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DUCKWEED: A tiny aquatic plant with enormous potential for agriculture and environment |
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DUCKWEED: A tiny aquatic plant with enormous potential for agriculture and environment |
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A considerable proportion of the world's surface is covered by saline waters, and the land areas from which the salts of the sea mostly originated are continuously leached of minerals by the run-off of rain water. Aquatic habitats abound; these may be temporary following rains or permanent largely through impediments to drainage . From the beginning of time these aquatic habitats have been harvested for biomass in many forms (food, fuel and building materials) by animals and man. From the time of the Industrial Revolution and with the onset of intensive land use enormous changes occurred. Agriculturists harvested both water and dry lands for biomass and minerals were applied to stimulate biomass yields, the aquatic habitats often became enriched (or contaminated) and water bodies were more temporary because of water use in agriculture or were lost through drainage or the establishment of major dams for irrigation, human water supplies and/or hydro-electric power generation. On the other hand other human activities, created aquatic areas for such purposes as the control of soil erosion, for irrigation, storage of water, sewage disposal and industrial waste storage or treatment and for recreational use. Aquatic habitats have, in general, degenerated throughout the world because of pollution by both industry and other activities. Human activities have, in general, resulted in much higher flows of minerals and organic materials through aquatic systems, often leading to eutrophication and a huge drop in the biomass produced in such systems. The lack of dissolved oxygen in water bodies, through its uptake by microbes for decomposition of organic compounds, produces degrees of anaerobiosis that results in major growth of anaerobic bacteria and the evolution of methane gases. Despite this in the areas of high rainfall, particularly in the wet-tropics, there remain major aquaculture industries, which vary from small farmers with 'manure fed' ponds producing fish through to large and extensive cultivation of fish and shellfish that are replacing the biomass harvested from the seas. The distribution of global aquaculture is shown in Figure 1. Fish production from ocean catches appear to be reduced, but production from farming practices are increasing which clearly demonstrates how important aquaculture is (and will become) in protein food production. This trend is illustrated by the trend in world prawn (shrimp) production shown in Figure 2.
Although traditional or staple crops can be produced from water bodies and in many situations traditional people often harnessed these resources, the aquatic habitat has been considered too costly and to difficult to farm other than for extremely high value crops such as algae harvested for high value materials such as b -carotene or essential long-chain fatty acids. Intensive aquaculture (hydroponics) of crops in highly mechanised farms have been developed but require highly sophisticated management systems and are expensive. Throughout the world, and particularly in Asia, farmers have harvested naturally produced aquatic plants for a number of purposes including animal feed, green manure and for their family feed resources. The best known of these include the free floating plants; water lettuce (Pistia), water hyacinth (Eichhcornia), duckweed (Lemna) and Azolla and some bottom growing plants. Azolla, which is a member of the fern family grows extensively in association with nitrogen fixing bacteria, which allows it to produce on waters low in N but containing phosphorus. Azolla has been comprehensively discussed by van Hove (1989). In recent years a commonly occurring aquatic plant, "duckweed", has become prominent, because of its ability to concentrate minerals on heavily polluted water such as that arising from sewage treatment facilities. However, it has also attracted the attention of scientists because of its apparent high potential as a feed resource for livestock (Skillicorn et al., 1993; Leng, et al., 1994). Duckweed grows on water with relatively high levels of N, P and K and concentrates the minerals and synthesises protein. These are the nutrients which are often critically deficient in traditional fodders and feeds given to ruminants and to pigs and poultry particularly where the former depend on agro-industrial byproducts and crop residues. The growing awareness of water pollution and its threat to the ecology of a region and agriculture per se has also focussed attention on potential biological mechanisms for cleansing water of these impurities making it potable and available for reuse. In general, water availability is becoming a primary limitation to expanding human activities and also the capacity of agricultural land to feed the ever increasing population of the world. Another pressure that has stimulated interest in aquatic plants has been the over-use of fertilisers, particularly in Europe that has led to contamination of ground water supplies that can no longer be tolerated. In the early 1960's a number of scientists warned of the pending shortage of fossil fuels, the expanding population and the potential for mass-starvation from an inability of agriculture to produce sufficient food. The prophesies have proved wrong in the short term, largely because of the extent of the then undiscovered fossil fuel, but also because of the impact of the development of high yielding crop varieties, particularly of cereal grain. The "Green Revolution" whilst increasing cereal crop yields faster than human population increase has had serious side effects such as increased erosion and greater water pollution in some places and a huge increase in demand for water and fertiliser. Fertiliser availability and water use are often highly dependent on fossil fuel costs. Water resources in many of the world's aquifers are being used at rates far beyond their renewal from rainfall (see World Watch 1997). At the present time it appears that potentially the application of scientific research could maintain the momentum for increased food production to support an increasing world population, but it is rather obvious that if this is to occur it must be without increased pollution, and with limited increases in the need for water and fertiliser and therefore also fossil fuel. GLOBAL WARMING, FOSSIL FUEL AND NUTRIENT RECYCLE NEEDS. Global warming has now been accepted as inevitable. It is now a major political issue in most countries. Governments are now considering the need to reduce the combustion of fuels which contribute most to the build up of greenhouse gases and thus the increase in the thermal load that is presently occurring. A second problem for fossil fuel devouring industries is the potential for scarcer, and therefore, more costly oil resources in the near future. As Fleay (1996) in his book "The decline in the age of oil" has pointed out there have been no major discoveries of oil in the last ten years. This suggests that we have already discovered the major resources. Many of the oil wells are approaching or have passed the point at which half the reserves have been extracted. At this stage the cost in fuel to extract the remaining fuel increases markedly. The need to reduce fuel combustion and the potential for large increases in costs of extraction of oil from the major deposits all indicate major increases in fuel costs and the need to stimulate alternative energy strategies for industry and agriculture alike. Fuel is a major economic component of all industries, and in particular, industrialised agriculture. Therefore food prices are highly influenced by fuel prices. The energy balance for grain production has consistently decreased with mechanisation as is illustrated by the fuel costs for grain production which is approaching 1MJ in as fossil fuel used in all activities associated with growing that crop to 1.5MJ out in the grain. A major component of the costs are in traction, fertiliser, herbicides and water use, particularly the energy costs of irrigation. In recent times, a movement has begun to examine a more sustainable future for agriculture, particularly in the developing countries. The need in developing countries of Asia and Africa where most of the world's population lives and where population growth is the highest is to:
This suggests that small farmers need to be targeted and that farming should be integrated so that fertiliser and other chemical use is minimised together with lowered gaseous pollution. At the same time a country must ensure its security of food supplies. In the 1998 financial crisis in Asia, the small farmer was seriously effected because of the relatively high cost of fuel. This is bound to have serious effects on food production in the next few years if fertiliser applications are restricted. This will show up as a decline in crop yields over the next few years. The problem of decreasing world supplies of fuels, increased legislation to decrease use of fossil fuel to reduce pollution, and the economic disincentive to use fertilisers in developing countries indicates to this writer that there is a massive need to consider a more integrated farming systems approach, rather than the monocultures that have developed to the present time. Integrated farming systems use require three major components to minimise fertiliser use:
It also requires incorporation of animals into the system to utilise the major byproducts from human food production. Duckweed aquaculture is an activity that fits readily into many crop/animal systems managed by small farmers and can be a major mechanism for scavenging nutrient loss. It appears to have great potential in securing continuous food production, particularly by small farmers, as it can provide fertiliser, food for humans and feed for livestock and in addition decrease water pollution and increase the potential for water re-use. The production and use of duckweed is not restricted to this area and there is immense scope to produce duckweeds on industrial waste waters, providing a feed stock particularly for the animal production industries, at the same time purifying water. In this presentation, duckweed production and use, particularly in small farmer systems, is discussed to highlight its potential in food security, particularly in countries where water resources abound and have been misused. On the other hand, duckweed aquaculture through its water cleansing abilities can make a greater amount of potable water available to a population living under arid conditions, providing certain safeguards are applied. CHAPTER 2: The plant and its habitat Duckweed is the common name given to the simplest and smallest flowering plant that grows ubiquitously on fresh or polluted water throughout the world. They have been, botanical curiosities with an inordinate amount of research aimed largely at understanding the plant or biochemical mechanisms. Duckweeds have great application in genetic or biochemical research. This has been more or less in the same way that drosophila (fruit flies) and breadmoulds have been used as inexpensive medium for genetic, morphological, physiological and biochemical research. Duckweeds are small, fragile, free floating aquatic plants. However, at times they grow on mud or water that is only millimetres deep to water depths of 3 metres. Their vegetative reproduction can be rapid when nutrient densities are optimum. They grow slowly where nutrient deficiencies occur or major imbalances in nutrients are apparent. They are opportunistic in using flushes of nutrients and can put on growth spurts during such periods. Duckweeds belong to four genea; Lemna, Spirodela, Wolfia and Wolffiella. About 40 species are known world wide. All of the species have flattened minute, leaflike oval to round "fronds" from about 1mm to less than 1cm across. Some species develop root-like structures in open water which either stabilise the plant or assist it to obtain nutrients where these are in dilute concentrations. When conditions are ideal, in terms of water temperature, pH, incident light and nutrient concentrations they compete in terms of biomass production with the most vigorous photosynthetic terrestrial plants doubling their biomass in between 16 hours and 2 days, depending on conditions. An idea of their rapid growth is illustrated by the calculation that shows that if duckweed growth is unrestricted and therefore exponential that a biomass of duckweed covering 10cm2 may increase to cover 1 hectare (100 million cm2) in under 50 days or a 10 million fold increase in biomass in that time. Obviously when biomass doubles every 1-2 days, by 60 days this could extend to a coverage of 32ha. In natural or farming conditions, however, the growth rate is altered by crowding, nutrient supply, light incidence and both air and water temperature in addition to harvesting by natural predators (fish, ducks, crustaceans and humans). In addition to the above limiting factors there also appears to be a senescence and rejuvenation cycle which is also apparent in Azolla. Vegetative growth in Lemna minor exhibits cycles of senescence and rejuvenation under constant nutrient availability and consistent climatic conditions (Ashbey & Wangermann, 1949). Fronds of Lemna have a definite life span, during which, a set number of daughter fronds are produced; each of these daughter fronds is of smaller mass than the one preceding it and its life span is reduced. The size reduction is due to a change in cell numbers. Late daughter fronds also produce fewer daughters than early daughters. At the same time as a senescence cycle is occurring an apparent rejuvenation cycle, in which the short lived daughter fronds (with half the life span of the early daughters) produce first daughter fronds that are larger than themselves and their daughter fronds are also larger, and this continues until the largest size is produced and senescence starts again. This has repercussions as there will be cyclical growth pattern if the plants are sourced from a single colony and are all the same age. Under natural conditions it is possible to see a mat of duckweeds, apparently wane and explode in growth patterns. The cyclic nature of a synchronised duckweed mat (i.e. all the same age) could be over at least 1 month as the life span of fronds from early to late daughters can be 33 or 19d respectively with a 3 fold difference in frond rate production (See Wangermann & Ashby, 1950). The phenomena of cyclical senescence and rejuvenation may cause considerable errors of interpretation in studies that examine, for example, the response of a few plants to differing nutrient sources over short time periods. In practice this cycle may be responsible for the need to restock many production units after a few weeks of harvesting. In Vietnam, with small growth chambers the duckweed required reseeding every 4-6 weeks (T.R. Preston personal communication) to be able to produce a constant harvestable biomass growing on diluted biogas digestor fluid. There is also the possibility in such systems of a build up in the plant of compounds that eventually become toxic or at least diminish their growth rate. Root length appears to be a convenient relative measure of frond-age. The senescence-rejuvenation cycle is increased by high temperatures through a decrease in individual frond life span but there is a concomitant increase in daughter frond production so that the biomass of fronds produced in a shorter life span is the same. The rejuvenation cycle appears to be unaffected by either light density or temperature. The cyclical changes appear to be mediated by chemicals secreted by the mother frond and growth patterns may be modified greatly by harvesting methods which mix water, wind effects and shelter as well as light intensity and temperature. The increased death rate of duckweed mats exposed to direct sunlight has been recognised in work in Bangladesh where workers are set to cool duckweed mats by splashing them with water from below the surface and in Vietnam, Preston (personal communication) observed that the incidence of showers stimulated very rapid growth of duckweed in small ponds. Duckweeds appear to have evolved, so as, to make good use of the periodic flushes of nutrients that arise from natural sources. However, in recent times they are more likely to be found growing in water associated with cropping and fertiliser washout, or down stream from human activities, particularly from sewage works, housed animal production systems and to some extent industrial plants. For the many purposes related in this publication, the selection of duckweed to farm will depend on what grows on a particular water body and the farmer has little control over the species present. The various duckweeds have different characteristics. The fronds of Spirodela and Lemna are flat, oval and leaf like. Spirodela has two or more thread-like roots on each frond, Lemna has only one. Wolffiella and Wolfia are thalloid and have no roots; they are much smaller than Spirodela or Lemna. Wolfia fronds are usually sickle shaped whereas Wolffiella is boat shaped and neither has roots. Differentiation and identification is difficult and is perhaps irrelevant to the discussion. This is mainly because the species that grows on any water is the one with the characteristic requirement of that particular water and the dominant species will change with the variations in water quality, topography, management and climate, most of which are not easily or economically manipulated
The structure of the fronds of duckweed is simple. New or daughter fronds are produced alternatively and in a pattern from two pockets on each side of the mature frond in Spirodela and Lemma. In Wolffiella and Wolfia only one pocket exists. These pockets are situated in Spirodela or Lemna close to where the roots arise. Each frond, as they mature, may remain attached to the mother frond and each in turn, under goes this process of reproduction. In all four genea each mother frond produces a considerable number of daughter fronds during its lifetime. However, after six deliveries of daughter fronds, the mother frond tends to die. Colonies produced in laboratory or naturally are always spotted with brown dead mother fronds. The bulk of the frond is composed of chlorenchymatous cells separated by large intracellular spaces that are filled with air (or other gases) and provide buoyancy. Some cells of Lemna and Spirodela have needle like raphides which are presumably composed of calcium oxalate. The upper epidermis in the Lemna is highly cutinized and is unwettable. Stomata are on the upper side in all four genea. Anthocyanin pigments similar to that in Azolla also form in a number species of Lemnacae. Both Spirodela and Lemna have greatly reduced vascular bundles. Roots in both Spirodela and Lemna are adventitious. The roots are usually short but this depends on species and environmental conditions and vary from a few millimetres up to 14cm. They often contain chloroplasts which are active photosynthetically. However, there are no root-hairs. The plant reproduces both vegetatively and sexually, flowering occurs sporadically and unpredictably. The fruit contains several ribbed seeds which are resistant to prolonged desiccation and quickly germinate in favourable conditions. The Lemnacae family is world wide, but most diverse species appear in the subtropical or tropical areas. These readily grow in the summer months in temperate and cold regions; they occur in still or slowly moving water and will persist on mud. Luxurious growth often occurs in sheltered small ponds, ditches or swamps where there are rich sources of nutrients. Duckweed mats often abound in slow moving backwaters down-stream from sewage works. In the aquatic habitat of crocodiles and alligators, duckweeds often have luxurious growth on the nutrients from the excrement of these reptiles and the local zoo can often provide a convenient source of duckweed for experimental purposes (see Photo 2).
Some species appear to tolerate saline waters but they do not concentrate sodium ions in their growth. The apparent limit for growth appears to be between 0.5 and 2.5% sodium chloride for Lemna minor When the aquatic ecosystem dries out or declining temperatures occur, duckweeds have mechanisms to persist until conditions return that can support growth. This occurs through late summer flowering, or the production of starch filled structures or turin which are more dense than the fronds so the plants sink to the bottom of the water body and become embedded in dried mud. The four species of Lemnacae are found in all possible combinations with each other and other floating plants. They are supported by plants that are rooted in the pond. They effect the light penetration of water resources and depending on their coverage of the area they can prevent the growth of algae or plants that grow emersed in water. They provide habitat and protection for a number of insects that associate with the plant but they appear to have few insects that feed on them. The main predators appears to be herbivorous fish, (particularly carp), snails, flatworms and ducks, other birds may also feed on duckweeds but reports are few in the literature. The musk rat appears to enjoy duckweeds and the author suggests that many other animals may occasionally take duckweeds such as pigs and ruminants. The appearances of duckweed species not previously seen in areas of Europe have been attributed to global warming and/or a strong indication of rising water temperature throughout the world from global warming (Wolff & Landolt 1994). HISTORY OF DUCKWEED UTILIZATION This is a most difficult area to review since much of the information is by way of the popular press or is only mentioned in scientific papers. However, after a lecture given at the University of Agriculture and Forestry in Ho Chi Minh City in which the potential of duckweed biomass for animal production was discussed, as a novel concept, the writer was most chastened to find that duckweed was used extensively by local farmers as feed for ducks and fish and there was a flourishing market for duckweed. The duckweed based farming system in Vietnam depended largely on manure and excrement being collected in a small pond where some eutrophication takes place; the water from this pond runs into a larger pond about 0.5m deep on which duckweed grows in a thick mat. This was harvested on a daily basis and immediately mixed with cassava waste (largely peelings) and fed to ducks which were constrained in pens on the side of the pond or lagoon (see Photo 3). The ducks were produced for the local restaurant trade. In Taiwan, it was traditional to produce duckweeds for sale to pig and poultry producers. There are reports that Wolfia arrhiza, which is about 1mm across has been used for many generations as a vegetable by Burmese, Laotions and Northern Thailand people. Thai's refer to this duckweed as "Khai-nam" or "eggs of the water" and it was apparently regarded as a highly nutritious food stuff. It could have been a valuable source of vitamins particular of vitamin A to these people. This would have been particularly important source during the long dry season of Northern Thailand when green vegetables may have been scarce. It is also a good source of minerals, again its phosphorous content could have been vital in areas where there are major deficiencies, such as occurs in Northern Thailand. There are references in the literature to duckweed as both a human food resource and as a component of animal and bird diets in traditional/small farmer systems in most of South Asia.
CHAPTER 3: Nutrient requirements of duckweed Like all photosynthetic organisms, duckweeds grow with only requirements for minerals, utilising solar energy to synthesise biomass. They have, however, the capacity to utilise preformed organic materials particularly sugars and can grow without sunlight when provided with such energy substrates. In practice the ability to use sugars in the medium as energy sources is irrelevant, as in most aquatic systems they do not exist. However, they could be of some importance where industrial effluent's need to be purified and duckweed is considered for this process (e.g. waste water from the sugar industry or waste water from starch processing). Most research on nutrient requirements have centred on the need for nitrogen, phosphorus and potassium (NPK). However, like all plants, duckweeds need an array of trace elements and have well developed mechanisms for concentrating these from dilute sources. From the experience of the Non-Government Organisation PRISM (based in Colombia, Maryland, USA, see chapter 6) in Bangladesh, it appears that providing trace minerals through the application of crude sea salt was sufficient to ensure good growth rates of duckweeds in ponded systems. However, considerable interest has been shown by scientists in the capacities of duckweed to concentrate, in particular, copper, cobalt and cadmium from water resources where these have economic significance. Mineral nutrients appear to be absorbed through all surfaces of the duckweed frond, however, absorption of trace elements is often centred on specific sites in the frond. The requirements to fertilise duckweeds depends on the source of the water. Rainwater collected in ponds may need a balanced NPK application which can be given as inorganic fertiliser or as rotting biomass, manure or polluted water from agriculture or industry. Effluent's from housed animals are often adequate or are too highly concentrated sources of minerals and particularly because of high ammonia concentration may need to be diluted to favour duckweed growth. Run-off water from agriculture is often high in P and N but the concentration may need to be more appropriately balanced. Sewage waste water can be high or low in N depending on pretreatments but is almost always adequate in K and P. Industrial waste water from sugar and alcohol industries for example are always low in N. Little work has been done to find the best balance of nutrients to provide maximum growth of duckweed. The duckweed has been provided with mechanisms that allow it to preferentially uptake minerals and can grow on very dilute medium. The main variables that effect its growth under these circumstances are light incidence and water and air temperatures. The growth rate and chemical composition of duckweed depends heavily on the concentration of minerals in water and also on their rate of replenishment, their balance, water pH, water temperature, incidence of sunlight and perhaps day length. Its production per unit of pond surface also depends on biomass present at any one time. Duckweeds grow at water temperatures between 6 and 33° C. Growth rate increase with water temperature, but there is an upper limit of water temperature around 30° C when growth slows and at higher temperature ceases. In open lagoons in direct sunlight duckweed is stressed by high temperature created by irradication and in practice yields are increased by mixing the cooler layers of water low in the pond and splashing to reduce surface temperature of the duckweed matt. Duckweed survives at pH's between 5 and 9 but grows best over the range of 6.5-7.5. Efficient management would tend to maintain pH between 6.5 and 7. In this pH range ammonia is present largely as the ammonium ion which is the most readily absorbed N form. On the other hand a high pH results in ammonia in solution which can be toxic and can also be lost by volatilisation. Duckweeds appear to be able to concentrate many macro and micro minerals several hundred fold from water, on the other hand high mineral levels can depress growth or eliminate duckweeds which grow best on fairly dilute mineral media. There is a mass of data on the uptake by duckweed of micro-elements which can be accumulated to toxic levels (for animal feed). However, their ability to concentrate trace elements from very dilute medium can be a major asset where duckweed is to be used as an animal feed supplement. Trace elements are often deficient in the major feed available to the livestock of small resource poor farmers. For example, in cattle fed mainly straw based diets both macro and micro mineral deficiencies are present. Duckweeds need many nutrients and minerals to support growth. Generally slowly decaying plant materials release sufficient trace minerals to provide for growth which is often more effected by the concentrations of ammonia, phosphorous, potassium and sodium levels. There is by far the greatest literature on the requirements of duckweed for NPK and the ability of the plant to concentrate the requirements of micro nutrients from the aquatic medium is usually considered not to be a limitation. In the work in Bangladesh by PRISM, crude sea salt was considered to be sufficient to provide all trace mineral requirements when added to water at 9kg/ha water surface area when duckweed growth rates were high at around 1,000kg of fresh plant material/day. Depth of water required to grow duckweed under warm conditions is minimal but there is a major problem with shallow ponds in both cold and hot climates where the temperature can quickly move below or above optimum growth needs. However, to obtain a sufficiently high concentration of nutrients and to maintain low temperatures for prolonged optimal growth rate a balance must be established between volume and surface area. Depth of water is also critical for management, anything greater than about 0.5 metres poses problems for harvesting duckweeds, particularly by resource poor farmers. Whereas, where water purification is a major objective in the production of duckweed, it is impractical to construct ponds shallower than about 2m deep. Incident sunlight and environmental temperatures are significant in determining the depth of water as undoubtedly duckweed is stressed by temperatures in excess of 30° C and below about 20° C growth rate is reduced. In practice, depth of water is probably set by the management needs rather than the pool of available nutrients and harvesting is adjusted according to changes of growth rate, climate changes and the nutrient flows into the system. REQUIREMENTS FOR NPK AND OTHER MINERALS Duckweeds evolved to take advantage of the minerals released by decaying organic materials in water, and also to use flushes of minerals in water as they occurred when wet lands flooded. Duckweeds now appear to have the potential to be harnessed as a commercial crop for a number of purposes. Water availability is likely to limit terrestrial crop production particularly of cereals in the coming years (see World Watch 1997). Water purification and re-use particularly that water arising from sewage works, industrial processing and run-off from irrigation appears to be mandatory in the future, both to reduce pollution of existing water bodies and to provide reusable water for many purposes including that required by humans in some places as drinking water. Duckweeds appear to be able to use a number of nitrogenous compounds either on their own or through the activities of associated plant and animal species. The ammonium ion (NH4+) appears to be the most useful N source and depending on temperatures duckweeds continue to grow down to extremely low levels of N in the water. However, the level of ammonia N in the water effects the accretion of crude protein in the plant (see Table 1).
The value of duckweed as a feed resource for domestic animals increases with increasing crude protein content. In studies at the University of New England, Armidale, Australia, the crude protein content of duckweed growing on diluted effluent from housed pigs increased with increased water levels of N from about 15% crude protein with trace levels of N (1-4mg N/l) to 37% at between 10-15mg N/l. Above 60mg N/l a toxic effect was noticed perhaps due to high levels of free ammonia in the water. Whilst few experiments have been undertaken on the optimum level of ammonia required, these results give a guide-line for the levels of N to be established and maintained in duckweed aquaculture to obtain a consistently high crude protein level in the dry matter.
In most practical situations the approach to growing duckweed is to find the dilution of water where N is not limiting growth and supports high levels of crude protein in the plant. This is usually done by an arbitrary test. Serial dilutions of the water source with relatively pure water (rain water) is carried out and duckweed seeded into each dilution and weight change recorded after, say, 4 weeks. In this way the appropriate N concentration is established. A useful indicator of whether conditions in the pond are appropriate for growth of duckweed (Lemna spp) of high protein content in the length of the roots. Many experimental observations (Rodriguez and Preston 1996a; Nguyen Duc Anh et al 1997; Le Ha Chau 1998) have shown that over short growth periods there is a close negative relationship between root lenght and protein content of the duckweed and with the N content of the water. Data taken from the experiment of le Ha Chau (1998) are illustrated in Figure 4. In most small-scale farmsituations it is not feasible to determine the protein content of the duckweed that is being used; nor can the nutrient content (especially nitrogen) of the water be estimated easily. To determine the root length of duckweed is a simple operation and and requires neither equipment nor chemicals. By monitoring this characteristic, the user can have an indication of the nutritive corrective measures when the lenght of the roots exceeds about 10mm.
In duckweed aquaculture a source of N essential and in many start-up systems, based on water effluent from sewage or housed animals, the project has been considered by pre-treatments that denitrify the water and reduce ammonia concentrations. Most forms of aeration in sewage works are highly efficient in de-nitrification of waste waters, but this process compounds pollution peoblems. for instance where the effluent is high in P this promotes the growth of algae that fix N. In Australia the contamination of river systems with phosphorus often led to massive blooms of blue-green algae that rae toxic to humans and animals. Although there is an association of N fixing cyanobacteria with duckweeds, these are certainly not important from a standpoint of farming duckweeds. Duong and Tiedje (1985) were able to demonstrate that duckweeds from many sources had heterocystous cyanobacteria firmly attached to the lower epidermis of older leaves, inside the reproductive pockets and occasionally attached to the roots. They calculated that N fixation via these colonies could amount to 3.7-7.5kg N per hectare of water surface in typical Lemna blooms, but the association of cyanobacteria with Lemna trisulca was 10 times more effective. Probably, under most practical situations ammonia is the primary limiting nutrient for duckweed growth and the establishment of the optimum level for maximum growth of duckweeds needs research, particularly in the variety of systems the plant may be expected to grow. The effects of time and lowering of N content of sewage water on yield and crude protein content of duckweed is shown in Figure 5 and Figure 6. From recent research it appears that duckweed require about 20-60mg N/l to grow actively and from two studies [(those of Sutton & Ornes, (1975) compared with those of Leng et al., (1994)] it is apparent that there is a complex relationship between, the initial composition of the duckweed used in research and the level of nutrients required. Stambolie and Leng (1993) showed with duckweeds harvested from a backwater of a river and with an initial low crude protein content, it was only when the duckweed protein increased to the highest level that rapid growth of biomass commenced (i.e. at 3 weeks after introduction to the water) (Figure 6) even though by that time the N content of the water had declined to levels that were below the optimum that appears to be necessary for maximum protein levels (Figure 3). In the work of Sutton and Ornes (1975), however, duckweed of a higher protein content was initially used and growth rate again peaked at about the third week (Fig. 6) but by this time the crude protein content had declined to below 15%. This apparent opposite result can be rationalised if there is a stress factor involved which requires 3 weeks to overcome, and under these circumstances its growth may be considered
to be optimised at about 20mg ammonia N/l but to obtain maximum crude protein content it requires ammonia levels to be about 60 mg N/l (see Figure 5). A further implication is that where a high protein content is present in duckweed at the commencement of a growth study, the duckweed can grow through mainly synthesis of only carbohydrate. However, the variable results using duckweeds harvested from the wild and the slow "adaptation" to new conditions is obviously a confusing factor in interpreting any data of the requirements for duckweed for nutrients in such short term studies. The most important issue is that duckweed increases its protein content according to the ammonia level in an otherwise adequate medium up to levels of 60mg N/l (see Figures 3, 4, & 5). For food or feed purposes there is a vast difference in the value of duckweed biomass depending on its protein content (see later). Rapid growth of duckweed is also associated with high protein accretion and low fibre content and fibre content increases where root growth occurs.
Duckweeds appear to concentrate P up to about 1.5% of their dry weight and as such are able to grow on high P waters provided the N concentrations are maintained. The plant also appears to be able to draw on the pool of P in its biomass for its biochemical activities and once P had been accumulated it will continue to grow on waters devoid of P. On the other hand the P in duckweed appears to be highly soluble and is released rapidly to the medium on death of the plant (Stambolie & Leng 1994). The relationship between P content of sewage water and P content of duckweeds growing on such water are shown in Figure 6. Leng et al (1994) found a higher concentration of P in duckweed at high water P levels than that found by Sutton and Ornes (1975). The time course of uptake of P by duckweed in static sewage water is shown in Figure 6. The differences in accumulation levels in the two studies cited possibly resides in the rates of growth when the samples were taken. The capacity of duckweed to concentrate P is clear and maximum P levels in tissues (10-14mg P/kg dry weight) are achieved with water P levels as low as 1.0 mg P/litre.
The important issue here is that duckweeds concentrate P when water levels are enriched with P and it appears to be readily available once the plant is disrupted or dies. The P level in duckweed is sufficiently high to be a valuable source of this nutrient for both plants and animals. Vigorously growing duckweed is a highly efficient K sink, but only low concentration of K in water are needed to support good growth when other mineral requirements are satisfied. Most decaying plant materials would easily produce the K requirements of duckweed. Little work has been done to examine the S requirements of duckweeds. The mechanisms for sulphate uptake have been studied since uptake of sulphate is the first step in the biosynthesis of S-amino acids. Such biosynthesis needs the integration of pathways providing carbon building blocks and reduced sulphur (Datko & Mudd, 1984). It is possible that S levels are at times limiting to growth or protein accretion because of the high level of S-amino acids in the plant when growth rate is high and ammonia in the medium is non-limiting. Salts of sulphate appears to meet the requirements. As S is so readily leached from soils it is an unlikely candidate for deficiencies in systems that may be established to farm duckweed except where huge dilutions of the water are needed to obtain a suitable N level. The uptake of NPSK by duckweed from sewage water is shown in Figure 7 and the experimental design for such studies are shown in Photo 4. Sodium requirements Sea salt (9kg/ha/d) has been applied as part of a fertiliser program in pilot studies of duckweed farming in Bangladesh (see the discussion of PRISM's work in Chapter 6). This work suggested a good ability of duckweed to accumulate sodium as there was no apparent problems with salination. It appeared possible that duckweed removed up to 9kg salt/ha/d when grown under fairly optimal conditions, suggesting a potential for duckweed to rehabilitate saline land and water.
In studies undertaken at the University of New England it soon became apparent that the requirement for salt and the capacity of the plant to concentrate sodium was not significant in relation to salt levels that accumulated in lagoons fed by open cut coal mines. However, the exercise pointed a way for the potential use of such waters for duckweed production as duckweeds tolerated the salt levels and grew substantially when additional nutrients were provided. Using small galvanised iron tanks (see Photo 4) the effect of growing duckweed on saline mine waters with or without extra added nutrients was studied. Growth rate and protein content of duckweed is shown in Figure 8 together with the effects on mineral levels in the water. Duckweed grew on the water with or without added fertilisers but the uptake of sodium was low. The quality of the duckweed (indicated by its crude protein content) was maintained for some time by fertiliser application. The phosphorous requirement for growth was apparently low.
This data is introduced here to show that even saline waters can be used to grow duckweed, although research is needed to investigate the needs for additional nutrients on saline waters. Figure 8a. The effects of growing duckweed on saline mine waters with (D ) or without (o) added NPK fertiliser to optimum levels on crude protein and salt content. (Sell et al., 1993; Sell, 1993). Figure 8b. The effects of growing duckweed on saline mine waters with (D )or without (o) added NPK fertiliser to optimum levels on dry matter harvest and P content. (Sell, et al 1993, Sell, 1993).
8(c) Duck weed dry matter harvested: cumulative and residual pool 8 (d) Phosphorus content of mine waste water growing duckweedConclusions on mineral requirements The well developed system of concentrating minerals in duckweed allows them to grow under a wide range of conditions. The concentrating ability of duckweed for trace elements has been estimated to be many hundreds of thousands times. A simple comparison indicates some of the potential for duckweed to accumulate nutrients by comparing water levels and tissue dry matter levels of a number of minerals. Table 2: Some mineral compositions of duckweed and their potential to remove minerals from water bodies (calculated from the literature).
Along with this advantage of mineral removal is obviously the potential detrimental effects of accumulation of heavy metals. Heavy metal accumulation by duckweeds All members of the duckweed family concentrate heavy metals in particular cadmium, chromium and lead which may at times reach levels in the plant which are detrimental to both the health and growth of the plant in addition to creating problems where the plant is used in any part of a food chain eventually leading to human consumption. The accumulation of heavy metals by duckweed is not normally a problem for those wishing to use duckweeds from natural water resources or effluent from human or intensive animal housing as these metals are normally at extremely low concentrations. Duckweeds, however, are contaminated by such heavy metals from industries such as tanning (chromium) leachates from mining (e.g. cadmium) and great care is needed where water is contaminated to be sure that heavy metals do not get into the human food chain. On the other hand, duckweeds may find use in stripping heavy metals from industrial water. Also their content of heavy metals can be used to indicate potential pollution levels of waters. Cadmium appears to be absorbed by both living and dead duckweed plants and the cadmium is actively taken up by the plant (Noraho & Gour 1996). Cadmium at high concentrations, that is the concentration that prevents vegetative reproduction (EC-50) was found to be 800ppb but duckweeds grown in medium of 2.2ppm still accumulated most of the cadmium over 7d and when fed to crayfish increased cadmium in the hepatopncreata 26 fold and in muscles almost 7 fold (Devi et al., 1996). It is therefore, extremely important to be sure of low cadmium levels in water prior to any large scale use of duckweeds as feed for domestic animals or humans. Many reports are available on the uptake of metal ions by duckweeds and the numerous interactions that occur. Duckweeds will uptake and concentrate Cd, N, Cr, Zn, Sr, Co, Fe, Mn, Cu, Pb, Al and even Au. To attempt to define the rates of accumulation is not important here, except to point out that as the levels of these minerals rise to higher than normal in general they may directly inhibit growth of the plant and any animal that consume significant quantities. At low level accumulation the plants become a very useful source of trace minerals particularly for livestock and fish. Problems of heavy metal contamination obviously arise where duckweeds grow on industrial and mining waste where the contaminating elements are known and therefore the problem should be apparent from the beginning of any study. In conclusion, it is only where heavy metals are washed out in effluents from industry and mining that there is potential for duckweeds to become toxic to livestock, and in these situations duckweeds harvested from such sources should be disposed of differently. A most useful disposal method being as a mulch for non-food crops such as trees. A commercial balanced NPSK with sea salt to provide trace minerals can undoubtedly be used with relative unpolluted waters to meet the growth requirements of duckweeds. In the Mirazapur project, (see chapter 6) muriate of potash (KCl), urea and superphosphate (supplying P+ S) were successfully used to produce duckweeds on inundated lands that also collected the effluent waters from the local hospital. Fertilisers were applied on a daily basis, which together with the need for regular harvesting had a high labour cost. Sea salt was added as a source of trace minerals. Slow decomposition of manure and other organic materials are good ways to continuously supply a water body with nutrients required for duckweeds to grow. The skill here resides in somehow controlling the nutrient inflow into the growth ponds. In many instances this can be established by trial and error. This has been apparently highly successfully in Vietnam where, commercial producers of duckweed used ruminant and pig excrement. This was collected in a small settling pond, the water from which then passes through a series of duckweed ponds before either entering the river or being used for irrigation. These systems appear to work because of long established experience with growing duckweed. The series of ponds all apparently produced a good harvest of duckweed. A further extension of this method was seen in Bangladesh. The system was based on a simple toilet block, which may be just a hole in a concrete slab from which human excrement could be directed by gravity through a plastic pipe into a basket (usually split bamboo) situated at the centre of the pond and from which nutrients slowly diffused into the duckweed pond (see Photo 6). The effluents from biogas digestors, suitably diluted are very effective media for growing duckweed. These can be extremely simple systems, easily incorporated into a small farming areas based on home-biodigestors, constructed from plastic (see Photo 6) through to industrial size biodigestors made of metal. In all cases the excrement plus washings from animals held under penned conditions are collected, held in some form of settling pond to remove solids and then into an enclosed container which allows anaerobic microbes to grow and convert the residual carbohydrates to carbon dioxide and methane. The gaseous effluent containing methane and carbon dioxide is collected and combusted for various purposes including household cooking. The water leaving the biodigestor retains the minerals and with suitable dilution is a good media for the duckweed farm. Photo 5: A young boy harvests duckweed as a protein source for ducks in Vietna
Figure 9: Schematic representation of present duckweed framing in Vietnam.
Photo 6. Duckweed mats fed from faecal materials through a small basket which collects the solids in the middle of the pond
Biodigestor effluent from animal production is usually pH neutral and has a relatively high ammonia content. The mineral component of the diet effects the levels of nutrients in the water and therefore the need to dilute the effluent depends on the animal's diet. Ammonia treated straw results in an effluent from cows fed this that is high in ammonia. A simple system is shown in Figure 10. The system advocated by Dr. Preston, (Photo 7) is relatively simply to apply on small farms. A cow and calf, mainly fed crop residues provide both urine and faeces to a biodigestor suitably diluted with wash water. The biodigestor in this case is a simple polyethylene tube placed in a ground pit. The washings from the stall enter the digestor and have a half time of 10-15 days during which time most of the organic matter is converted to carbon dioxide and methane. The effluent is diluted and run into narrow plastic lined channels or concrete cannels in which the duckweeds are seeded and grow for several weeks, harvesting the duckweed occurs every few days. The duckweed is then fed either fresh as a supplement to pigs and poultry or is sun dried for the same use. Figure 10: Diagrammatic representation of flow of nutrients through simple biogas digestor to feed duckweed.
Photo 7: Duckweed growing on small plastic lined containers fed by biogas digestor fluid
Another system that has been proposed uses the effluent (washings) from large numbers of housed animals under intensive management or from abattoirs. The effluent is channelled through a lagoon covered with a 5-10cm thick plastic film and methane is collected from a convenient site beneath the plastic. The effluent being run into ponds, diluted and duckweed produced on the effluent (Figure 11) Wherever there is an effluent of polluted water associated with industry or agriculture there is potential to purify the effluent waters with duckweeds. Each process, however, requires particular attention and it is beyond the scope of this presentation to make recommendation here for such systems. Some examples of where duckweed cleansing systems might have application are listed Table 3. Figure 11: Schematic Diagram showing abattoir or intensive animal production waste processing and biogas flow
Table 3: Some examples of where duckweed might be used to cleanse wastewater of mineral pollution and produce a feedstock of duckweed biomass
Possibly the best case can be put for the use of duckweeds to remove P from human sewage which is mostly collected strategically at a point site in a township and although treated to varying degrees is often finally exported via rivers to the sea. There are now a number of commercially viable duckweed based sewage systems that have been developed. These systems are expensive because of the obvious need for high technology to ensure success in treatment. It appears, however, that both chemical and microbiological treatment plants are much more costly. On the other hand the use of aquaculture does not necessarily replace such systems and they can often be incorporated into or added on to a number of sewage purification plants. However, in small communities in the tropics the cultivation of duckweed on lagoons may be the only treatment necessary for simple sewage treatment. The literature contains a great deal of information on the potential growth rates of duckweeds. In many early studies the growth rates were measured under controlled conditions for short periods of time. In the absence of large scale field data obtained over 12 month periods these data have been used to estimate potential production rates. The data needs to be treated with reservations as the data in Figures 4 and 5 point to serious problems in doing growth trials with duckweed under laboratory conditions. The results in Table 4 are from research with near optimum conditions for duckweed growth. Landolt and Kandeler (1987) concluded that under such conditions a 73ton of dry matter are possibly produced per hectare per year or 20g DM/m2/d. Results up to 180ton DM/ha/y have been recorded. Under less than optimum conditions it is more realistic to target between 5 and 20ton DM/ha/y (Table 5). In practice the yields of duckweed often depend on the skill of the farmer in solving the problem of how to balance the mineral requirements of duckweeds and to identify with time the need for continuing and varying mineral supplementation. Waters that are high in P and K and trace element need minimal but repeated inputs of an ammonia source, keeping ammonia at around the 60mg N/l when growth and protein accretion is greatest. Table 4: Field results of duckweed growth in near-optimal conditions
Table 5: Field results of duckweed growth in sub-optimal conditions.
Yield of duckweed will depend on how the farmer monitors the duckweed system and whether a high protein meal is the objective. Industrial waters can only be discharged when the P levels have been depleted by consistent cropping and fertilisation with urea as ammonia levels decrease and become limiting to growth. How often to feed urea into the system and how long can duckweed growth be maintained can only be understood from research in the locality. Where the production of clean water is a major objective then it becomes necessary often to balance other nutrients as well as ammonia in order to end-up with water that has had its total mineral composition decreased to levels that will allow its re-use. In Table 6 the residual P in wash water from piggeries shows that with skill, a number of fertilisations with urea followed by harvesting of duckweed could result in very low P levels occurring. Recharging ammonia levels could be expected to stimulate growth so that P levels can drop below 1mg P/litre. The growth rate of duckweed under ideal light, temperature and pH would be exponential if there were no limitation in terms of mineral (including ammonia) deficiencies or excesses. However, in practice many issues reduce the biomass yield. One of the most important is obviously plant density. The rate of harvesting duckweed is important since there is a minimum biomass at which yields will decrease and an upper biomass where yield will be limited by crowding, all other variables being equal. In a study where most of the conditions for growth were unlimited the effect of harvesting indicated that, above about 1.2kg/m2 duckweed (fresh) growth decreased and below 0.6 kg/m2 Table 6: Removal of nutrients by Lemna from a flow through pond fed by aerated piggery waste water (Instituto de Investigaciones Porcinas, Havana, Cuba (unpublished observations)
duckweed (fresh) biomass limited growth potential. It appeared that if 1.0kg fresh duckweed/m2 could be maintained by frequent harvesting then an extrapolated yield of 32 tonnes DM/ha/yr could be produced under other non-limiting conditions. The data is shown in Figure 12. Figure 12: The range of densities of duckweed biomass on the water surface after harvesting at which duckweed grows optimally (Stambolie and Leng, 1994). In this case the average yield was 32 tonnes DM/ha/year. The upper density (filled squares) appears to be that at which crowding limits growth and the lower density (unfilled squares) is the density at which growth is insufficient to prevent algal blooms (Stambolie 1994 reported by Leng et al 1994)
CHAPTER 4: Integrated farming systems
WHY DO DEVELOPING COUNTRIES NEED TO EXAMINE THE POTENTIAL FOR INTEGRATED FARMING SYSTEMS? Integrated farming systems, so long as they improve soil fertility (or at least maintain the same soil fertility) in the long term have major advantages which can improve both the overall production of land without losing sustainability. The World Commission on Environment and Development defines sustainability as: "ensuring that development meets the needs of the present without compromising the ability of future generations to meet their own needs". However, development opportunities and aspirations change with changing economic considerations. Major increases in the cost of food production is likely to arise where cost of fuel (fossil) increases relative to income. Fuel prices must surely increase in the future in response to:
The cost of food production in a country is highly dependant on fuel prices, and food prices in the 1998 Asian financial crisis rose and must continue to rise. The two important agricultural cost factors that will be effected most are, mechanisation, where fuel is directly consumed in crop farming, and fertiliser availability and application since the cost of NPK is highly related to gasoline prices. The fuel crisis in Cuba brought about by removal of economic support from Russia and the embargo by the United States has seen a return to animal traction in the past 6 years and a massive decline in crop yields through decreased use of inorganic fertilisers. Farms export considerable mineral nutrients in products and also effluent from many sources. In the future and for continuing sustainable food production, these minerals must be replaced. In industrialised farming systems this is done largely by inorganic fertilisers produced and delivered at an increasing cost of fossil fuel combustion. High level use of NPK have resulted in the sustained yields of feed crops in industrialised countries and in the last 20 years greatly improved yields in developing countries where these inputs have been used together with improved crop varieties. The other major issues in terms of crop production has been the increased use of water resources, some of which are irreplaceable. Levels of fertiliser and water application are almost always in excess of plant needs and water run-off contaminated with minerals has created great problems with salination and eutrophication in river and pond systems throughout the world, changing the aquatic ecology of whole regions in places. Integrated systems are aimed at minimising (or preventing) loss of nutrients from a farming system and in many situations conserving water for reuse (Preston & Murgueitio, 1992). Integrated farming systems to be employed by small farmers in developing countries, require considerable skills in operation in order for them to be economic and/or sustainable. Integration may be developed on a single land holding or may be more easily applied where a number of farms combine their requirements to develop an integrated system where the minerals leached from the land by farming are returned to the land via good conservation practices involving a number of farms. In Australia the Land-Care Movement involves usually a number of land-holders to combine their efforts to conserve a whole catchment area. Similarly in India a catchment area approach to sustainability has been implemented through ICRASAT. Integrated and sustainable systems must be developed in order to prevent land degradation, minimise external (costly) inputs, conserve resources otherwise lost through effluents and potentially increase the income and standard of living of the farmer and at the same time maintain the fertility of the land. Integrated farming systems, that are also sustainable require that:
Integrated systems were traditional in most developing countries prior to the "green revolution" and many ancient societies recognised and put into practice sustainable cropping systems often through application of taboos against practices that caused degeneration of food supplies. In more recent times (say the 1920's) this took the form of integration of crop and animal production. The animals being an intermediate in conversion of crop residues and other wastes to dung which was then returned to the land. In some countries composting and biogas digestors were instrumental in recycling nutrients within the farm(s). The net result was that the minerals in biomass produced on the farm were recycled by re-incorporating the nutrients back to the land via human or animal excrement. Often the animal was a draught animal which is now being replaced by tractors. However, even in these systems effluent loss was considerable in water run off and in product where crops and animal products were sold from the farm. FIXATION OF N AND MOBILISATION OF P FROM PLANT GROWTH Within the integrated farming systems, strategies to encourage N fixation and for increasing P availability are primary targets. N accumulation in land or maintenance of levels through N fixing plants (e.g. legumes) or the extraction from effluent waters by aquatic plants are strategies that were used by small farmers only a few decades ago. Similarly P fertilisers have often been developed from aquatic plants. For example, in Kashmir, aquatic bottom growing plants are harvested from the lakes for use as fertilisers, and seaweeds have been used where ever they are washed ashore for application to soils. Ruminants in general, if they graze non cultivatable land harvest considerable N and P and this then can find its way into crops via manure and therefore potentially avoid downstream problems. The major constraint to the establishment of integrated farming systems is the level of management that must be exerted by small farmer. This can often be beyond his presently developed skills. An exception to this statement however, was seen in Vietnam and Bangladesh where the collection of all animal and human wastes into ponds and the subsequent growth of duckweeds has proven to be relatively free of problems and is very skilfully managed by a number of cooperating farmers. However, if these tropical systems had to be managed for production of quality water as well as feed for ducks or fish a greater degree of control of the duckweed growth would need to be exerted. INTEGRATED FARMING (THEORETICAL CONSIDERATIONS) One of the major reasons for the development of livestock production in cropping systems in developing countries was to utilise crop residues efficiently, thus, eliminating waste and optimising the use of the total biomass produced within the small farm system. The small farmer has often a requirement for draught power with animal products (milk and meat) as secondary considerations. Integrated crop and livestock production systems can be highly efficient; potentially crop residues are used as livestock feed; the waste products (e.g. faeces and urine) are fed into a biogas digestor and the effluent used to fertilise ponds for aquatic plant/algae production, with fish farming as the terminal activity. These systems are worthwhile pursuing as a means of providing nutrients/fuel for the family, minimising fuel combustion and reducing environmental pollution (Preston, 1990). The array of integrated strategies that could be developed is large. They all have as a central core a basic flow of nutrients through a number of systems. At each of these steps research can be brought to bear to optimise the partitioning of the available biomass into food, fuel and residues (see Figure 13). The environmental attributes of such systems are the methane emissions into the atmosphere and fuel (fossil fuel and fire wood) use are minimised. In addition the efficient and also Figure 13: Flow diagram showing the potential recycling of feed and faeces biomass from crop residues in an integrated farm.
total harnessing of the energy from high producing crops reduce the land areas required per unit of product (see Preston, 1990). A complete discussion of these systems is beyond the scope of this document but two examples are:-
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(a)
Crude Protein in duckweed
(b)
Changing biomass on water
