92

18: Plants producing exudates

Hevea brasiliensis (Willd. ex Juss.) Müll.Arg.

Linnaea 34: 204 (1865).

EUPHORBIACEAE

2n = 36

Natural rubber, para rubber (En). Caoutchouc (Fr). Indonesia: karet. Malaysia: getah asli. Cambodia: kausuu. Laos: jaang. Thailand: yang phara. Vietnam: cao su.

The centre of origin of natural rubber covers part of the Amazon Basin, parts of Matto Grosso (Upper Orinoco) and the Guianas. Geographically, wild and semi-wild Hevea is found in the northern part of South America, from Brazil to Venezuela and Colombia to Peru and Bolivia.
Natural rubber was first introduced into South-East Asia from the Neotropics in 1876. Early attempts to encourage its planting were not well received. However, with the arrival and expansion of the motor car industry and the increased demand for natural rubber, it soon grew into an important plantation crop in a number of tropical and subtropical countries. Today, rubber is grown in Malaysia, Indonesia, Thailand, Vietnam, Sri Lanka, China, India and Papua New Guinea in Asia, as well as in Ivory Coast, Nigeria, Cameroon, Liberia and Gabon in Africa.
In South America, particularly in Brazil, despite massive opening of new land for rubber cultivation, production continues to be hampered by a major rubber leaf disease caused by Microcyclus ulei, known as South American Leaf Blight.

When tapped, the rubber tree produces a milky liquid (latex). This can be processed into latex concentrate, sheet rubber or block rubber; it is marketed as natural raw rubber. The main users of natural raw rubber are the tyre manufacturers who consume 50-60% of the total world natural rubber produced. The balance is divided among manufacturers of rubber car components (producing e.g. engine mountings, bushes, weather strips, V-belts, hoses, joint rings), manufacturers of engineering components (e.g. building mounts, anti-vibration mounts, dock fenders, flooring, high quality sheeting), and manufacturers of consumer products (e.g. footwear, sport goods, toys, gloves, latex threads, catheters, swimming caps, condoms). Moreover, when felled for replanting, the rubber tree is also sawn to give rubberwood (i.e. timber). With proper treatment, this can be used for high value added products like furniture, chipboard, medium density fibre board, parquet and many other wood products. Furthermore, rubber wood can be converted into charcoal. Seeds contain a semi-drying oil that can be used in making paints and soap.

Of the total world consumption of about 16.4 million t of rubber in 1998, 6.6 million t or 40% was natural rubber, 9.8 million t or 60% was synthetic rubber. Almost all (95%) of the world natural rubber supply comes from Asia, with Malaysia, Indonesia, Thailand and Vietnam as major producers, together accounting for about 80%. The total area under natural rubber plantations is estimated to be around 7 million ha.
The most important group of rubber producers are smallholders who cultivate more than three quarters of the world acreage. In Thailand more than 95% of all rubber is grown on smallholdings. In Indonesia and Malaysia these proportions are about 80% and 85% respectively. The estates are now planting oil palm, since this is more profitable. Most natural rubber is exported to industrialized countries. This explains why the commodity is actively traded on the international markets in Singapore, Tokyo and Kobe, where quotations of spot and future prices are readily available on every trading day.
Prices of natural rubber move in tandem with the level of industrial activities and in response to short-term imbalances of supply and demand in industrialized countries. Historically, price movement follows the rubber trade cycle averaging 48 months. Currently, natural rubber of all types and grades is very depressed, with the average price of US $ 0.89 per kg (1998), the lowest for 21 years.
The United States is still the world's largest consumer of natural rubber. In 1998, it consumed 1.16 million t or 17.5% of the world's total production; then China (839 000 t or 12.7%), Japan (707 300 t or 10.7%), Malaysia (334 100 t or 5.1%), India (254 330 t or 3.8%). Germany, France, Italy, the United Kingdom and Spain accounted for 916 400 t or 13.9%. All other countries accounted for 2.4 million t or 36.3%.

Latex consists of a colloidal suspension of rubber particles in an aqueous serum. The rubber content of latex may vary from 25-40% but is usually between 30-35%. Properties of rubber depend on the processing of the raw product after collection in the field.
The natural rubber molecule is made up of many isoprene units forming a polymer with a high molecular weight, chemically known as cis-1,4-poly- isoprene (C₅H₈)_n. Rubber generally has a high viscosity which, for freshly prepared natural rubber, ranges from 55-90 centipoise. In storage and during transit, the viscosity of natural rubber increases to 70-100 centipoise depending on the duration.
Owing to its high structural regularity, natural rubber tends to crystallize when stored at low temperature or when stretched. The strain-induced crystallization behaviour gives natural rubber its unique high tensile strength in pure gum or in non-reinforcing filler vulcanisates.
Natural rubber has an intrinsic density of about 0.92 g/cm³ and a bulk density of 0.85 g/cm³. It has a tendency to cold-flow unless restricted by physical constraints.
Properties of latex concentrate are specifically defined by the dry rubber content (d.r.c.), the volatile fatty acid number (V.F.A.), mechanical stability time (M.S.T.), the KOH number, alkalinity and colour. The properties for latex concentrate specify that the dry rubber content (%) should have a minimum of 60, the difference between d.r.c. and t.s.c. (total solid content) should not exceed 2%; the volatile fatty acid number should not exceed 0.20 but a typical latex concentrate can be kept at a low level of V.F.A. (e.g. <0.05) with good preservatives; the minimum requirement of mechanical stability time is 650 seconds; the KOH number (g), which determines the ionic content in latex, should not exceed 1.0, although immediately after production it is usually 0.4-0.5; the alkalinity of the latex with low ammonia type is 0.2% and with high ammonia type 0.6%; the coagulum content (%) should not be equal to or greater than 0.05; the dried latex film should be pale in colour. The properties of raw rubber are subject to the Standard Malaysian Rubber (S.M.R.) grades. Currently, there are 9 S.M.R. grades (see reference 7 for their properties).
The heartwood of rubberwood is pale cream- coloured, often with a pink tinge, weathering to pale straw-coloured or pale brown, not distinctly demarcated from the sapwood. The density is 560-640 kg/m³ at 15% moisture content. The grain is straight to shallowly interlocked, texture moderately coarse and even.

A deciduous, monoecious tree, 30-40 m tall, about 15(-25) m in cultivation; root system with a well-developed taproot of 1-2 m long, laterals spreading to about 10 m; bole usually straight but tapered, up to at least 50 cm in diameter, without buttresses; bark surface smooth to slightly corky, hoop marked, pale to dark brown, inner bark pale brown, with abundant white latex; crown conical, branching pattern highly variable, stem leader dominant or soon divided into several heavy branches. Leaves arranged spirally, trifoliolate; petiole long with apical glands; stipules deciduous; leaflets entire, elliptical to obovate, 4-50 cm x 1.5-15 cm, acuminate, pinnately veined. Inflorescence a many-flowered, axillary, short- pubescent panicle on the basal part of a new flush; male and female flowers in the same panicle, small, without petals, female flowers less numerous and distributed at the apex of main and lateral branches; male flowers with a bell-shaped, 5-lobed perianth, yellow; stamens united into a column with 10 sessile anthers in 2 rows; female flowers with a green disk at base, ovary superior, 3-celled, with 3 sessile white stigmas. Fruit an exploding, 3-lobed capsule, 3-5 cm in diameter, pale brown when mature, with a thin rind and bony inner wall breaking into 6 pieces, each lobe with 1 seed. Seeds ovoid, about 2-3.5 cm long, testa waxy, with very numerous small dark brown spots and a variable number of irregularly shaped patches; endosperm abundant, almost completely enveloping the straight embryo. Seedling with hypogeal germination; hypocotyl elongating; cotyledons thin, leaf-like, green with pink or purple tinge.

Rubber seeds usually germinate 7-10 days after sowing. Seedlings and buddings exhibit growth periodicity. Terminal buds of main stems produce long internodes with leaves clustered towards the end of them. The shoot pushes out vertically, slowly for 2-3 days, then rapidly before tailing off for 1-2 days. The energy for growth is then diverted into leaf development. Leaf petioles and leaf blades show the same kind of growth as the shoot, but the blades go on growing for 3-4 days longer than the petioles. When their growth ceases, the blades change colour from dark reddish to light green, and continue to droop. During the next stage the leaves rise to the horizontal position after which they become dark green. A complete cycle takes about 36 days, 18 for extension growth and 18 for leaf development. Subsequent growth proceeds in similar cycles and, as the plant grows, the leaves appear in whorls. After a period of about 2 months during which the plant becomes established, the growth rate is about 30 cm per month during the first year. A healthy seedling can reach a diameter of 2.5 cm in one year.
Branching is from the axils of leaves with average-sized petioles and begins after about 9 flushes; the branches are distinctly tiered. After the first year of growth, the plants then go through a phase of rapid vegetative growth for the next 4 years before they start flowering and fruiting. However, trees have been reported to flower when only 20 months old and to fruit in just over 3 years. Inflorescences appear in the axil of scale leaves towards the base of flushes on high-level branches of older trees. The development of flowers does not affect the growth of the shoot. The tree consequently has Rauh's architectural growth model, determined by a monopodial trunk which grows rhythmically, thus developing tiers of branches. 'Lampbrush' is a modification without ramification and with continuous growth according to Corner's architectural model. It can be induced by eliminating about one-third of each maturing leaf.
After branching, diameter increment starts and growth periodicity is less pronounced. Diameter increment decreases when trees are tapped. The trees attain a height of about 18 m in 8 years. To prevent wind damage, a rather short tree with a symmetrical crown starting about 3 m above ground level is preferred. When trees reach a certain age they shed all or some of their leaves, usually once a year. Before they fall, old leaves turn vivid orange or red, sometimes bright yellow. In Peninsular Malaysia, trees are briefly deciduous at the beginning of the year, and there is often a second complete or partial leaf-change in August-September. The intensity of leaf shedding, usually called wintering, depends on climatic conditions and varies with clone. A dry period of one month or longer causes partial or complete leaf fall. This causes a drop in latex production, especially during refoliation. Flowers are produced along with the new leaves. Both self- and cross-pollination is carried out by small insects. Self-incompatibility occurs in some clones. Natural pollination is poor and leads to only 1-4% of the female flowers setting fruit. Even with hand pollination no more than 5% of the pollinated female flowers develop into mature fruit. Fruits ripen in about 5 months. Seeds are actively dispersed for 10 m or more by the exploding capsules. Seeds are viable only for a few days, but storage in sealed containers with damp sawdust can extend the viability period to one month.

Of the 10 presently recognized Hevea species only Hevea brasiliensis, Hevea guianensis Aubl. and Hevea benthamiana Müll.Arg. yield usable rubber; the latex of other species is undesirable because of its high resin and low rubber contents. There are numerous cultivars (often clones) of Hevea brasiliensis.
From the periphery towards the centre the bark consists of cork layers, hard bark, and soft bark. Soft bark mainly consists of vertical rows of sieve tubes and latex vessels. Latex vessels are modified sieve tubes. They are formed from the cambium in concentric rings as cells which fuse longitudinally by gradual disintegration of the cross-walls. Within each ring, vessels are laterally interconnected but the connections are disrupted as the trunk expands. Latex vessels of stem, branches and leaves are interconnected. The latex-vessel cylinders generally run clockwise at an angle of about 3.5° to the vertical, which is why tapping cuts are made from upper-left to lower-right. The diameter of the latex vessels, the number of vessels per ring and the number of rings in the virgin bark are important characteristics, because they largely determine the content of the latex vessel system of a tree.

Rubber is a crop of the per-humid lowland tropics between 6°N and 6°S. Attempts to cultivate rubber as far south as the Sao Paolo Region in Brazil and as far north as Mexico and the Guangdong Province in China have met with some success. The optimum day temperature is 26-28°C. Rubber should preferably not be planted at altitudes above 400-500 m because the low ambient temperature retards diameter increment, delays tapping, and reduces latex production.
The annual rainfall requirement ranges from 2000-3000 mm with 170-200 rainy days. A well distributed annual rainfall of 1500 mm is considered the lower limit for commercial production. In Indonesia the best rubber areas have annual rainfall totals between 2500-4000 mm. In high rainfall areas soils should have good drainage. A large number of rainy days, especially with rain in the morning, is undesirable, because it disrupts the tapping schedule. In some areas rubber can also tolerate a 2-3 month drought. Wind is an important factor because it may snap trunks and branches.
Owing to its extensive root system rubber needs a well drained, root-penetrable soil, at least 1 m deep with an adequate moisture storage capacity. Temporary waterlogging with flowing water causes little damage. It can be grown in soils ranging from sandy to red lateritic and yellow podzols, young volcanic soils, alluvial clays and peat soil. Rubber is less demanding in terms of soil fertility and topography than other tree crops such as oil palm and cocoa and is often planted on land which is not suitable for these crops. In West Malaysia, rubber-producing areas have been classified into zones on the basis of factors limiting growth and production such as strong winds, disease incidence, soil type and topography.

Rubber can be established by planting seed at stake or by raising plants in nurseries and later transplanting them to the field. Seedling trees are used, but improved vegetatively propagated planting material is often preferred. This can easily be obtained by bud grafting rootstock by a technique developed in 1916 in Indonesia.
Seed from vigorous high-yielding parents is used to produce rootstock. As seeds are viable for only a short time, they must be sown soon after harvesting. They are first germinated on shaded beds and transferred to the nursery soon after germination, where they are either planted in the ground or in perforated polythene bags.
Budwood is grown in special nurseries in which trees budded with the desired clone are closely spaced. Green budstick is obtained by cutting back the buddings, which then start producing numerous shoots. About 4 crops of budsticks can be obtained per year. About 1 crop of brown budwood can be harvested a year.
Bud grafting is carried out by making an inverted U-shaped incision on the rootstock 4-5 cm above soil level and inserting the bud patch without a petiole under the bark of the bud panel. It is essential that the rootstock and scion are at an active stage of growth and that their cambial tissue should be closely appressed and tied in place. About 3 weeks after budding the strips are opened and the successful stock stems cut back above the bud patch to allow the new bud to sprout.
'Brown budding' is the traditional bud grafting method in which 12-18 month old rootstock is budded with budwood of about the same age. This method was later superseded by the 'green budding' technique. This refers to budding 4-6-month-old, still green stock with buds from green budsticks. The advantage of this method is the short nursery period and the economic production of budwood. Green budding, however, requires greater skill than budding older stocks. Further improvement has been achieved by budding of 7-8(-10) week old rootstock. This is an early form of green budding called 'young budding' and it is used in raising advanced planting material. 'Crown budding' is a method of producing a 3-component tree which, for example, combines a disease-resistant crown with a high-yielding production trunk budded on a seedling rootstock. This technique is used in South America where Microcyclus ulei is a serious problem, and sometimes in Malaysia to overcome leaf disease.
After bud grafting the planting material can be nursed as bare-root stumps (e.g. budded stumps, stumped buddings and mini-stumps) or as polybag-raised buddings (2-whorl polybag-raised buddings, large polybag-raised buddings and soil-core whorled buddings). Raising bare-root planting material requires a suitable and well-prepared soil, whereas for polybag plants, only the potting medium and a good water supply matter. Lifting and root pruning of bare-root plants is time-consuming and laborious, but once this is done the material is easy to handle and to transport. Polybag plants need constant attention. They are ready for transplanting immediately but great care must be taken during their transport, to prevent root damage. Large polybag plants are cumbersome. Polybag plants, however, develop more quickly after planting. Soil-core buddings have much the same advantages and disadvantages as polybag plants, but as they are raised in the ground they are less susceptible to drought in the nursery.
'Clonal' seed obtained from monoclonal or polyclonal plantings which are known to produce high-yielding families is used for the production of seedling trees. These 'clonal' seedlings are cheaper to produce and they may have greater wind resistance and may reach maturity earlier than brown-budded rubber, but they are more variable and seedling plantings usually give lower yields. Germination and nursery procedures are essentially the same as for raising stock.
All planting material, buddings and seedlings are pruned to restrict development to one single stem free from any branches up to 3 m, to ensure enough tappable bark for high panel tapping.
In smallholdings temporary intercropping of young rubber with food crops is a common practice to provide cash income when the trees are still immature. On flat or undulating land intercropping can be carried out during the first 1-3 years after planting without adversely affecting rubber plants.
Budded stumps with bare roots are planted in holes of 45 cm x 45 cm x 45 cm. These are normally dug in advance, refilled and allowed to settle naturally with time and rain. Rock phosphate is added at a rate of about 100 g per hole during refilling. A similar procedure is used for planting advanced planting material (maxi-stumps) with bare roots, except that larger planting holes are used. Because of their susceptibility to drought, cylinders of polythene sheeting (sarongs) are temporarily placed in the planting hole around the upper half of the taproot. It is filled with a mixture of good soil and rock phosphate, watered and then mulched with grass. When the first leaves are properly hardened the sarongs are removed. For polybag plants, planting holes are made at the time of planting.
The preference for planting patterns has varied over the years between square spacings of about 5 m to avenue plantings with 8-10 m between the rows and 2-3 m in the row. The former has the advantage of optimal use of soil and space, early closure of the canopy and less wind damage, the latter of cheaper maintenance, lower tapping costs and space for temporary intercropping. The current recommendations of the Rubber Research Institute of Malaysia for smallholders practising intercropping is to plant rubber in east-west rows at distances of 9 m x 2.7-3 m.
High planting densities give the highest yields/ha but trees take a longer time to reach a tappable size and give lower yields per tree and per tapper. This is why smallholders, who are usually interested in maximization of yield/ha, plant at higher densities (500-600 trees/ha) than estates (400-450 trees/ha) which are interested in maximization of net revenue.
Cover crop establishment is a standard practice in both new planting and when replanting on estates and is done just before planting the rubber. Drainage is required in areas which are waterlogged. The most common leguminous species used are Calopogonium mucunoides Desv., Centrosema pubescens Benth. and Pueraria phaseoloides (Roxb.) Benth. Though legume cover crops compete rather strongly with the rubber in the first year of establishment, their overall effect on the rubber trees is beneficial and may extend over a 20-year period.

The economic life cycle of rubber in plantation is 30-35 years. After each cycle, replanting is necessary to realize optimum use of the land. Land preparation for replanting is done mechanically, which involves cutting old stands, stacking and burning. This is followed by ploughing, rotavating, preparation of planting holes and planting. This last operation must coincide with the rainy season. If rubber is to be planted on land under forest, trees of economic importance are extracted first and then all other trees are felled and stumps are removed along the lines of the future planting. Burning follows, and then the non-burnt vegetation is wind-rowed. Recent public awareness of the importance of a clean environment and reduced air pollution has led to a zero-burning technique used in Malaysia. The wood of the stem is used as timber and remnant wood debris is stacked in alternate planting rows prior to lining and holing for planting.
Maintenance of young plants during immaturity includes weeding, manuring and sometimes mulching. Weeding is the most important and is also costly. Frequent weeding is required. Initially, only the tree circles to a radius of about 1 m are weeded, but later on this is done for the whole tree row or rubber strip. At the same time noxious weeds should be controlled or removed in the inter-row legume cover. Once the rubber trees reach maturity, the number of weeding rounds can be reduced due to shading by the tree canopy. It is then sufficient to weed the rubber strip and to slash the inter-row vegetation once or twice a year. On estates chemical weed control has replaced manual weeding except during the first year after planting when green scions and leaves are still found below a height of 1 m.
During the immature phase, branch pruning or controlled branch pruning is routinely carried out. The amount of fertilizer applied to the trees is determined after assessing the nutrient status of both plant and soil. The method of fertilizer application varies with terrain. For flat to undulating terrain, a general broadcast of fertilizer is advocated, on hilly terrain the fertilizer should be applied along the planting rows after strip spraying. For immature rubber, the fertilizer should be evenly applied in a ring or broadcast along the planting strips.
In the nursery and during the first few years after field planting, fertilizers are frequently applied in small quantities. Subsequently, applications are made twice a year and, when trees have reached maturity, usually only once a year when the new leaves have emerged after wintering. The amount of nutrients removed in the latex is low, but may increase considerably when yield stimulants are used. To compensate for these losses and for the immobilization of nutrients in trunks and branches, annual fertilizer rates per ha used are in the order of 50 kg N, 20 kg P, 60 kg K and 20 kg Mg. Fertilizer recommendations for young trees are based on soil type, and for mature trees, on soil and leaf analysis, stimulation and on the specific requirements of clones. In Malaysia, young rubber receives mainly N and P, and fertilizer recommendations only differentiate between sandy and clayey soils. Mature trees receive N and K, but P and Mg are only given when leaf analysis indicates a need for them. Both organic and inorganic fertilizers are used; the former is preferred on sandy and lateritic soils.

There are several important diseases and pests which attack rubber both in the nursery and in the field. The 3 most important fungi in South-East Asia which cause root disease are, in order of significance, Rigidoporus lignosus, Ganoderma pseudoferreum and Phellinus noxius, giving rise to white, red and brown root disease respectively. They cause much destruction and total tree losses in new plantings and replanted areas of rubber. Hence proper control of these diseases during pre- and post-planting is essential. Pre-planting control is accomplished by removing all infected inoculum sources and post-planting control is achieved by regular inspection and by treating the affected plants with calixin. Early establishment of cover crops is also effective in controlling root diseases.
Important fungal leaf diseases are Colletotrichum and Oidium, causing secondary leaf fall, Corynespora (leaf spot) and Phytophthora (leaf fall). Oidium attack can be controlled by protective sulphur dusting. In large areas of disease it can effectively be avoided by aerial spraying with defoliants a few weeks before wintering. Refoliation then takes place in a relatively dry period. Bird's eye spot disease caused by Helminthosporium heveae is also common, but is confined to the nursery. The most damaging and most feared leaf disease is South American Leaf Blight (SALB) caused by Microcyclus ulei. So far the disease has been confined to South and Central America. Infected trees lose their leaves after every new flush resulting in dieback and ultimately the death of the trees. At present the best solution to the SALB problem is an integrated approach combining the use of tolerant clones, judicious application of fungicides (e.g. carbamates), correct manuring and maintenance, and planting in somewhat drier areas.
Pink disease, caused by Corticium salmonicolor, is also of economic importance as it attacks the trunk and branches and causes branch snap. However, it is easily controlled by calixin or bordeaux mixture.
Underground pests are important and require attention at all stages of rubber growth. They include termites (Coptotermes curvignathus) and grubs of certain Melolonthis beetles. Among the aboveground pests, yellow tea mite (Hemitarsonemus latus) and thrips (Scirtothrips dorsalis) are commonly present in nurseries, where they cause defoliation of tender leaflets. Other common sap-sucking insects, such as scale insects and mealy bugs, may occasionally cause sufficient damage to warrant treatment in nurseries and of young plantings. Other pests that may require occasional attention in young rubber are slugs and snails, and a variety of mammals ranging from rats to elephants. Giant snails (Achatina fulica) cause damage in parts of Indonesia.
The first symptom of 'brown bast' or dryness is increased yield of watery latex, followed by a drying up of part or whole of the tapping cut. The cause is not known, but is generally considered to be a physiological condition due to overtapping.

Tapping of rubber starts 5-6 years after planting. Trees are opened for tapping when 50-70% of the trees in a given area, measured at 150 cm height from the base, have attained girth size of at least 45 cm (15 cm diameter). Tapping involves cutting the bark from top left (at 150 cm height) to bottom right. The slope of the tapping cut is at an angle of about 30° to the horizontal. The amount of bark consumed is determined by the frequency of tapping. Cutting is carried out using a knife with a V-shaped cutting edge leaving a grooved channel along which the latex can flow (excision method).
The tapping system is characterized by a combination of the number of cuts per tree, the length of the cut and the frequency of tapping. According to an international notation the length of the cut is given as a fraction of the circumference: S/1 or S is a full spiral, S/2 a half spiral, S/4 a quarter spiral, S/R a reduced spiral. The frequency of tapping is expressed as d/1 for daily tapping, d/2 for alternate day tapping. The system S/2 d/2 is considered as standard. With the current labour shortage experienced by many countries and depressed rubber price, however, less intensive short cut tapping e.g. S/4 d/4 and S/4 d/6 is preferred. An alternative to low frequency tapping is periodic tapping, a system with nine months tapping and three months tapping rest e.g. S/2 d/3 (9/12 m). With appropriate stimulation these systems give yields comparable to those obtained with conventional methods.
At each tapping a thin slice of bark is removed. The latex runs along the cut and then down a vertical grove to a metal spout driven into the tree, which channels the latex into a cup. The conventional tapping method is one in which subsequent cuts move downwards until about 5 cm above the join in buddings. The bark above the cut is renewed from the cambium. To ensure good bark renewal tapping cuts should stop about 1.5 mm away from the cambium. Normal bark consumption for a half spiral cut tapped on alternate days is 2-2.5 cm a month. When there are no periodic resting periods it takes 5-6 years to tap the bark of a 150 cm high panel. After completing the first panel, the second one is opened at the same height on the opposite side of the trunk. When this panel has been used, tapping continues on the renewed bark of the first panel. Later, the renewed bark of the second panel is retapped. In this system about 10 years is allowed for bark renewal before tapping can resume. When this cycle is complete the trees are about 30 years old and are considered ready for replacement. However, they are intensively tapped (several panels per tree and high stimulant concentrations) for 3 years before being cut out.
In smallholdings where the trees are often tapped daily, bark consumption is much greater. After the first and second panels have been tapped twice the bark higher up the tree is exploited. A high panel, as it is called, is also used on estates, where it is often exploited in combination with a low, regenerated bark panel, either on the same or on the opposite side of the tree. In these double cut systems, periods in which only the upper or only the lower panel is tapped alternate. Upward tapping of high panels gives higher yields than downward tapping, but it requires greater skill to control the tapping knife. On older trees, control upward tapping (CUT) on the high panel is now an accepted practice.
Yield can be stimulated by applying ethylene-releasing chemicals (e.g. ethephon) on the tapping cut of young trees and on the bark of older trees either directly below (downward tapping) or above (upward tapping) the tapping cut. In young trees stimulation is usually not recommended, although in Ivory Coast and Malaysia, where labour is scarce, stimulation starts with the first panel. Non-intensive methods of stimulation involving low ethephon concentrations, low frequency of application plus periodic stimulation rest, may be used at an earlier stage. Stimulation should, in fact, always be combined with a lower tapping intensity than was used before the stimulation started. However, stimulation of trees requires increased fertilizer use. In practice, stimulation is considered as a means of maintaining or obtaining reasonably high yields at low tapping frequency, thus as a method of saving labour and costs.
As it is now common practice to use stimulants at a later stage of exploitation (probably after about 10 years of tapping) it is advisable from the outset to use systems that can be converted to lower intensity systems when stimulation is introduced. In view of this, S/2 d/2 is recommended for clones not susceptible to dryness ('brown bast') and S/2 d/3 for clones more prone to dryness. To avoid exhaustion of trees, special S/4 systems are recommended for smallholders tapping daily and using stimulants.
The advent of stimulants has enabled the development of unconventional methods using a needle instead of a tapping knife and requiring fewer and less skilled labourers. The most promising, called micro tapping techniques, are puncture tapping and micro-X tapping. In the first method 4-6 punctures are made per tap on a vertical strip of bark, 60-100 cm long and 1-2 cm wide, previously scraped and treated with ethephon. The second method combines puncture tapping and the conventional excision tapping methods. Here, 3 punctures are made on the existing half spiral 9 times on d/2 followed by 3 successive conventional tappings at the same frequency. In Malaysia a new puncture tapping method has been developed, in which ethylene gas is directly applied to a lightly scraped bark area on the panel and latex is extracted from the tree in a closed system into sealed receptacles containing a powerful anti-coagulant. This so-called RRIMFLOW system has several advantages compared to conventional tapping: trees can be exploited by unskilled labour, no yield is lost from rain interference, trees can be tapped overnight, thus maximizing productivity, and the entire yield can be harvested as latex, provided collection is completed within 72 hours after puncture tapping. This system, which is primarily intended for trees entering the last phase of their economic life, is still awaiting commercial application.
The number of trees a tapper is assigned to tap in a day is called a tapping task. It depends on the number of cuts per tree, their length, and on the age and condition of the trees and the topography of the land. A skilled tapper can tap (with half spiral downward) 400-500 trees in 3-4 hours. If a tapper begins at 6 a.m. he can start collecting the latex about 5 hours later when the latex flow from the cut has stopped. No tapping can be done during rain or when the panel is wet. If there is frequent morning rain then recovery tapping is carried out in the afternoon. An alternative solution to cope with rain interference is to use rainguards. These are used, for instance, in dense stands of young rubber, where tapping panels remain wet even after rain has long since stopped.
The latex is collected in buckets and brought to a central point for bulking and transport to the factory. On some estates the latex runs from the spouts into polythene bags which are collected at regular intervals by ordinary labourers. The bags contain coagulated latex from several tappings, which requires special processing. Because of the shortage of tappers, management research and development is currently paying special attention to the division of tapping and collecting labour and to optimizing tapping tasks.

Yield of rubber is largely dependent on the cultivar planted and the agro-management inputs given to the trees during the periods of immaturity and production. Because of the superior management and better inputs, yields are normally higher on estates than on smallholdings.
In general, latex yield is expressed in kg/ha per year. In South-East Asia, average estate yield is about 1500 kg/ha per year, ranging from 1200-2000 kg/ha per year. The average yield from a smallholding is about 800 kg/ha per year and ranges from 400-1500 kg/ha per year. In Malaysia, the average national yield is about 1150 kg/ha per year.

When rubber latex arrives at the factory, it is filtered and bulked before coagulation. After coagulation, it is processed into either sheet rubber, crepe rubber or block rubber. Generally, formic acid is used to coagulate the latex. Under normal factory conditions and depending upon the concentration of the latex and the acid used, this process takes a few hours.
If the production line is set up for sheet rubber, the coagulated rubber is milled through 3-4 different pairs of rollers. The first few rollers are usually smooth and the last is ribbed. The milled sheets are then dried in a smoke house for 4 days to produce ribbed smoked sheets (R.S.S.).
If crêpe rubber or air-dried sheet is required, the coagulated rubber is milled using a battery of power-driven crêpers to produce a well-knitted thin crêpe. After milling, the crêpe can then be dried in suitably constructed hot air rooms or chambers.
To produce SMR (Standard Malaysian Rubber) block rubber, the coagulum is generally crêped and hammer-milled to produce crumb rubber. The crumbs are dried in hot air at 110°C in deep-bed driers. The dried crumbs are then baled into 33,333 kg bales, wrapped in polythene sheets and packed into 1 t wooden crates.
For latex concentrate production, the filtered latex is subjected to one of the following methods of processing: centrifugation, evaporation, or creaming. In South-East Asia, centrifugation is the most widely used. During centrifugation, the lighter rubber particles are separated from the heavier serum to produce a concentrated fraction of about 60% dry rubber content. These are then stored and tested before export. For export, the latex is either shipped in bulk in the ship's deep tank, or in containers, or in flexible bags, usually of 1 t capacity.

The surviving Hevea seedlings from Wickham's introduction in 1876 into South-East Asia provide only a narrow genetic base. Subsequent introductions into Indonesia made by the Dutch (1896, 1898 and 1913-1916) and by the British into Peninsular Malaysia (1951-1954) were added as genetic resources but did not have much impact on breeding progress. Another small introduction of various Hevea species was made into Malaysia in 1966. Further augmentation of genetic resources in the South-East Asian region was implemented through the introduction of large numbers of wild Hevea germplasm from Brazil in 1981 and 1992. Germplasm collections are maintained in Malaysia and Ivory Coast.

Selection and breeding of new rubber clones or cultivars are still the most efficient means of reducing the cost of production. However, it has now been realized that further spectacular yield increases as occurred during the early years of controlled breeding are most unlikely to occur again. Over the last 60 years, 6-fold yield increases, i.e. from 500 to 3000 kg/ha per year, have been achieved. Modern clones like RRIM (Rubber Research Institute of Malaysia) 600, 712, 901 and 2001, PR (Proefstation voor Rubber, Indonesia) 255 and 261, PB (Prang Besar, Peninsular Malaysia) 217, 235, 255, 260 and 350, and GT (Gondang Tapen, Indonesia) 1 are products of this achievement, with yield averaging about 2 t/ha per year after the first 5 years of tapping. However, present-day breeders recognize that emphasis should not be placed on yield alone but on other desirable characteristics as well, such as vigour, quality of virgin and renewed bark, colour and stability of latex, resistance to leaf and bark diseases and to wind damage and timber volume produced. Response to stimulation and to low intensity tapping has become an additional criterion in selection. The use of other Hevea species to incorporate resistance to leaf diseases (in particular South American Leaf Disease) has also been pursued in breeding.
Another Hevea species, Hevea spruceana Müll.Arg., was introduced into Indonesia in 1913 for breeding purposes, without any positive results. It easily hybridizes with Hevea brasiliensis and therefore seeds collected from areas containing Hevea spruceana progeny might constitute a danger to the established Hevea brasiliensis plantations, as both Hevea spruceana and its hybrid with Hevea brasiliensis yield only small amounts of rubber of poor quality. Hevea spruceana differs from Hevea brasiliensis e.g. in the veins on the lower leaf surface being hairy, the flowering shoots being situated at the end of strong twigs, the larger flowers which are tinged with red or mauve, and the fruit being pear- shaped.

The prospects for natural rubber are very good. Demand is expected to increase in view of the demands of the automobile industry and the possible diversification of rubber in manufacturing. This would help to stabilize prices at a favourable level on the world market and persuade planters to continue planting rubber. Rubberwood has important uses in the manufacture of medium density fibreboard, furniture and parquet which strengthen the economic viability of rubber growing.
In most rubber-producing countries, rubber will continue to be cultivated, although the scale and emphasis will vary from country to country. This is because in most countries rubber is still an important cash crop for planters. Various governments have increased the budgets for research and development and given new emphasis to the transfer of technology to smallholders. Incorporating new germplasm into Hevea breeding programmes has improved the prospects for further yield improvement.

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Mohd Noor A. Ghani & M. Wessel

Ghani, M.N.A. & Wessel, M., 2000. Hevea brasiliensis (Willd. ex Juss.) Müll.Arg.. In: Boer, E. and Ella, A.B. (Editors): Plant Resources of South-East Asia No 18: Plants producing exudates. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea