the crop. As the total number of leaves produced during the life of the plant is constant and the rate of leaf emergence is affected by temperature and rainfall, the time from planting to the first harvest, the total production period and the number of cuttings depend on environmental conditions. An early start of cutting is conducive to better yields, provided the plants are not cut too severely. If cutting is delayed, plants pole earlier and heavy leaf losses occur through withering. Overcutting results in the formation of more but smaller leaves with lower fibre content, leading to reduced fibre yields and higher cutting costs. In general the first harvest takes place when leaves over 60 cm long start to touch the ground. Leaves shorter than 60 cm are normally not used for fibre extraction, because mechanical decorticators cannot handle them. The time from planting to the first harvest depends on the rate of leaf production, which in turn depends on climate and soil conditions. Under East African lowland conditions (high temperature) cutting usually starts 2–3 years after planting and cutting is then repeated annually. In the Kenyan highlands (lower temperature) cutting usually starts 4 years after planting. In Brazil (low rainfall, low soil fertility) the first harvest usually takes place when the plants are 3 years old and subsequent harvesting is twice a year. Generally harvesting continues for about (5–)8(–12) years. At the last cutting, when about 80% of the plants are poling, all suitable leaves (more than 60 cm long and sufficiently succulent) are cut. Harvesting is usually done throughout the year.

Leaves are usually cut manually at 2.5–5 cm from the bole. It is essential to leave sufficient leaf area at each cutting to enable the plant to continue growing. About 20–25 leaves are left on the plant at the first cutting, and this number is usually decreased to 15–20 leaves at subsequent cuttings. The terminal spines are removed before or after the leaves have been cut. The leaves are tied in bundles and transported to the processing site, which must be done as soon as possible after harvesting, because cut leaves deteriorate rapidly if exposed to the sun.

Mechanization of leaf cutting is complicated and not economical, but gains can be made in transportation of the leaves to the mill.

Yield

From 100 kg sisal leaves about 3.5 kg extractable fibre is obtained, of which about 92–96% is line fibre and 4–8% tow. For the best Chinese plantations annual yields of 4.5 t/ha are claimed. On East African plantations annual fibre yields of 2.0–2.8 t/ha have been obtained, whereas on poorer soils annual yields were about 1 t/ha. Yields in Tanzania have shown a strong downward trend since the 1960s, but they have recovered since. Average annual fibre yields in Brazil, Kenya and Tanzania are now nearly 1 t/ha. Sisal processed for pulp production in Brazil yields about 5.5 t of dried fibre per ha per year, with 6 kg of fibre obtained per 100 kg fresh leaves.

Handling after harvest

Fibre extraction of sisal should be carried out as soon as possible after cutting, because leaf juices tend to harden, making fibre extraction more difficult. Where sisal is grown for local use, the fibre is manually extracted by scraping away the leaf parenchyma with a blunt knife or piece of wood. Retting is also done, e.g. in India, where the leaves are immersed in water for about a week, after which the leaves are beaten on a stone to remove the remaining extraneous matter, and the separated fibre is washed, dried in the sun and baled. In commercial production, decortication is usually mechanical. Before the advent of high-speed automatic decorticators this was done by semi-automatic raspadors, consisting of 1–4 open rotating drums with blades or bars on the periphery, into which the leaves are fed manually and end-on. Decorticators consist of decorticating drums, chains or rope for leaf-gripping and belts, into which the leaves are fed sideways, and have a much higher productivity than the raspador. Extraction with decorticators involves crushing and scraping, removing and washing away the parenchymatous leaf tissue and leaving the fibre strands to continue through the machine. During decortication 15–20% of the total leaf fibre (‘flume tow’) is lost and enters the waste effluent. Mobile decorticators have been introduced into Tanzania. After decortication and washing, the fibre is dried, either in the sun or in drying machines, the latter giving fibre of a more uniform quality. Excessive drying in the sun may lead to deterioration in colour. The dried fibre, which has become stiff and congealed, may be beaten lightly by metal beaters (‘brushing’) to free the individual bundles and to remove dirt and other extraneous matter. This process also combs out the shorter fibre strands, 7.5–12.5 cm in length, which constitute the ‘brush tow’.

Sisal fibre is mainly graded according to length, colour and presence of impurities, but designations vary by country and even within countries. The moisture content of packed fibre should not be more than 10–12%. If it is too wet, it becomes stiffly matted and there is a danger of spontaneous combustion in the bales. The fibre is baled in hydraulic presses to produce unwrapped bales of the desired size. Spinning of yarn is usually done on special machines able to cope with the long fibres. The yarns produced are coarse, spiky and harsh to the hands. They are used singly as harvest twine, 3-folded into packaging twine or further multiplied into ropes of different sizes. Processing sisal fibre may cause lung problems in factory workers.

For paper production in Brazil, the leaves, with the terminal spine removed, are transversely cut into pieces about 5 cm long and passed through a hammer mill. The juice and other residues are removed through vertical screens; the fibres are passed through a dryer, after which they are pulped using an alkaline soda process.

To obtain hecogenin, sisal leaf juice collected from the decorticator is allowed to ferment for several days, after which the sludge is hydrolysed into a dark brown solid (‘coffee grounds’) with a hecogenin content of 10–20%. Alternatively, air is blown upwards through a tank containing fresh juice, and the resulting foam, containing most of the saponins, is transferred to a vessel for immediate hydrolysis to hecogenin.

Genetic resources

About 70 sisal accessions are kept at the Centro Nacional de Pesquisa de Algodão (CNPA), Campina Grande, Brazil. The Instituto Agronômico de Campinas (IAC), Campinas, São Paulo, Brazil, maintains a collection of about 300 Agave accessions. Germplasm is also kept at the Mlingano Agricultural Research Institute in Tanzania.

Breeding

Sisal has a narrow genetic base and offers little opportunity for breeding and selection. Furthermore, the plants have a long life cycle and it is almost impossible to synchronize flowering of prospective parents. Most breeding work has been carried out in East Africa, where it focused on developing a long-fibre Agave with a more rapid growth and higher leaf number potential than sisal, but resembling sisal in other respects (non-spiny leaf margins; long, heavy and rigid leaves of good configuration; good fibre yield per leaf; resistance to diseases and pests; good fibre quality). Several Agave species have been incorporated in sisal breeding. Examples are the diploids Agave amaniensis, which has smooth margins and finer and more numerous fibre bundles in its leaves than Agave sisalana but has leaves tending to be corrugated, making mechanical processing difficult, and Agave angustifolia, which produces many but short leaves with spiny margins. Crosses between Agave sisalana and these species resulted in progenies with spiny margins, but crosses between Agave amaniensis and Agave angustifolia are fertile and combine a high number of leaves with a good leaf size, some of them having smooth margins. Backcrossing of these hybrids with Agave amaniensis gave very good results, in particular Hybrid 11648, which may produce more than 600 leaves and give annual fibre yields twice as high as sisal, with a longer life cycle. The leaves are of good configuration and have smooth margins, and the fibre is as strong as sisal fibre, though finer. However, Hybrid 11648 is susceptible to zebra disease caused by Phytophthora spp., to which Agave sisalana is mainly resistant. At altitudes higher than 600 m the leaves are short and the leaf-number potential is not realized because of early poling. Other cultivars released in Tanzania include ‘H-1300’ and ‘Mlola 1’, which is grown in Kenya.

New cultivars have been released in China, such as ‘South Asia No.1’ and ‘South Asia No.2’. These new cultivars are resistant to zebra disease while maintaining similar fibre production as Hybrid 11648 and outweighing it in terms of fibre quality and cold resistance. Breeding work in Brazil focussed on backcrosses between Agave amaniensis and hybrids of Agave amaniensis and Agave angustifolia. Successful crosses between sisal and cantala have been made, e.g. in Indonesia, but usually seed set is poor and the progeny has spiny leaf margins.

Prospects

There is scope for increased utilization of sisal, including agaves such as Hybrid 11648, in view of the resurgence of demand for natural fibres for their biodegradability and unique appearance and texture. Non-traditional uses of sisal and sisal-like fibre, especially for the production of pulp, offer promising new possibilities for producers. The development of highly productive cultivars suited to local needs, improved management practices, efficient fibre extraction and pulping technologies and further promotion of the use of natural fibres may open a new frontier to the profitable cultivation of sisal in tropical Africa. However, labour shortages due to low wages paid in sisal plantations may hamper developments. The development of mechanical harvesting methods would enhance their prospects as fibre crops, especially if it could be combined with fibre extraction. Better utilization of the by-products (short fibres, poles and boles for pulping; leaf waste for feed or hecogenin extraction) would help to make their cultivation more profitable.

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Sources of illustration

• Dahal, K.R., Utomo, B.I. & Brink, M., 2003. Agave sisalana Perrine. In: Brink, M. & Escobin, R.P. (Editors). Plant Resources of South-East Asia No 17. Fibre plants. Backhuys Publishers, Leiden, Netherlands. pp. 68–75.

Author(s)

• L.P.A. Oyen, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article

Oyen, L.P.A., 2011. Agave sisalana Perrine. [Internet] Record from PROTA4U. Brink, M. & Achigan-Dako, E.G. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.

Accessed 23 December 2024.

• See the Prota4U database.