Agave sisalana (PROSEA)

Plant Resources of South-East Asia
Introduction

Protologue: U.S. 25th Congress, 2nd Session House of Reps Rep. 564 (Tropical Plants): 8, 9, 16, 47, 60, 86; Senate Rep. 300: 36, 105, 140, pl. 1, 2, 4 (1838).

Family: Agavaceae

Chromosome number: 2n= 150 (pentaploid)

Synonyms

Agave rigida Mill. var. sisalana Engelmann (1875).

Vernacular names

Sisal, sisal hemp, sisal agave (En). Sisal, langue de boeuf, pite sisal (Fr)
Indonesia: sisal
Thailand: sapparot thet (central), pan-sonnarai
Vietnam: dửa sợi trơn, dửa sợi không gai.

Origin and geographic distribution

Sisal is probably of southern Mexican origin, but this is not entirely certain, as wild forms are not known. Sisal has been widely introduced and cultivated in the tropics and subtropics. It was introduced into Florida in 1836-1843, Tanzania in 1893, Brazil at the end of 19th Century and Kenya in 1903-1908. In Asia it was introduced into India between 1885 and 1892, Malaysia a few years later, the Philippines around 1905 and Indonesia in 1913. At present it is cultivated in a range of countries, including Indonesia, Thailand, Burma (Myanmar) and India.

Uses

The main sisal product is the long fibre ("line fibre") from its leaves, which form the major part of the "hard fibres" of commerce. Sisal fibre is mainly used to make twines, ropes, strings, fishing nets and hammocks. In Madura (Indonesia), for instance, sisal cordage is used for making ship cords, fishing gear and for packing tobacco. Sisal has been widely used for the manufacture of binder and baler twines, but this use has declined steadily in 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. Sisal fibres serve 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. Sisal waste material after fibre extraction and discarded sisal products may also be utilized for papermaking.

Hecogenin from sisal leaves has been used as a precursor in the partial synthesis of corticosteroids such as cortisone, hydrocortisone and prednisone, whereas waxes and pectin can also be obtained from sisal leaves. The inflorescence stalks are used for house construction, fencing and thatching. Waste material after fibre extraction and boles may serve as animal feed, either directly or after ensilage.

Production and international trade

World sisal production reached its peak of over 600 000 t per year in the 1960s, but has declined gradually since then with the introduction of synthetic fibres such as polypropylene. The annual sisal production in Tanzania peaked in 1964 at about 230 000 t from about 230 000 ha, whereas in Brazil the peak of about 300 000 t was reached in the 1970s. The average annual world sisal fibre (line fibre and tow) production in 1996-2000 was about 300 000 t, the major producers being Brazil (153 000 t), China (35 000 t), Kenya (24 000 t), Tanzania (22 000 t) and Madagascar (17 000 t). Indonesia produced only about 450 t annually in this period, which is very little compared to the estimated estate sisal production of about 80 000 t in 1939. In Thailand 35-40 t was produced annually in 1996-2000.

The international trade in sisal amounted to about 72 000 t per year in 1996-2000, the principal exporters being Brazil (31 000 t), Kenya (17 000 t), Tanzania (12 000 t) and Madagascar (5 000 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 fetches a higher price on the world market. The main importing countries were Portugal (23 000 t) and Spain (9 000 t). South-East Asian countries were net importers in this period, with the Philippines importing about 600 t annually, Thailand 200 t and Indonesia 100 t.

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 divide 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 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 greatly thickened walls (about 6(-9) μm) 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 show wide variation in, for example, tensile strength of the fibre (170-640 N/mm²) and elongation at break (2-29%). Typical values of the tensile strength, elongation at break, and Young's modulus of sisal fibre are 510-635 N/mm², 2.0-2.5% and 9.4-22.0 GPa, respectively. 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 contains hecogenin, with the 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. Sisal has shown molluscicidal activity against the schistosomiasis vector Biomphalaria glabrata . The protein content of sisal waste is about 5-6% and that of the bole only about 2%.

Adulterations and substitutes

Abaca ( Musa textilis Née) and henequen ( Agave fourcroydes Lem.) may be substituted 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 and slightly thinner 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

A robust, monocarpic, xerophytic perennial herb, 3-9 m tall when flowering, producing subterraneous stolons (rhizomes) and suckers, and numerous crowded leaves in a 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 deep, but concentrated in the upper 30-40 cm of the soil; anchoring or bearer roots, 2-4 mm in diameter, extend horizontally for 1.5-3(-5) m in free range; feeder roots, 1-2 mm in diameter, arise from the bearer roots. Stem short and thick, 120 cm × 20 cm, with an apical meristem and a close rosette. Rhizomes 1.5-3 cm in diameter arise from buds in the axils of leaves below ground level, numbering 5-10 at one time and about 20 in the total life span, grow about 2 m in length before coming to soil surface and producing suckers used as propagation material. Leaves succulent, sessile, arranged in an ascending spiral, linearlanceolate, 75-185 cm × 10-15 cm × 2-4.5 cm, base fleshy, bulbous, triangular in cross section, margin usually spineless, blade gradually broadening to middle and narrowing towards terminal sharp, lignified, dark brown spine up to 3 cm long, concave above and convex below, dark green but covered with a white waxy layer. Inflorescence a panicle on long peduncle, 2-8 m tall, branches widely patent, 30-100 cm × 2 cm, apically 5-6 times branched trichotomously, bearing about 40 flowers per branch; pedicel short; flowers erect, protandrous, containing much nectar; 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; pistil with inferior, 3-locular ovary containing numerous ovules, 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. Seed 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. Seedling with epigeal germination.

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 shed pollen. Pollination is mostly by insects, mainly bees, but wind-pollination can also occur. Although the pollen is viable, the flowers usually abscise. Where sisal has produced seed, this may have been due to contact with pollen from A. vivipara L. or related species. It has also been suggested that fruiting in sisal 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 abscise 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 normally start to form when plants are about 1 year old, are most prolific in the 2nd and 3rd 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, each new leaf is shorter than the preceding. Based on these characteristics the life cycle can be divided into 4 phases:

the immature period from planting to first cut;

the period during which relatively short leaves are cut (usually the 1st and 2nd cut);

the period during which long leaves (about 120 cm) are harvested;

the period just before poling, when leaves are becoming shorter.

Sisal follows the Crassulacean Acid Metabolism (CAM) pathway. CAM plants are able to fix CO₂at 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.

Other botanical information

Agave L. comprises about 200 species distributed mainly in arid and semi-arid regions from the south-western United States to western Panama, the Caribbean and Venezuela. However, the taxonomy of the genus is very complicated and different views exist on its delimitation and classification. 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 9 000 years. A. sisalana is probably of hybrid origin, with A. vivipara (synonym: A. angustifolia Haw.) and A. kewensis Jacobi as possible parents.

Several Agave spp. are cultivated for their long fibres, which form about 85% of the hard fibres of commerce. A. sisalana is the most important of these on a world scale, followed by henequen ( A. fourcroydes ), which is only grown in Mexico and some Central American and Caribbean countries, with Mexico and Cuba as main producers. Other long-fibre agaves are A. cantala and A. vivipara L. var. letonae (Taylor ex Trel.) P.I. Forst., which are cultivated to a limited extent, the former being of some importance in South-East Asia (see separate article). Hybrid 11648, obtained in East Africa by backcrossing a hybrid of A. amaniensis Trel. & W. Nowell and A. vivipara with A. amaniensis , has largely replaced A. sisalana in Tanzania. However, its fibre is similar to that of A. sisalana and commercially no distinction is made. No information exists on the extent to which this hybrid has replaced A. sisalana in South-East Asia.

A. lurida Ait. (synonym: A. vera-cruz Miller) gives an inferior fibre used in the "longyi" (a garment) industry in Burma (Myanmar) and it is sometimes grown in Malaysia.

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 Java it is grown up to about 600 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 in Indonesia.

Propagation and planting

Sisal is propagated vegetatively with bulbils or suckers. Though suckers are directly available from the field, bulbils are more often preferred, because they are produced in greater numbers, making selection possible and thus 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 at 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 ranges from 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 between plants 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. In Indonesian estates the usual density is 7200 plants/ha, obtained by planting in double rows 2.7 m apart, with 0.9 m between the rows of each pair and 0.75 m between plants within the rows. 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 different concentrations of 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 interplanted with maize, beans or cotton without adverse effects on the sisal crop unless the other crops are planted very close to the sisal rows. In Indonesia sisal has been grown in rotation with, for instance, cassava (East Java) and abaca (Sumatra), but rotation is not necessary if sisal processing waste is returned to the field.

Husbandry

Weeds should be controlled in the first 2-3 years after transplanting sisal, by hand, or by mechanical or chemical means. Later, 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.

The only serious insect pest of sisal is the Mexican sisal weevil ( Scyphophorus interstitialis ), first recorded in Tanzania in 1914 and in Java in 1916. 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 insect.

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 with cutting is conducive to better yields, if 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 a 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. On estates in Indonesia (high temperature, high soil fertility) it was common practice to have the first cutting at 1.5-2 years after planting with 12-13 further cuttings until the plants were 6-8 years old. 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.

Usually the leaves are 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.

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. On the best Indonesian and 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. However, yields in Tanzania have shown a strong downward trend since the 1960s. Present yields in Madura (Indonesia) are less than 0.5 t/ha per year, with a fibre recovery from the leaves of only 2-2.5%. 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 in Indonesia, the fibre is manually extracted by scraping away the leaf parenchyma with a stump 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.

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 drier and 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 Station 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 other Agave spp. have been incorporated in sisal breeding. Examples are the diploids A. amaniensis , which has smooth margins and finer and more numerous fibre bundles in its leaves than A. sisalana but has leaves tending to be corrugated, making mechanical processing difficult, and A. vivipara , which produces many but short leaves with spiny margins. Crosses between A. sisalana and these species resulted in progenies with spiny margins, but crosses between A. amaniensis and A. vivipara 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 A. 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 A. 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. Breeding work in Brazil also focussed on backcrosses between A. amaniensis and hybrids of A. amaniensis and A. vivipara . 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 and sisal-like agaves such as Hybrid 11638, 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 and sisal-like agaves in South-East Asia. 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 (e.g. 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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Authors

K.R. Dahal, B.I. Utomo & M. Brink