African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 5, Num. 3, 1997, pp. 309-324

FORUM - Managing waterhyacinth invasion through integrated control and utilisation: perspectives for Lake Victoria

P. L. WOOMER

Department of Soil Science, University of Nairobi, PO Box 29053, Nairobi, Kenya

(Received 11 April, 1997; accepted 16 August, 1997)

Code Number: CS97038
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ABSTRACT

Achieving control of the biological invasion of waterhyacinth (Eichhornia crassipes) in Lake Victoria has become a major priority in East Africa. Waterhyacinth was first introduced to Africa about 1890 in Egypt but its spread upriver was restricted. A later introduction in 1942 resulted in its spread along the entire length of the Congo River within 14 years and it is believed to have crossed into the upper Nile shortly thereafter. Integrated management of waterhyacinth consists of chemical and biological control, and mechanical and manual clearance. The smooth waterhyacinth weevil (Neochetina eichhornia) is widely utilised in biological control programmes throughout the tropics and sub-tropics, including East Africa. This insect was observed feeding on banana, cabbage and vanilla under laboratory conditions, but did not deposit eggs into these crop plants as it does with waterhyacinth. The herbicide 2,4-D is commonly employed for chemical control. Mechanical and manual clearance occupies an important role as an emergency measure, especially in the decongestion of harbours and hydroelectric reservoirs. The Owen Falls hydroelectric facility on the River Nile (Uganda) is currently protected from waterhyacinth by mechanical harvesting using four push boats with front-mounted hydraulic rakes and two floating conveyer belts. Waterhyacinth may be used as an organic input to soils, livestock feed, source of crude fibre, substrate for biogas generation and in waste water treatment. Waterhyacinth is rich in plant nutrients, particularly phosphorus and potassium, but its high moisture content (95%) restricts recovery and transport. Fresh waterhyacinth offers less potential as an animal feed than when it is ensiled or mixed with other rations. Utilisation of waterhyacinth as a fibre or in biogas production in East Africa requires that processing constraints be overcome and industrial capacities improved. Waterhyacinth should be regarded as a weed foremost and an under-exploited resource secondly. Any attempts to better utilise waterhyacinth must be nested into our attempts to destroy it.

Key Words: Aquatic weed, biological control, East Africa, Eichhornia crassipes, herbicides

RESUME

La lutte contre la propagation de la jacinthe d'eau (Eichhornia crassipes) sur le lac Victoria est devenue une priorite en Afrique de l'Est. La jacinthe d'eau etait introduite en Afrique aux alentours de 1890 en Egypte mais sa propagation etait limitee. La reintroduction ulterieure de l'herbe aquatique en 1942 a resulte en sa propagation tout le long du fleuve Congo dans l'espace de 14 ans. Et il est estime que la propagation aurait atteint la region du Haut Nil quelques temps apres. Le programme integral pour la luttle contre la jacinthe d'eau consiste l'eliminer par moyens biologiques, chimiques, mecaniques et le desherbage manuel. Le charancon de la jacinthe d'eau (Neochetina eichhornia) est d'une grande utilite dans le cadre du programme de la lutte contre l'herbe aquatique dans toutes les regions tropicales et sub-tropicales, y compris l'Afrique de l'Est. Les observations au laboratoire ont etabli que le charancon ne ponde pas sur les bananes ni les choux ou vanilles malgrequ il se nourrit de ces cultures. L'herbicide 2.4 est d'habitude utilise dans la lutte chimique contre la propagation de la jacinthe d'eau. Le desherbage manuel et mecanique constitue une mesure importante surtout dans le cadre du degagement de la plante de ports et reservoirs electriques. La centrale hydroelectrique d'Owen sur le fleuve Nil (en Ouganda) est actuellement, protegee contre la jacinthe d'eau par les methodes de desherbage mecanique en utilisant quatre remorqueurs avec les grilles montees en avant et avec deux transporteurs courroies. La jacinthe d'eau peut etre utilisee comme matiere organique du sol, fourrages du betail, source des fibres vegetales, dans la generation substratique du biogaz et dans l'epuration des eaux usees. La jacinthe d'eau est riche en nutritifs de plantes, en particulier en phosphore et en potassium, mais sa teneur elevee en eau (95%) limite sa recuperation et transportation. La jacinthe d'eau fraiche est faible en nutritifs si elle est utilisee comme fourrages du betail plu que lorsqu' elle est ensilee ou melangee avec d'autres rations alimentaires. L'usage de la jacinthe d'eau dans la production des fibres ou du biogaz en Afrique de l'Est exige que des problmes lies la transformation du produit soient regles et que la capacitede transformation et traitement soit amelioree. La jacinthe d'eau devrait tres d'abord consideree comme une mauvaise herbe et ensuite comme une resource inexploitee. Donc, toute tentative d'utiliser la jacinthe d'eau devrait s'orienter vers sa destruction.

Mots Cles: Herbe aquatique, lutte biologique, Afrique de l'Est, Eichhornia crassipes, herbicides

INTRODUCTION

Over the past decade, the biological invasion of waterhyacinth (Eichhornia crassipes), an aquatic weed, has posed a serious obstacle to the exploitation of Lake Victoria's resources. The floating weed is pushed by wind, predominantly from the southeast, to the northern lake shore where its accumulation limits access to lake waters by fishing communities, depriving them of livelihood and protein. This problem is now being addressed by an integrated control programme which includes the release of biological agents and harvesting near harbours. The release of the smooth waterhyacinth weevil (Neochetina eichhornia) has proven to be an effective control agent in programmes in other parts of the world including Florida, India, Thailand, Indonesia, Egypt and Zambia (Bennett, 1984). Mechanical clearance (Baruah, 1984) and chemical control using herbicides, particularly 2,4-D (Julian, 1984) have also been employed in developed (Australia, USA) and intermediately developed nations (Malaysia), but developing nations find manual clearance to be more cost effective (Baruah, 1984) particularly when populations along shorelines are large and adversely affected (Soerjani, 1984). Following the implementation of manual control programmes along Lake Victoria, large mounds of waterhyacinth began to accumulate.

But mounds of waterhyacinth in Uganda and Western Kenya were observed to dry too slowly for removal by burning (M. Bekunda and B. Jama, personal communications). This difficulty has been documented elsewhere, including Fiji. Manually cleared waterhyacinth in Fiji initially contained 95.8% water, which was reduced to only 72% after 15 days with temperatures and humidity averages of 25 C and 68%, respectively (Solly, 1984), conditions similar to the Lake Victoria Basin (Hargreaves and Samani, 1986). Strategies of waterhyacinth utilisation have been developed in many parts of the world including its use in biogas generation, paper-making, livestock and small animal feed and as an organic addition to soils (NAS, 1977) but an important initial step in processing is de-watering, which allows for harvested material to be more efficiently transported (Bagnall et al., 1973). Given the immediacy of waterhyacinth removal along the shores of Lake Victoria and the industrial capacities of Kenya and Uganda, the most practical short term utilisation strategies are as animal feed (Easley and Shirley, 1974) and compost (Karim, 1948; Kamal and Little, 1970). While the moisture content of waterhyacinth is very high, and its drying and decompositions rate slow, the nutrient and nutritional contents of waterhyacinth are favourable with 24% protein, 3.8% nitrogen, 1.1% phosphorus, 5% potassium and 21% fibre (John, 1984). How then may the vegetation mats blockading small harbours be best controlled and the smouldering mounds accumulating along the lake shore be of greatest benefit?

THE BIOLOGY OF WATER HYACINTH

Taxonomy, anatomy and habitat. Water-hyacinth is a perennial, free-floating freshwater plant originating from South America and classified as Eichhornia crassipes (Martins) Solms (NAS, 1976). It belongs to the family Pontederiaceae, which contains eight other genera (Gopal, 1987). Seven other species of the genera are reported, six of which are native to South America and one other, E. natans, is native to Africa (Gopal, 1987). Only E. crassipes is regarded as a pan-tropical weed of lakes, rivers and canals. The widespread distribution of waterhyacinth is partly attributable to an attractive purple flower, prompting its establishment within many botanic gardens during the late nineteenth century (e.g. see Soerjani, 1984) prior to its recognition as a noxious aquatic weed. Water-hyacinth consists of a fibrous root system, a basal rhizome, elongated, buoyant petioles and a small, simple leaf. Flowers are lavender and borne on a terminal inflorescence bearing up to 60, but usually 8-15, flowers. The fruit is a thin walled capsule containing up to 450 seeds. The plant reproduces sexually, producing small dark seeds, and by vegetative propagules. The roots are adventitious, unbranched, darkly pigmented ending in a conspicuous root cap and may extend to 3 m (Gopal, 1987). Waterhyacinth often grows with other aquatic weeds including Salvinia molesta (Tjitrosoedirdjo and Wiroatmodjo, 1984), the grasses Panicum repens and Paspalum distichum (Batanouny and El-Fiky, 1984). Some of the largest, most complex free-floating aquatic stands are referred to as "floating islands" reaching 45 ha in size and used for rice cultivation and animal grazing(TjitrosoedirdjoandWiroatmodjo, 1984).

TABLE 1

Productivity. Waterhyacinth is one of the most productive plants on earth. Reports of productivity by this aquatic plant include 173, 123 and 106t ha^-1 yr^-1 in Florida, Guyana and Indonesia, respectively (Gopal, 1984). Wolverton and McDonald (1978) reported a growth rate of 800 kg ha^-1 day^-1. Productivity tends to be less with decreasing temperature and greater in nutrient rich waste waters (Haider, 1984; John, 1984). A stand of 19.7 t ha^-1 (d.w.) was reported in India, containing 2415 kg N and 465 kg P ha^-1 (Baruah, 1984). Batanouny and El-Fiky (1984) reported a 30-fold increase in biomass which produced 43 offsets (vegetative propagules) over 50 days. The coverage of waterhyacinth in the Curug Reservoir in Java changed from 3 to 48 ha in 50 days (Tjitrosoedirdjo and Wiroatmodjo, 1984). A waterhyacinth production experiment in Florida reported 67 kg fresh weight m^-2 recovered over 13 months and 21 harvests (Reddy and D'Angelo, 1990), equivalent to 620 t ha^-1 yr^-1. Other reports of waterhyacinth biomass and productivity, suggesting a wide range of performance in different aquatic systems and climates, are presented in Table 1. Indeed, the great productivity of waterhyacinth is responsible for both its threat as an aquatic weed and its potential as a vegetation resource.

Root physiology. A unique feature of water hyacinth is its capacity to absorb chemical species from solution, resulting in biological approaches to waste water treatment. Haider (1984) reported selectivity in copper, zinc and iron uptake by waterhyacinth with most of these metal ions stored in the root and stem. For example, Cu^+2 increased from 19 and 91 ppm to 131 and 1500 ppm within two days in the hyacinth stem and root, respectively. Waterhyacinth has also been reported to remove mercury (Lenka et al., 1990), chromium (Saltabas and Akcin, 1994), cadmium (Rai et al., 1995) and phenolics (Nor, 1994) from solution.

THE BIOLOGICAL INVASION OF AFRICA

The first introduction of waterhyacinth to Africa was in Egypt around 1890, originally to plant in public garden in Cairo. It escaped into the Nile Delta but has not spread beyond (Batanouny and El Fiky, 1984). Another early introduction was to Natal, South Africa in 1910. It is also reported in the Transvaal, southern Mozambique and Zimbabwe (Gopal, 1987), probably assisted by man. The greatest spread of waterhyacinth is thought to have resulted from its introduction to the River Congo in 1942. It was established along the entire length of that river by 1956, and is believed to have crossed over into the Nile Basin through an interconnective swampy area in south-western Sudan (Bebawi, 1972). This route is most likely responsible for the current biological invasion in Lake Victoria. Gopal (1987) cites the spread from the Congo to the Nile as the only example where waterhyacinth has invaded a maj or, new watershed unassisted by humans. During colonial times, the cultivation and transportation of waterhyacinth was prohibited in East Africa, but these restrictions appear to not have been enforced following independence. Waterhyacinth is reported in several other countries including Angola, Madagascar and Senegal, although it is not widespread in West Africa (Gopal, 1987).

THE CONTROL OF WATER HYACINTH

There are four individual means of controlling waterhyacinth invasions, through biological control, herbicides, mechanical removal and manual clearance. These methods may be combined in various ways into integrated control programmes. The intensity of biological invasion, type of waterway, availability oflabour and access to specialised equipment dictate which combination is most effective for a given circumstance.

Biological control. Both arthropods and microorganisms have been collected, tested and released to control waterhyacinth. The most widely released organism is the smooth hyacinth weevil (Neochetina eichhorniae) belonging to the beetle family (Coleoptera). A closely related species, N. bruchi, has also been studied but tends to be less effective and to compete with N. eichhorniae (Harley, 1984). Both larvae and adults of these species feed upon hyacinth. but N. eichhorniae has also been documented to feed and oviposition upon otherplant species, including a few economic ones (Table 2). Two moths which produce larvae that feed upon the leaves and petioles of hyacinth, Sameodes albiguttalis and Acigona infusella, have also been released in Queensland. Control levels resulting from these arthropods reported in Australia include 57% of the leaf area and 62% of the original hyacinth coverage (Wright, 1984). Other arthropods which are reported to attack waterhyacinth include grasshoppers (Napompeth, 1984), other Lepidoptera and mites (Harley, 1994).

TABLE 2

Kajjansi is the site of a brick factory along the Entebbe Road in Uganda. A large clay excavation called Pan Lake (about 8 ha) has unintentionally flooded and is now completely colonized with waterhyacinth. This is a release site near Lake Victoria of the smooth hyacinth weevil (Neochetina eichhonziae)in 1995. Weevil damage was widespread, and weakened plants also appear more susceptible to a leaf disorder (probably Cercospora). While it is important to note that the weevil is well established, and the waterhyacinth was visibly damaged by them, it is also important to note that the stand of waterhyacinth and other aquatic weeds continue to occupy 100% of the artificial lake surface.

Herbicides. Use of herbicides offers the most complete form of control (Julian, 1984) particularly in confined (vs open) waterways such as narrow rivers and irrigation and drainage canals (Yusof, 1984). The most commonly applied herbicide is 2,4-D, which may be mixed with kerosene so that the formulation floats on the water surface. Gopal (1987) has assembled reports on the efficacy of 2,4-D on waterhyacinth and finds 40-100% control obtained in as few as 6 days, although complete control usually requires two applications. As little as 1.0 kg 2,4-D ha^-1 has resulted in complete control of water hyacinth in Australia (Julian, 1984). The herbicide 2,4-D decomposes rapidly. United States limits of acceptability in fish and drinking water are 1 ppm and 0.1 ppm, respectively (Thomas Moorehouse, personal communication). Other herbicides have been evaluated for waterhyacinth control including Diquat, Paraquat, Amitrol and Ametryne but these compare unfavourably to 2,4-D due to higher application rates and costs and/or reduced and delayed effects (Gopal, 1987). One promising alternative is Glyphosate (Roundup) which acts more slowly, but results in complete kill and reduced environmental impacts (Chen et al., 1989) with acceptable levels in potable water set at 0.7 ppm (Thomas Moorehouse, personal communication). The advantages of herbicide application include the ease of application and low costs but some constraining disadvantages include high start-up costs, damage to non-target and beneficial plants, particularly when applied by air, and the inability to remove dead materials irom water bodies (Baruah, 1984). Kaijansi is also a site of herbicide research in Uganda. Several 5 m x 6 m pools were dug and planted with waterhyacinth. These pools were sprayed with 2,4-D, Glyphosate and Diquat. It was observed that 2,4-D resulted in rapid leaf necrosis while Glyphosate had a slow but more complete and lasting effect (M.K. Magunda, personal communication).

Mechanical harvesting. Baruah (1984) cites mechanical clearance as offering advantage because it is rapid and physically removes the weed from the water body. Several harvesters of aquatic weeds have been developed based on bucket or rake designs (NAS, 1976). Forexample, the Grinwald-Thomson Harvester can clear 2.77t of weeds hr^-1 and costs $64 hr^-1 to operate (Baruah, 1984). There are several disadvantages to control through mechanical harvesting, other than initial costs, including difficulties of access by conveyors and trucks and the requirement for disposal following harvest. The greatest constraint for developing nations is the initial and maintenance costs of the heavy equipment. Nonetheless, mechanical harvesting has proven effective in developing countries, such as Malaysia, where dragline excavators clear irrigation and drainage canals of heavy waterhyacinth growth (Yusof, 1984).

Owen Falls is the site of the hydroelectric facility on the River Nile near Jinja, Uganda. As of last September, the waterhyacinth mat was estimated to be 55 ha. Chemical control is not feasible because killed waterhyacinth will sink and threaten the function of the hydroelectric turbines. The Government of Uganda has mounted a very successful campaign of mechanical harvesting and as of March 1997 the waterhyacinth stand has been removed. This effort relies on four Conver push boats with front-mounted hydraulic rakes and two floating conveyer belts which are used to load trucks at water's edge. One estimate the waterhyacinth harvest to be at least 900 t (d.w.) over six months. The hyacinth is transported to two nearby fields, dumped and bull dozed into wind rows (Thomas Moorehouse, personal communication).

Manual clearance. Manual control, that is the physical removal of aquatic weeds using manual labour, has been cited as the least desirable control measure by some authors (Baruah, 1984; Yusof, 1984) and the most practical by others (Tjitrosoedirdjo and Wiroatmodjo, 1984). Yusof cites difficulties in job completion and problems in supervision as limitations in Malaysia, while Tjitrosoedirdjo and Wiroatmodjo (1984) cite successes in small lakes and reservoirs when hundreds of villagers are mobilised to "clean up" their local aquatic resources. But manual removal is slow, as low as 0.26 t hr^-1 and the costs, estimated to be $83-$111 ha^-1 (Baruah, 1984) are greater than chemical control ($20-60 ha^-1). Nonetheless, when communities are confronted with waterhyacinth invasion of their waterways and harbours, and no other means are available, manual clearance becomes the only option available to them to regain access to their aquatic resources.

A 650 ha stand of waterhyacinth is established in Lake Victoria near Port Bell. When the mat impeded the harbour in the past, manual clearance was conducted by a Ugandan Veterans Association. Harbour operators consider the wind direction to be the cause of waterhyacinth movement within the bay. Another manual clearance operation is under way in Western Kenya, where fishermen are removing the waterhyacinth that blocks their small barbours using small boats and drag lines.

Integrated control. In the above discussion, we have focused on the strengths and weaknesses of individual control measures. While some single measures have proven effective (Julian, 1984) or necessary (Tjitrosoedirdjo and Wiroatmodjo, 1984), effective control of water hyacinth requires well-thought combinations of control measures linked with a strategy to utilise harvested weed biomass. The release of hyacinth weevils has proven partially effective in many parts of the world (Bennett, 1984) with little risk to economic plants (Nagarkatti and Jayanth, 1984). Chemical control of remaining stands is highly effective but must be weighed against environmental consequences and impacts on non-target plants. Wright (1984) reports that biological and chemical control measures may interact, as the moth S. albigutta was observed to selectively attack plants weakened by sub-lethal doses of herbicides. Gupta et al. (1989) reported 38% toxicity to N. eichhorniae six weeks after application of 2,4-D but still observed active damage to leaves and petioles, concluding that the biological and chemical control measures were compatible.

Technologies exist for mechanical clearance of congested waterways, reservoirs and harbours. Communities most affected by invasions have been successfully mobilised to manually remove congestions of hyacinth that deprive them access to aquatic resources. Integrated control campaigns have greatly reduced hyacinth stands in Australia (Smith et al., 1984), Florida (Bagnall et al., 1973) and Malaysia (Bakar et al., 1984) but have proven less so in Indonesia (Tjitrosoedirdjo and Wiroatmodjo, 1984), Thailand (Napompeth, 1984) and Sudan (Obeid, 1984) usually due to lack of financial resources. The control of waterhyacinth is complicated by the longevity of seed stocks, which are reported to maintain viability for seven years (Gopalakrishnan and Joy, 1977). The biological invasion of waterhyacinth into Lake Victoria and other smaller waters of East Africa is not a unique phenomenon, and one may even wonder why this is so late in occurring. Technologies exist to combat this outbreak but what remains is to place the components of an integrated control campaign into a structure that is appropriate to the conditions of Lake Victoria.

UTILISATION OF WATER HYACINTH

TABLE 3

Humankind's appreciation of waterhyacinth has changed over time. It was first described in South America by plant explorers as a beautiful flower and transported to botanic gardens in Europe (Gopal, 1987). From Europe, it was transported to botanical gardens in the tropics from which it invaded neighbouring freshwaters and became recognised as a noxious weed (NAS, 1977). When legislative, chemical, biological and manual control measures failed to eradicate waterhyacinth, greater emphasis was placed upon its economic utilisation (NAS, 1977; Thyagarajan, 1984), since it has a fairly high nutritive value (Table 3). To some extent, the utilisation of waterhyacinth may be regarded as making the best of a bad situation, yet this plant has found useful application in agriculture, animal industry, energy generation, paper and board making and other applications. Less optimistic reports in the literature suggest that the value of waterhyacinth is consistently less than the costs of recovery and processing (see Mara, 1976) but this approach assumes that waterhyacinth is being produced as a crop rather than removed as a weed with accompanying disposal problems.

Soil additive. Waterhyacinth is used as an organic input to agricultural systems as a fresh or dried mulch, and as a compost. Kamal and Little (1970) reported that waterhyacinth applied as a surface mulch greatly suppressed Cyperus rotundus and Cyandon dactylon, two pan-tropical weed species, within two months. Waterhyacinth mulch is applied to tea estates in India (Gopal, 1987). However, the use of waterhyacinth as a source of plant nutrients leads to several complexities. Fresh water hyacinth contains 95% water, and is slow to dry and decompose. Yet on a dry matter basis, it contains relatively high levels of plant nutrients (Table 2) and has a C:N ratio favourable to N release (Dhar and Srivastava, 1976) resulting in 2.5-fold increases in cereal yields. Farmers have successfully included waterhyacinth as an important ingredient in small-scale composting operations, along with wood ash and livestock manure (Watson, 1947). Haider et al. (1984) reported that compost prepared from water-hyacinth contained greater amounts of N, P, K and Ca than that prepared from cattle manure. They also reported that a finished compost was produced within 30-35 days. One serious constraint to the transportation of water hyacinth as an organic input is its high water content, but several methods of rapid dewatering, including rolling, pressing and chopping have been developed (NAS, 1977). Waterhyacinth appears in abundance along the Lake Victoria shoreline adjacent to banana-based farming systems undergoing severe yield declines (see Bekunda and Woomer, 1996) and utilisation of this aquatic weed may offer an important source of organic inputs to those systems.

Few reports on the litter decay and nutrient release of waterhyacinth are available, especially in aerobic and terrestrial systems. Ayyappan et al. (1992) report anaerobic decomposition of 2.2% daily. Hammerly et al. (1989) found decomposition rates of submerged hyacinth to range between 1.2-4.3% daily, depending upon the extent of litter fragmentation. In contrast, Sastroutomo (1991) reported delayed decomposition by water-hyacinth with 70% of the shoot biomass remaining after 28 days. Guar et al. (1989) found that leaves decay more rapidly than petioles and later suggested that bacteria, and not fungi, predominate degradation (Guar et al., 1992). Because of the site specificity of decomposition processes (Brown et al., 1994), these reports do not provide useful guidelines to the decay rates and nutrient release patterns likely to occur in terrestrial systems in the Lake Victoria region. Waterhyacinth has also been identified to possess specialised applications in agriculture. Dried wastes were found a suitable media for the production of straw mushrooms in Thailand (Tansukul and Klitsaneephaiboon, 1983). Blended waterhyacinth controlled the root knot nematode on tomatoes in Nigeria to the same degree as a pesticide application (Onifade and Egunjobi, 1994).

Animal feed. Much attention has been directed toward the use of waterhyacinth as an animal feed. Ruminants and pigs are observed to feed on fresh waterhyacinth (Gopal, 1987). In general, weight gains of animals fed hyacinth are less than those observed with grains, yet waterhyacinth mixed with other materials is both palatable and nutritive. Solly et al. (1984) concluded that 25% waterhyacinth is an acceptable supplement to high grade feed for piglets and rabbits. When alfalfa was substituted with 20% pelleted waterhyacinth, there was no difference in the weight gain of young rabbits (Moreland et al., 1991). Reddy and Reddy (1974) reported that a 20% supplement of dried waterhyacinth added to regular feeds did not reduce the consumption or weight gain of young cattle. Silage was prepared by mixing chopped and dewatered waterhyacinth with 5% molasses and 0.5% urea (Chakraborty et al., 1991) and proved a suitable feed for milking cattle when combined with para grass (Bracharia mutica). It is important to note that the nitrate, oxalate and cyanide content of waterhyacinth is not detrimental (Shirley et al., 1973).

In many cases, waterhyacinth was not a suitable complete ration. Gopal (1987) cites several examples where livestock are unable to maintain weight in absence of other feeds. Solly et al. (1984) state that goats will consume waterhyacinth, but the results are only acceptable under low growth rate programme. Poultry are less suited to waterhyacinth feeds with rations above 7.5% waterhyacinth reducing weight gains (Soesia-wangingrini et al., 1979). Solly et al. (1984) reported that waterhyacinth was detrimental to ducklings. It was also a poor replacement for maize or soy meal in the diets of laying hens, where only 3% could be substituted without negative effects on egg laying (Benicio et al., 1993). Yet most of the concerns over the use of waterhyacinth stand with respect to weight gains attainable under high quality feeds rather than the feeding conditions common within smallhold farming systems where even low quality roughage is often in short supply.

Biogas generation.

TABLE 4. Biogas generation and percentage methane from anaerobic fermentation of waterhyacinth "fuel"

The anaerobic fermentation of waterhyacinth wastes is best studied in three progressive stages. First, a simple laboratory digester similar to that of Ghole et al. (1984) are constructed and the biogas yields measured from waterhyacinth prepared in different manners and mixed with other organic substrates (Fig. 1). Then largerscale fermenters are designed similar to that of Polisetty et al. (1984) and Guha et al. ( 19 84) and the biogas yields compared using semi-continuous feeding regimes (Fig. 1 ). Finally, prototype pilot biogas fermenters for cooking fuel are populafised in community-ofiented development programmes, as was done in the Karimnagar Biogas Project in India (Murty and Thyagarajan, 1984).

FIGURE 1

Fibre products. Waterhyacinth is better suited to the production of coarse rather than fine fibre products. A process was first patented in the US in 1926 (Callahan, 1926). Later, Azam (1941) reported that only petioles were suitable for pulping because leaves result in brittleness and roots in dark discolouration. Research teams at Universities in Florida and California have reported failure in overcoming processing constraints, one widely experienced processing problem is the high moisture retention of fibres, rendering it unacceptable for high-speed pressing (Gopal, 1987). The pulp appears better suited to the fabrication of particle board, although hyacinth fibres alone tend to be brittle and disintegrate (Gopal, 1987). One alternative approach has been its incorporation into cement (reinforced) pressed boards or into waxed paper (Ghosh etal., 1984). It is difficult to foresee waterhyacinth pulp as a substitute fibre product in the Lake Victoria Basin until significant processing constraints are overcome elsewhere and industrial capabilities are increased within the sub-region.

Wastewater treatment. Waterhyacinth plays an important role in the upgrading of settling ponds containing sewage effluents (Wolverton and McDonald, 1978), livestock manures (Stanley, 1974) and the by-products from sugar (Parashar, 1970), tannery (Prashad et al., 1984), palm oil (Yosuf et al., 1984), cheese (Monray et al. 1995) and rubber factories (John, 1984). Rogers and Davis (1972) reported that 1 ha of waterhyacinth is capable of removing effluent from 800 persons. The uptake and deactivation of heavy metals, organic compounds and human pathogens has also been reported (Gopal, 1987). The great tolerance of waterhyacinth to metal ion imbalance and its storage mechanisms of toxic materials (Haider, 1984) continues to be an emerging field of plant physiology, but is of less concern to its utilisation when the aquatic plant is recovered from non-polluted freshwater. While opportunity exists to improve water treatment within settling ponds colonised by waterhyacinth, this is not seen as a viable area of study for the Lake Basin unless stringent safeguards of its containment are in place.

Other uses. Other uses of waterhyacinth are reported in the literature including its complete combustion into "charcoal black" pigments (Sumathi et al., 1984), production of activated carbon (Mahtabuddin Ahmed et al., 1984), and extraction for liquid-based proteins (Kashem et al., 1984). While these avenues of utilisation may offer potential in other areas of the world, there appears to be little immediate opportunity given the present industrial base and markets of the Lake Victoria Basin.

HYACINTH CONTROL IN LAKE VICTORIA

Given the findings of researchers presented in this paper and the current status of waterhyacinth control and utilisation under study in the Lake Victoria Basin, some predictions concerning future success and impacts may be drawn. Biological control agents will continue to be released and will successfully colonise congested shoreline populations of waterhyacinth reducing population densities and biomass by up to 70%, but will have little effect on source populations occurring in open waters of Lake Victoria. Biocontrol agents will periodically fail to control aquatic weed "blooms". The smooth waterhyacinth weevil will occasionally be observed feeding on banana and vanilla, but will not become an economic pest. Herbicides will be applied to the remaining shoerline populations on an irregular basis, but application to source populations will prove impractical. Herbicidws will not prove effective in protecting the hydroelectric plant at Owen Falls because dead, submerged plants pose a greater threat than floating communities. Some fish "kills" will be observed in shallow protected waters receiving applications of herbicide but this condition will have resulted from high biological oxygen demand by decaying residues rather than herbicide by-product toxicity and will not become widespread.

The hydroelectric plant at Owen Falls and larger harbours such as Port Bell and Kisumu, will continue to rely on a combination of mechanical harvesting and manual clearance. Difficulties in maintaining and replacing large mechanical harvesters will result in periodic congestion of reservoirs and larger harbours. Large piles of waterhyacinth wastes will accumulate in these areas and require large-scale, mechanised utilisation strategies such as processing into compost or animal feed. Manual clearance of hyacinth communities reduced by biological control agents and shoreline spraying programmes will continue to be the major means of providing access to smaller fishing harbours. Waterhyacinth wastes will accumulate, and will at first be dried and burned until local farmers become aware of the benefits from using the wastes as organic inputs and animal feeds.

Despite the potential applications of waterhyacinth to agriculture as an organic input to soils and a feed for livestock, we must remember that waterhyacinth is a weed and biological invader first, and a resource second. Any programme of waterhyacinth must be nested into the mechanical and manual clearance components of an integrated control strategy. Yet, waterhyacinth will not be eradicated from Lake Victoria, but rather communities will be reduced to 20-40% of their current populations. Occasionally, combinations of strong northerly winds, troughs of biocontrol agent activity and breakdown of mechanical harvesters will result in congestion of harbours. As much as 30,000 t yr^-1 of waterhyacinth will be recovered from 20 km^2 of harbour areas by mechanical and manual clearing, much of this will be processed and applied to nearby agricultural soils or fed to livestock, mainly to swine and goats.

CONCLUSION

The shorelines of Lake Victoria are being attacked. Not a military but rather a biological invasion is under way. The enemy has many names, the beautiful blue devil, Eichhornia crassipes and the water violet, but we know it best as waterhyacinth. The first few infiltrators arrived exploring new territory and we barely took notice. Then larger forces appeared along the horizon and we naively blamed the wind direction and assumed that threat would dissipate. Today the harbours are under blockade and hydroelectric stations under attack. Local militias of fisherman have been mobilised to capture thousands of the enemy daily and place them in smouldering piles along the shoreline. Strategic facilities, such as the Jinja hydroelectric plant, are protected by armoured harvesters that rake and chop the enemy at will without casualties except for metal fatigue from the slaughter. Because this enemy does not actively fight back, rather it relies on its ability to grow more rapidly than any foe can destroy it, this strategy until now is winning. Some optimists among us view the assault as a blessing, citing the many potential uses for waterhyacinth such as an organic fertilizer, a livestock feed or a processed fibre. But we should not be deceived by pacifists, this is first and foremost an aggressive and terrible weed resulting in ecological imbalance and lost aquatic resources and only then an organic resource. Any strategy to derive benefits from waterhyacinth must be built within our attempts to destroy it. Past works in other parts of the world suggest that waterhyacinth can never be eradicated, only weakened, and our battle against it will never end unless we lose courage and surrender our harbours and lake resources. But is this not too great a price to pay? Now is the time to engage in full naval, biological and chemical warfare and we only need be concerned with international opinion through our indecision and inactivity.

ACKNOWLEDGEMENT

The literature review and field visits were conducted while the author was a grantee of The Rockefeller Foundation, financial support which is greatly appreciated. The observations concerning waterhyacinth control efforts are based upon helpful discussions with Bashir Jama (ICRAF, Kenya), M.K. Magunda (NARO, Uganda) and Thomas Moorehouse (Aquatics Unlimited, USA).

REFERENCES

Ayyappan, S., Tripathi, S.D., Basheer, V.S., Das, M. and Bhandari, B. 1992. Decomposition patterns of manurial inputs and their model-simulating substrates. International Journal of Ecology and Environmental Sciences (India) 18:101-109.

Azam, M.A. 1941. Utilization of water hyacinth in the manufacture of paper and pressed boards. Science of Cultivation 6:656-661.

Bagnall, L.O., Shirley, R.L. and Hentges, J.F. 1973. Processing, chemical composition and nutritive value of aquatic weeds. Publication No. 25. Water Resources Research Center, University of Florida. Gainsville, USA. 55 pp.

Bakar, B.B., Amin, S.M. and Yousof, E.B. 1984. Water hyacinth (Eichhornia crassipes (Mart.) Solms) in Malaysia: Status of distribution, problems and control. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 944-958. UNEP, Nairobi.

Baruah, J.N. 1984. An environmentally sound scheme for the management of water hyacinth through its utilization. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 96-125. UNEP, Nairobi.

Batanounty, K.H. and El-Fiky, A.M. 1984. Water hyacinth in Egypt. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 127-144. UNEP, Nairobi.

Bebawi, F.F. 1972. Studies on the ecology of Eichhornia crassipes (Mart.) Solms. in the Sudan. MSc. Thesis, University of Khartoum.

Bekunda, M.A. and Woomer, P.L. 1996. Organic resource management in banana-based cropping systems of the Lake Victoria Basin, Uganda. Agriculture Ecosystems and Environment 59:171-180.

Benicio, L.A.S., Fonseca, J.B., Silva, D.J. da, Rostagno, H.S. and Silva, M. de A. 1993. Use of water hyacinth (Eichhornia crassipes) in pelleted diets for commercial laying hens. Review of the Society Brasileira Zootecnia 22:155-166.

Bennett, F.D. 1984. Biological control of aquatic weeds. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 14-41. UNEP, Nairobi.

Boyd, C.E. 1970. Chemical analyses of some aquatic vascular plants. Archives of Hydrobiology 67:78-85.

Brown, S., Anderson, J., Woomer, P.L., Swift, M.J. and Barrios, E. 1994. Soil biological processes in tropical ecosystems. In: Biological Management of Tropical Soil Fertility. Woomer, P.L. & Swift, M.J. (Eds.), pp. 15-46. John Wiley & Sons, Chichester, UK.

Chakraborty, B., Biswas, P., Mandal, L. and Banerjee, G.C. 1991. The effect of feeding fresh water hyacinth (Eichhornia crassipes) or its silage on the milk production of crossbred cows. Indian Journal of Animal Nutrition 8:115-118.

Chen, Y.L., Chaing, H.C., Wu, L.Q. and Wang, W. 1989. Residues of glyphosate in an aquatic environment after control of water hyacinth (Eichhornia crassipes). Weed Research (Tokyo) 34:117-122.

Callahan, J.B. 1926. Self sized sheets or webs from water hyacinth. US Patent 19.1595.448. 10 August 1926. Washington D.C., USA.

Dhahiyat, Y., Siregar, H. and Salim, F. 1984. Studies on the usage of water hyacinth as biogas energy resource in the dam of Curug (West Java). In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 604-614. UNEP, Nairobi.

Dhar, N.R. and Srivastava, S.P. 1976. Studies on crop yield by applying water hyacinth, rock phosphate in the presence and absence of N, P, K. Proceedings of the National Acadamy of Science (India) 41A:263-279.

Easley, J.F. and Shirley, R.L. 1974. Nutrient elements for livestock in aquatic plants. Hyacinth Control Journal 12:82.

Galbraith, J.C. 1984. The role of microorganisms in the biological control of water hyacinth in Australia. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 823-833. UNEP, Nairobi.

Gaur, S., Singhal, P.K. and Hasija, S.K. 1992. Relative contributions of bacteria and fungi to water hyacinth decomposition. Aquatic Botany 43:1-15.

Gaur, S., Singhal, P.K. and Hasija, S.K. 1989. Process of decomposition in Eichhornia crassipes (Mart.) Solms: 1. Early decomposition in different plant parts and effect of site variation. Journal of Environmental Biology 10:23-33.

Ghole, V.S., Hedge, M.V., Gunale, V.R. and Nair, S. 1984. Utilization of water hyacinth for biogas and particle board production. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 559-565. UNEP, Nairobi.

Ghosh, S.R., Saikia, D.C., Goswami, T., Chaliha, B.P., Baruah, J.N., Efrem, C., Jatkar, D.D. and Thyagarajan, G. 1884. Utilization of water hyacinth (Eichhornia crassipes) for paper and board making. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 436-460. UNEP, Nairobi.

Gopal, B. 1987. Water Hyacinth. Elsevier Science Publishers, Amsterdam. 471 pp.

Gopal, B. 1984. Utilization of water hyacinth as a new resource or for its control: some environmental considerations. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 193-206. UNEP, Nairobi.

Gopalkrishnan, R. and Joy, P.J. 1977. Aquatic weeds Salvinia and Eichhornia. Indian Farming 26:39-46.

Guha, B.R., Bandyopadyay, T.K. and Basu, S.K. 1984. Fuel gas from water hyacinth. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 615-623. UNEP, Nairobi.

Gupta, M., Jalali, M. and Pawar, A.D. 1989. A study to evaluate the compatibility of herbicides and a phytophagous insect to control water hyacinth weed. Plant Protection Bulletin (Faridabad) 41:40-42.

Hammerly, J., Leguizamon, M., Maine, M.A., Schiver, D. and Pizarro, M.J. 1989. Decomposition rate of plant material in the Parana Medio River (Argentina). Hydro-biologia 183:179-184.

Harley, K.L.S. 1984. Implementing a program for biological control of water hyacinth, Eichhornia crassipes. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 58-69. UNEP, Nairobi.

Haider, S.Z. 1984. Mechanisms of absorption of chemical species from aqueous medium by water hyacinth and prospects for its utilization. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 41-57. UNEP, Nairobi.

Haider, S.Z., Malik, K.M.A., Rahman, M.N. and Ali, M.A. 1984. Water hyacinth compost as a cheap fertilizer. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 751-758. UNEP, Nairobi.

Hargreaves, G.H. and Samani, Z.A. 1986. World Water for Agriculture. International Irrigation Center, Utah State University. 617 pp.

Howard, W.C. and Junk, W. 1977. The chemical composition of Central Amazonian aquatic macrophytes with special reference to their role in ecosystem. Achieves of Hydrobiology 79:446-464.

John, C.K. 1984. Use of water hyacinth in the treatment of effluents from rubber industry. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 699-612. UNEP, Nairobi.

Julian, A.C. 1984. Control of water hyacinth and water lettuce by the use of new formulations and application ideas. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 887-898. UNEP, Nairobi.

Kamal, I.A. and Little, E.C.S. 1970. The potential utilization of water hyacinth for horticulture in Sudan. Pest Articles and News Summaries 16:488-496.

Karim, A. 1948. Microbial decomposition of water hyacinth. Soil Science 66:401-416.

Kashem, M.A., Sen, K., Das, U., Islam, S., Khan, N.M., Huq, A.K.M.A. and Razzaque, A. 1984. Management of water hyacinth: Utilization of water hyacinth for animal feed. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 387-397. UNEP, Nairobi.

Knipling, E.B., West, S.H. and Haller, W.T. 1970. Growth characteristics, yield potential and nutritive content of water hyacinth. Proceedings of the Soil and Crop Science Society of Florida 30:51-63.

Lallana, V.H. 1981. Productividad de Eichhornia crassipes en una laguna islena de la cuenca del rio Parana medio. Ecologia 5:1-16.

Lenka, M., Panda, K.K. and Panda, B.B. 1990. Studies on the ability of water hyacinth to bioconcentrate and biomonitor aquatic mercury. Experimental Pollution 66:89-99.

Mahtabuddin Ahmed, Naim, A. and Mahtab, R. 1984. Preparation of active carbon from water hyacinth (Eichhornia crassipes). In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 785-792. UNEP, Nairobi.

Mara, M.J. 1976. Estimated values for selected water hyacinth by-products. Economic Botany 30:383-387.

Monray, O.H., Vazquez, F.M., Derramadero, J.C., Guyot, J.P., Orhon, D., Jiminez, B., Tilche, A. and Buitron, G. 1995. Anaerobic-aerobic treatment of cheese wastewater with national technology in Mexico: the case of "El Sauz". Water Science and Technology 32:149-156.

Moorhead, K.K., Graetz, D.A. and Reddy, K.R. 1990. Water hyacinth growth in anaerobic digester effluents. Biological Wastes 34:91-99.

Moreland, A.F., Collins, B.R., Hansen, C.A. and O'Brien, R. 1991. Wastewater grown waterhyacinth as an ingredient of rabbit food. Journal of Aquatic Plant Management 29: 32-39.

Murty, Y.S. and Thyagarajan, G. 1984. Development of low cost biogas plants under Karimnagar Project. In: Proceedings of the International Conference on WaterHyacinth. Thyagarajan, G. (Ed.), pp. 593-603. UNEP, Nairobi.

Musil C .F. and Breen, C .M. 1977. The application of growth kinetics to the control of Eichhornia crassipes and Salvinia molesta through nutrient removal by mechanical' harvesting. Hydrobiologia 53: 165-171.

Nagarkatti, S. and Jayanth, K.P. 1984. Screening biological control agents of water hyacinth for their safety to economically important plants in India. I. Neochetina eichorniae Warner (Col. Curculionidae). In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 868883. UNEP, Nairobi.

Napompeth, B. 1984. Biological control of water hyacinth in Thailand. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 811-822. UNEP, Nairobi.

National Acadamy of Sciences (NAS). 1977. Making Aquatic Weeds Useful: Some PerspectivesforDeveloping Countries. NAS, Washington D.C. 174 pp.

Nor, Y.M. 1994. Phenol removal by Eichhornia crassipes in the presence of trace metals. Water Research (Oxford) 28:1161 - 1166.

Obeid, M. 1984. Water hyacinth (Eichhornia crassipes (Mart.) Solms) in the Sudan. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 145-160. UNEP, Nairobi.

Onifade, A.K. and Egunjobi, O.A. 1994. Suppression of Meloidogyne incognita populations with waterhyacinth and water lettuce in Nigeria. Afro-Asian Journal af Nematology 4:96-100.

Parashar, D.R. 1970. Treatment of sugar factory effluent of high pollution load, self oxidation and use of waterhyacinth. Proceedings of the Annual Convention of the Sugar Technology Association of India 37:381-389.

Penfound, W.T. and Earle, T.T. 1948. The biology of water hyacinth. Ecology Monographs 18:447-472.

Pillai, K.R., Unni, B.G., Borthakur, A., Singh, H.D. and Baruah, J.N. 1984. Production of biogas from water hyacinth. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 507-525. UNEP, Nairobi.

Polisetty, R., Chandra, R., Reddy K.J. and Sirohi, G.S. 1984. Studies on biogas generation using water hyacinth (Eichhornia crassipes). In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan, G. (Ed.), pp. 540-549. UNEP, Nairobi.

Prashad, B.G.S., Madhavakrishna, W. and Nayudamma, Y. 1984. Utilization of water hyacinth in the treatment and disposal of tannery wastewater. In: Proceedings of the International Conference on WaterHyacinth. Thyagarajan, G. (Ed.), pp. 647-654. UNEP, Nairobi.

Rai, S., Barayanswami, M.S., Hasan. S.H., Rupainwar, D.C. and Sharma, Y.C. 1995. Removal of cadmium from wastewater by water hyacinth. International Journal of Environmental Studies 46:251-262.

Reddy, K.R.and D'Angelo, E.M. 1990. The biomass yield and nutrient removal by water hyacinth (Eichhornia crassipes). Biomass 21:27-42.

Reddy, P.V.S. and Reddy, M.R. 1979. Utilization of water hyacinth meal (Eichhornia crassipes) in the concentrate feeds of crossbred calves. Indian Journal of Animal Science 49:174-179.

Rogers, H.H. and Davis, D.E. 1972. Nutrient removal by water hyacinth. Weed Science 20:423-428.

Sahai, R. and Sinha, A.B. 1970. Contribution to the ecology of Indian aquatics. I. Seasonal changes in biomass of water hyacinth (Eichharnia crassipes) (Mart.) Solms). Hydrobiologia 35:376-382.

Saltabas, O. and Akcin, G. 1994. Removal of chromium, copper and nickel by waterhyacinth (Eichhornia crassipes). Toxicological and Environmental Chemistry 41:131-134.

Sastroutomo, S.S. 1991. Decay rates and nutrient release from decomposing aquatic weeds. BioTrop Special Publication 40:105-115.

Shirley, R.L., Easley, J.F. and Hentges, J.F. 1973. Toxic substances and chemical composition of hyacinths and other water plants. Annual Report of the Institute of FoodandAgricultural Science, University of Florida. 1971:251.

Smith, L .W., Williams, R.E., Shaw, M. and Green, K.R. 1984. A waterhyacinth eradication campaign in New South Wales, Australia. In: Proceedings of the International Conference on Water Hyacinth. Thyagarajan. G-(Ed.), pp. 925-935. UNEP, Nairobi.

Soerjani, M. 1984. The Indonesian experience in water hyacinth utilization. In: Proceedings of the International Conference on WaterHyacinth. Thyagarajan, G. (Ed.), pp. 165175. UNEP, Nairobi.

Soesiawangingrini, D.P., Soewardi, B. and Thohari, M. 1979. Waterhyacinth (Eichhornia crassipes) in broiler rations. Proceedings of the 6th Asian Pacific Weed Science Society Conference. pp. 623-627.

Soily, R.K. 1984. Integrated rural development with water hyacinth. In: Proceedings of the International Conference on Waterhyacinth. Thyagarajan, G. (Ed.), pp. 70-78. UNEP, Nairobi.

Soily, R.K., Singh, D. and Sharan, V. 1984. Waterhyacinth as feed supplement for small animals. In: Proceedings of the International Conference on Waterhyacinth. Thyagarajan, G. (Ed.), pp. 398-407. UNEP. Nairobi.

Stanley, R.A. 1974. Methods of biological recycling of nutrients from livestock waste. Bulletin of the National Fertilizer Development Center. Muscle Shoals, USA. 45 pp.

Sumathi, H., Lakshminarayana, G. and Thyagarajan, G. 1984. A preliminary report on the preparation of carbon black from waterhyacinth for use in paints. In: Proceedings of the International Conference on Water- hyacinth. Thyagarajan, G. (Ed.), pp. 782-784. UNEP, Nairobi.

Tansukul, R. and Klitsaneephaiboon, W. 1983. Utilization of waterhyacinth and chicken manure for straw mushroom (Volvariella volvacea) cultivation. Thai Journal of Agricultural Science 16:47-55.

Thyagarajan, G. (Ed.). 1984. Proceedings of the International Conference on Waterhyacinth. UNEP, Nairobi. 1005 pp.

Tjitrosoedirdjo, S.S. and Wiroatmodjo, J. 2984. Water hyacinth management in Java, Indonesia. In: Proceedings of the International Conference on Waterhyacinth. Thyagarajan. G. (Ed.), pp. 176-192. UNEP, Nairobi.

Watson, E.F. 1947. Utilization ofwaterhyacinth. Indian Farming 8:29-30.

Wolverton, B.C. aud McDonald, R.C. 1978. Nutritional composition of waterhyacinth grown on domestic sewage. Economic Botany 32:363-370.

Wright, A.D. 1984. The effect of biological control agents on waterhyacinth in Australia. In: Proceedings of the International Conference on Waterhyacinth. Thyagarajan, G. (Ed.), pp. 823-833. UNEP, Nairobi.

Wright, A.D. and Center, T.D. 1984. Biological control:Its place in the management of waterhyacinth. In: Proceedings of the International Conference on Waterhyacinth. Thyagarajan. G. (Ed.), pp. 793-802. UNEP, Nairobi.

Yusof, A.B.M. 1984. Status of water hyacinth infestation and management in Malaysia. In: Proceedings of the International Conference on Waterhyacinth. Thyagarajan, G. (Ed.), pp. 79-95. UNEP. Nairobi

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