Chup Rubber Plantation is a large Government public enterprise with a land area of 15,733 ha. The plantation produces on average from 12,000 to 13,000 tonnes of dry rubber annually. The production is expected to increase by a further 12,000 tonnes after old trees are replaced by young and more productive trees. A rubber plantation can be considered as a sustainable use of natural resources as the main output – the rubber – is the product of photosynthesis using solar energy, air and water as the inputs. The production of rubber comprises three principal activities: the growing of the rubber trees; the harvesting of the latex; and the dry rubber processing. This review of the available studies considers:

· Factors that influence the quantity and quality of the latex waste water

· Features of use of cow manure and chemicals as fertilizers

· Features of cassava and water spinach grown as forage crops

The latex and the effluent (waste water)

Harvesting the latex

Tapping the rubber tree consists in making an incision in the bark and collecting the sap that runs from the incision. A new incision is made on average three times per week (about 120 times per year). Sap is collected throughout the 48 hours. During the day the sap is of better quality as it can be coagulated by adding ammonia soon after it is collected, which is usually at intervals of 4 to 5 hours. During the night (from 6 pm to 6am) the sap is exposed to natural coagulation by bacteria and by the air and this results in rubber of poorer quality. The process of collecting the sap consists in the operator visiting each of 20 to 30 trees and pouring the sap from each cup (about 500 ml capacity) into a 15-litre container. When the container is full, a solution of ammonia (5% weight/volume) is added at the rate of 40 ml/litre of sap. The coagulated sap (latex) is then transferred to a tanker, which takes it to the factory. At the factory, the latex, which contains about 25 to 30% “dry rubber”, is discharged into a holding tank and water is added to dilute it to about 16 to 18 % of “dry rubber” content. The pH is measured (usually about 6.6 to 6.8) and then a formic acid solution (1.5% weight/volume) is added until the pH is reduced to 5.0.

The composition of the latex

Latex consists essentially of a colloidal emulsion of rubber particles in water. The rubber particles are approximately 0.5 to 1 micron (1/1000^th mm) in diameter. The particles are surrounded by a protective layer of proteins, which may be displaced by the addition of soaps or other protective agents. Fresh latex also contains at least two other types of particles: bright yellow spheres, constituting the yellow fraction and lutoid particles much larger than latex particles and about 2-10 micron in diameter. These lutoid particles contain within them, and protected by a membrane, numerous minute particles in rapid Brownian motion. Hevea latex also contains several enzymes, which are responsible for the enzymatic discolouration of the latex (Peries and Fernando 1983). Latex is the cytoplasm of an anastomosed cell system, which is specialized in the synthesis of cis-polyisoprene. The latex from the rubber tree is composed of 20 to 60% rubber, 0.3 to 0.7% ash, 1 to 2% protein, 2% lipids, 1 to 2% quebrachite, 0.3% lecithins and 33 to 75% water (Dijkman 1951).

The processing in the factory

Preliminary straining

On arrival at the factory (or collection station), the latex receives a preliminary straining through a sieve fitted with “Monel” gauze of 40 or 50 mesh; in order to remove lumps, bark shavings etc. Brass mesh is an unsuitable material for this purpose, owing to the deleterious effect of even minute traces of copper on the quality of the rubber. For this reason, there should be no equipment containing copper or brass which comes into contact with latex or rubber during its processing. Stainless steel is also very suitable and though more expensive than Monel metal is a more robust material (Peries and Fernando 1983).

Coagulation determination

There are four methods of carrying out coagulation determinations. The first and second methods are suitable for use in estates whilst the other two methods need a laboratory.

At Chup Factory, there are only two methods to carry out trial coagulation of the latex. These are used every day. One method is used in the factory laboratory (rapid determination) and another one (exact method) is used at the laboratory to compare with the factory laboratory result.

Estate method of trial coagulation: The “metrolac” reading of a sample from the bulked latex is taken after dilution with two volumes of water. A sample of the bulked latex is diluted with one volume of water and 4.5 litres of it is poured into a coagulating pan. The measure is rinsed out with 250 ml of water, which is added to the latex. A second quantity of 4.5 litres of diluted latex is measured in the same way and poured into another pan. The latex in each pan is then coagulated by adding 350 ml of a 1% solution of formic acid (made by adding 30 ml of undiluted acid to 2.5 litres of water) and stirring with a suitable paddle. When coagulation is complete the coagulum is rolled either in a sheet roller or in a “creping” mill. In the latter, care is required that none of the sample is lost; the rollers should be opened slightly to reduce the pressure on the rubber. After drying the samples are weighed together on a spring balance or scales, accurate to 25 g. The weight represents the rubber content of the original undiluted latex and is expressed as grammes per litre. Weighing the two samples separately can check the accuracy of the trial. If they differ by more than 25g the test should be repeated (Peries and Fernando 1983).
The Chee method: A dipper calibrated to deliver 40±0.5 g of field latex is used. This is immersed slowly in a representative sample of latex. One dipper full of latex is drawn out gently and transferred to a coagulating dish. A mixture of ten parts by volume of 10% formic acid and 90% parts by volume of methylated spirit is used for the quick coagulation of latex. The coagulum is fed through a pair of rollers a number of times at different gap settings. The coagulum is washed and pressed between blotting paper to remove as much water as possible, before it is weighed on a “Goldsmiths” balance. The dry rubber content (DRC) of the latex is obtained by multiplying the wet weight of the coagulum by a factor of 0.722 (Peries and Fernando 1983).
Standard laboratory method: About 10 to 15 g of a representative sample of latex is accurately weighed and poured into an aluminium coagulating dish from a 50 ml conical flask with a stopper. The latex is coagulated using sufficient acetic acid and heated over a steam bath until a clear serum is obtained before it is rolled to a thickness of approximately 2 mm. The coagulum is thoroughly washed and placed in a thermostatically controlled oven at about 70 degree C for about 16 hours. The dry rubber is cooled in desiccators and weighed on an analytical balance (Peries and Fernando 1983).
Rapid laboratory method: A representative sample of latex in a cylindrical plastic vial (about 2 ml capacity) is weighed together with a glass dropper using a torsion balance. From the glass dropper, approximately 0.5 g of the latex is added dropwise into a glass plate and immersed in a mixture of five parts by volume of glacial acetic acid and 95 parts by volume of methylated spirit in an aluminium tray. The dropper together with the balance of the latex is reweighed to obtain the weight of the latex transferred. Pieces of coagulum on the glass plate are pressed carefully with distilled water before it is placed over a steam bath to dry. The dry film on the glass plate is rolled off and weighed on a torsion balance (Peries and Fernando 1983).

The composition varies, depending upon the different organs from which it has been extracted. Furthermore, it also changes with the season, location according to clonal and soil variation. Therefore, it is impossible for a single instrument to give correct readings on all samples of latex. In order to obtain satisfactory results with the “metrolac”, it is necessary to determine from time to time the error on each location by trial coagulation and to make an allowance accordingly (Peries and Fernando 1983).

The effluent from the Chup rubber processing factory

The Chup Rubber Plantation (CRP) is located in Kampong Cham province. The total area is 15,733 ha with the annual production of 12,000-13,000 tons of dry rubber (CRP report 2000). To be able to process this quantity of dry rubber, 360,000 to 480,000 m³ of water are needed. With an average content of N of 95 mg to 110 mg/litre of waste water, this means that from 34,200 to 45,600 kg of N may be washed away in the waste water every year.

The main effluent from a raw rubber producing factory is rubber serum diluted with water. Latex contains about 30-40% rubber and the balance is rubber serum. The serum is a clear, slightly yellowish liquid with a distinct acid reaction caused by the acid used for coagulation (mainly formic acid) as well as by the acids formed spontaneously in the latex (e.g. lactic acid). The rubber serums contain proteins, amino acids, carbohydrates (polyhydric alcohols, myoinositol and quebrachitol) and various plant growth substances including a number of inorganic cations such as calcium, magnesium, sodium, potassium and phosphorus (Peries and Fernando 1983). The nature of the effluent also depends on very largely on the product to be manufactured such as RSS (Rubber Sheet Smokes), brown crep, pale crepe, TSR (Thai Standard Rubber), or latex concentrate. Of these the effluents from latex concentrate production contains the highest concentration of undesirable non-rubber constituents.

The pollution from rubber effluent

Water pollution is caused in the following manner when organic matter is discharged into a stream. The organic matter decomposes through the action of bacteria, which use it as a source of nutrients. The bacteria use the dissolved oxygen present in the water. When the oxygen has been exhausted the anaerobic bacteria are still able to live, as they have the biochemical capacity for utilizing the oxygen from the organic matter. This process of putrefaction gives rise to noxious gases such as hydrogen sulphide, and the stream develops a dirty appearance and produces foul odours. A stream gets its supply of oxygen from the air, and from the algae and aquatic plant present in it. If the stream flows rapidly, much oxygen is absorbed from the atmosphere. In the case of stagnant water the suspended matter tends to settle and thereby lets in more light, so that algae and higher aquatic plants then grow again producing oxygen in the water (Peries and Fernando 1983).

Rubber factory effluent (waste water) contains high concentration of organic and inorganic materials. According to the Department of Environment in Malaysia, the effluent coming from the rubber factory must be treated to reduce the biochemical oxygen demand (BOD) to levels of 50 mg/litre and of ammonia nitrogen to 50 mg/litre, before the effluent is allowed to be discharged into waterways (Maheswaran 1977). This author reported that an alternative to a conventional treatment system is the land application of effluent and that this has been proven to be beneficial to crops like rubber and oil palm. Furthermore, no adverse effect on the environment has been reported. In this article various aspects of land application of rubber factory effluents are reported.

Today in Cambodia the Ministry of Environment is also concerned about this problem and especially the pollution of ground water, which is used by people for cooking and industrial use also. The Chup Rubber Plantation wants to reduce the pollution caused by waste water from raw rubber processing, by encouraging farmers around the factory to use the waste as fertilizer.

Fertilizer value of effluent

Mohd et al (1979) reported that the rubber effluent contains substantial amount of nutrient elements particularly nitrogen and potassium (Table 1). The amounts of nutrients vary depending on the types of process and processing capacity of the factory. The effluent therefore has potential as fertilizer for crops such as oil and rubber. The fertilizer equivalent value per year of the total rubber effluent produced in Malaysia was estimated to be about 39 million Ringgit in 1983 (Yeow et al 1983).

Table 1. pH and nutrient content (mg/litre) of rubber factory effluents (Source: Mohd et al 1979)
Type of efluent	pH	N	P	K	Mg
Latex concentrate	5.50	563	60	386	44
Latex concentrate	(4.22-7.00)	(121-1310)	(72-149)	(68-1050)	(26-68)
SMR	5.99	95	20	47	13
SMR	(5.75-6.50)	48-339)	(10-30)	(30-95)	(8-20)
Conventional grade	4.7	230	87	280	56
Conventional grade	(3.95-5.20)	(71-492)	(7-189)	44-869)	(16-112)
(Figures in brackets are range of value while the rest are mean values)

Today at the Chup Rubber Plantation, the latex effluents are used by the farmers located around the factory, and also by the factory workers. The waste water is applied to vegetable crops particularly water spinach (Ipomoea aquatica).

The potential of effluent from rubber processing factory for crop farming

Peries and Fernando (1983) reported that more research is urgently needed in order to develop appropriate technology suited to developing countries to overcome the environmental hazard caused by latex waste water. Attempts have already been made to develop methods to utilize rubber factory serum for useful purposes. Use of rubber serum as a fertilizer, particularly for grasses and leafy vegetables, use of serum as growth media in single cell protein production, extraction of a sugar known as quebrachitol, which is used for pharmaceutical purposes, are some of them (Peries and Fernando 1983).

Traditional fertilizers

Cow manure

Cow manure is usually called “cold” manure, because it takes a long time to decompose and release the nutrients. Cow manure is an excellent soil conditioner for improving the texture and structure of the soil. If fresh manure is used it should be applied two months on the land before planting. As with most animal manures, additional phosphorous and potassium may be needed, and should be added in the form of chemical fertilizers. The recommended rate of usage is from 22 to 33 tonnes of cow manure per hectare for most kinds of crop. Cow manure is also very valuable as a soil texturizer.

Currently in Cambodia, chemical nitrogen fertilizer has a high price so its use by resource poor farmers is limited and this leads to low yields. Use of animal manure is widespread but this has not been the subject of detailed studies. Most farmers in [poor developing countries depend wholly or in part on animal wastes to enhance soil fertility. Generally, animal manure does not increase short-term crop yield to the extent of an equivalent amount supplied in refined chemical form. However, the differences in yield are with long-term usage.

Handling procedures can markedly influence the value of manures for soil fertility. An example of this is the processing of manure in an anaerobic biodigester. The introduction of biodigesters as components of Food Security and Poverty Alleviation projects (Khieu Borin 2001 personal communication) promises to be a means of focusing attention on the fertilizer value of the effluent. Some initial reports are encouraging. An experiment by Sophoun Sopheak (unpublished data 2000) confirmed the superiority of the effluent from a biodigester charged with pig manure compared with fresh goat manure as a fertilizer for Chinese cabbage

At the same level of N application, the effluent from the biodigester was better than the manure used to charge the biodigester, when used as fertilizer for both cassava (Le Ha Chau 1998a) and duckweed (Lemna spp) (Le Ha Chau 1998b).

Estimating fertilizer needs

Plant appearance and growth rate, together with markings on the leaves, are good indications of specific nutrient deficiencies (Bear et al 1949). Observing deficiency symptoms during the year may help in developing a fertilizer program for subsequent years. But in the same year, by the time plant symptoms are evident, it is usually too late to gain full value from fertilizer applications. Also plants may not show definite nutrient deficiency symptoms even though yields are being reduced due to inadequate supply of one or more nutrients. Nitrogen is one of sixteen elements that are essential for the growth of plants. It is a component of proteins and is therefore involved in regulating most process that occurs in plants. A deficiency of nitrogen causes poor growth, stunted plants and low yields. Because nitrogen is a component of chlorophyll, a yellow colour beginning with the lower leaves is a common symptom of nitrogen deficiency. Nitrogen tends to promote vegetative growth relatively more than reproductive growth. Plants given excess nitrogen tend to be tall with weak stems. Thus an oversupply of nitrogen can cause lodging (Morris and Stevenson 1997).

Plant analysis

The nutrient content of specific parts of plants is useful in determining needs for fertilizers. Two types of plant analysis have been used. One is a fresh tissue test, usually made in the field. The other is a total plant or tissue analysis performed with accuracy in a laboratory. Since such tests must be compared with other similar analyses for valid interpretations, directions for selection of plant materials, preparation and methods of analysis should be followed carefully. More detailed procedures are given in such references as Donahue et al (1977) and Tisdale and Nelson (1975).

Fertilizer application

In actual practice the application of fertilizers is usually a compromise between the ideal and the practical. Ideally, fertilizers should be applied when needed by plants and placed in strategic location in the root zone. In practice, the costs of extra or special field applications and of special equipment are weighed against the differences in fertilizer effectiveness. The appropriate time to apply fertilizer depends on the climate, soil, crop and nutrients to be applied. The amount of precipitation usually occurring between time of fertilizer application and utilization is important. If precipitation is normally high, if the soil is coarse in texture and if the nutrients are subject to leaching, then special split applications are usually advisable with part being side-dressed during the crop season. Later side dressing of fertilizer will ordinarily be limited to supplemental treatments with nitrogen carriers.

Where potassium and phosphorus are deficient, usually applications can be made before planting crops that are most responsive to these elements. Less demanding crops maybe adequately supported with the residual quantities remaining in the soil. Nitrogen fertilizers are usually planned for each crop, whereas phosphate and potash applications are frequently made on selected crops in the cropping sequence. Such considerations are especially common under conditions where modest quantities of fertilizer are used and in which high yields of all crops are not given top priority.

Cassava (Manihot esculenta)

Cassava in Cambodia

Cassava is a popular crop for farmers in the Northeast and Eastern regions of Cambodia. Several varieties have been used in different parts of the countries but no studies have been documented related to this crop. In general, cassava is cultivated for root production either commercially or on a small scale for home consumption. In Cambodia in 1998, cassava cultivation covered an area of 14,000 ha, excluding areas cultivated by small-scale farmers for their home consumption (MAFF 1999). The average root yield of the short-term variety is about 16 tonnes ha^-1(MAFF 1999), which is below the average yield of other countries in the region.

Cassava for root production

Among the entire tropical crops, cassava is capable of providing the highest yield of energy per hectare according to Oke (1978). Cassava has one important characteristic that it can be managed although soils are poor (sandy), to maximize production of carbohydrate (in the form of the roots), and protein (through the leaves) with acceptable yields in areas where other crops do not perform at all well (Khieu Borin, personal observation). The time taken for root production varies from 6 to 12 months according to the variety.

It is argued that the fertility of the soil decreases after the cultivation of cassava for root production. In general, better soils are almost always devoted to more profitable crops, leaving planting with cassava to those areas with soil problems (eg: high aluminium content, low exchangeable base content, high P fixation, and various degrees of erosion) (Howeler 1991, 1994; Romanoff and Lynam 1992). Although the blame is given to cassava as soil nutrients extractor, many poor resource farmers in many parts of the world have relied on this crop for their survival. Therefore, it is important to look for ways to improve the productivity of this crop as well as to maintain soil fertility.

When cassava was intercropped with ipil-ipil (Leucaena spp) root yields was significantly higher (Padullo 1983). In the sandy soils of northern Colombia when N, P and K fertilizer were applied in moderate amounts, root and top biomass yields were significantly increased and root HCN was reduced (Cadavid et al 1997).

Cassava for forage production

But the potential for yield of leaves depends on many factors such as cultivars, age of plant, plant density, soil fertility, harvesting frequency and climate (Ravindran 1993). When maximum protein production is the aim, the foliage is harvested at 2 to 3 month intervals by cutting the stems at 50 to 70 cm above the ground thereby encouraging the plant to re-grow. In this case the roots act as a nutrient reserve to facilitate the re-growth of the aerial part. Dual-purpose production systems are also possible whereby one or two harvests of the leaves are taken before the plant is allowed to continue the normal development of the root (Preston 2001).

Considerable amounts of cassava leaves are readily available as a by-product at the time of harvesting the roots. However, in the rainy season it is difficult to sun-dry, and extending the drying period diminishes the nutritional quality of the product. Ensiling could be a suitable alternative way of preserving the leaves. Numerous reports have shown that cassava leaf has a high but variable protein content (170 to 400 g/kg on a dry matter basis), with almost 0.85 of the crude protein fraction as true protein (Ravindran 1993). While cassava leaf protein is low in sulphur amino acids (Gomez and Valdivieso 1984), the content of most other essential amino acids is higher than in soya bean meal (Eggum 1970). The high protein content and a relatively good profile of essential amino acids are reasons for believing that cassava leaves could be a potential protein source for monogastric animals (Ly, J, personal communication).

The intercropping of cassava with Gliricidia septum or with Desmanthus gave a foliage DM yield of 10.6 tonnes ha^-1 year^-1 with 18.3% crude protein in the DM (Khieu Borin 2001, unpublished data). Inter-cropping has also been reported with cowpea (Vigna spp) (Polthanee et al 2001).

Cow manure and biodigester effluents have given good results with cassava grown as forage. When approximately 10 kg N ha^-1 from cow manure was applied, the DM yield and crude protein contents of cassava foliage were 3.6 to 4.4 tonnes ha^-1 and 20.6 to 22.0%, respectively (Poungchompu et al 2001). It is possible to obtain more than 6 tonnes of crude protein per hectare a year from cassava with proper agronomic practices directed toward foliage harvesting (Preston 2001). In a study by Khieu Borin (2001, unpublished data) on the effects of fertilization of cassava with effluent from biodigesters fed cow or pig manure, the DM yield was 20.4 tonnes ha^-1 year^-1with 16.2% crude protein in the DM.

Soil fertility, nutrient requirement and irrigation

Cassava can be planted on sandy or sandy loam soil and all types of soils, except water-logged soils. With a hardpan, soils about 30-40cm deep are desirable because they prevent deep penetration of roots (Polthanee et al 1998), which aids in harvesting. It grows well on the acidic (pH 5 to 5.5) or alkaline (pH 8 to 9) soils.

Cassava cultivation for several years usually results in a decline in soil fertility. This is due to (1) wide spacing, slow development of soil cover in the first three to four months, traditional soil tillage and clean weeding practices at the onset of the rainy season, which can result in high soil losses, (2) the above-ground part of the plant is not reincorporated into the soil (as the stem is used for planting material, (3) no-root residues remain in the soil (the root is removed and sold), (4) short turn around time for soil recovery (long growth duration), and (5) farmers apply small amounts of fertilizer. Polthanee et al (1998) determined nutrient balances for the cassava system under farmer management using a crop-cut study procedure. The results show that nitrogen (N) balance was slightly negative, phosphorus (P) balance slightly positive, but potassium (K) balances were highly negative (Table 2).

Table 2. Nutrient balance (kg/ha) for the cassava system at Nong Pak Top village (Polthanee et al 1998)
	N	P	K
Fertilizer (15-15-15)	28.1	12.2	23.3
Rainfall	2.21	1.72	1.03
Planting material	0.95	0.08	0.74
Total inputs	+ 31.2	+14.0	+25.1
Cassava roots	28.5	4.31	55.0
Cassava stems	15.5	1.33	12.0
Total outputs	-44.1	-5.64	-67.5
Balance	-12.8	+8.43	-42.3

Nutrient and chemical composition of cassava (leaf and root)

Nutrient composition of cassava leaf

The average of crude protein content in the cassava leaves is 210 g kg^-1 but this value ranges from 147 to 400 g kg^-1, according to Lancaster and Brooks (1983). This wide variability is primarily related to differences in stage of maturity. 20.6 to 36% according to Rogers (1959). The protein content in the leaves is related to many factors such as stage of maturity, soil fertility and climate. The crude protein content in DM decreased from 38.1% in very young leaves to 19.7 % in mature leaves.

Chemical composition of cassava root

Whole fresh cassava roots contain approximately (% fresh basis): water 65, protein (N*6.25) 1 to 2, ether extract 0.2 to 0.5, crude fibre 0.8 to 1.0, ash 1 to 2 and nitrogen free extract (NFE) 30 to 35 (Gomez 1979). Essentially they are sources of energy. Whole cassava root is rich in starch. The NFE in the tuber contain 80% starch and 20% sugars and amides according to Vogt (1966). The starch contains about 20% amylase and 70% amylopectin (Johnson and Raymond 1965). Jalaludin (1977) has reported that cassava roots are very low in protein. Also, Limon (1992) explained that the protein of cassava root is lacking in methionine, cysteine and cystine (the sulphur amino acids), and this deficiency is aggravated by the use of these amino acids in metabolic detoxification of HCN. There is great variation in the quantity and quality of minerals and vitamins due to different soil types and processing methods.

Cassava foliage as animal feed

Recent work in Cambodia has aimed to evaluate cassava as a protein supplement for cattle and goats (the fresh foliage) and pigs (the ensiled leaves) (Preston 2001). In this system, the roots are not harvested. Instead, the foliage is harvested by cutting the stems at about 70 cm above soil level. Provided there is adequate fertilization (return of extracted nutrients) successive harvests can be made at 50 to 70 day intervals with no loss of yield (Preston 2001). In integrated farming systems, the required fertilizer nutrients can be obtained by recycling biodigester effluent (Preston 2002). The use of the waste water from the rubber factory as fertilizer for cassava is an alternative that has application to the farms located close to the factory.

In contrast with the cassava research programme in Thailand, which is mainly based on production and utilization of cassava hay for dry season feeding Wanapat (2001), in Cambodia the emphasis is on use the fresh foliage on a year-round basis. The research with cassava reported in this thesis is therefore directed to the effect of fertilization on fresh cassava foliage yield and composition under repeated harvests, as it is the fresh foliage which has immediate use as feed for ruminants or, after ensiling, for pigs (Preston 2001).

Water spinach

Water spinach in Cambodia

Water spinach grows well at high temperatures, and is commonly cultivated in Southeast Asia (Gohl 1981) for human food, and sometimes for animals particularly poultry and pigs. It is an important feed resource for pigs particularly in Vietnam and Cambodia because of it availability and easiness to grow. In Malaysia and Fiji, it was reported that water spinach is used as feed for dairy cattle (Gohl 1981). Water spinach can also be given fresh to poultry and rabbits (Rodriguez Lylian, 2002, personal communication).

Two common types of water spinach are normally cultivated, the dry land and swamp spinach. The dry land-grown water spinach has long, narrow leaves with pointed ends bears with flowers. The succulent foliage and stem tips are light green in colour. To obtain seeds, harvesting of the plants is stopped to allow developing flowers to mature, from which seed bearing pods form. Two main cultivar groups can be distinguished: var. aquatica and var. replants. The first is an aquatic plant or paddy vegetable in the Southern part of India and Southeast Asia, propagated by cuttings and growing in the wild or cultivated in fish ponds and water courses. The second is an upland vegetable, cultivated on dry or marshy land and propagated by seeds and cuttings (Palada and Crossman 1999). It can be grown in beds provided there is plenty of moisture. Cambodia has the potential to grow both dry land and swamp spinach. In practice, large amounts of organic material (compost and manure) and water are used to get higher productivity from water spinach Stephens (1994).

Ecological considerations

The plant is a frost-sensitive perennial with essential no growth below 10^oC(50^oF). Optimum temperatures for growth are between 24 to 30^oC(75-85^oF). With the moist soil procedure, raised beds are usually used. Seedlings or stem cuttings are transplanted into these beds, or seed is sown directly at relatively close spacing 10 to 15 cm in order to maximize the yield of tender shoot and continuing productivity and quality. The crop is harvested several times as re-growth of shoots readily occurs. Growth is usually rapid with harvest beginning at 4 to 5 weeks. The crop is responsive to supplemental fertilization (Rubatzky 1991). Water spinach is ready for harvesting 30 days after planting.

Fertilization of water spinach

Water spinach is highly responsive to fertilizer N application. Thus a linear response in yield of water spinach, in accordance with level of N application as biodigester effluent, was reported by Kean Sophea and Preston (2001), who used up to 140 kg N/ha, and Ngo Tien Dung (2001), with up to 40 kg N/ha.

References

Bear, F.E. et al. 1949. Hunger Signs in Crop. American Society of Agronomy and National Fertilizer Association, Washington, D.C

Cadavid, L.F., El-Sharkawy M.A., Acosta, A. and Sanchez, T. 1997. Long-term effects of mulch, fertilization and tillage on cassava grown in sandy soils in northern Colombia. Field Crops Research 57: 45-56.

Donahue, R.L., Miller, R.W. and Schickluna. J.C. 1977. An introduction to soils and Plant Growth. Prentice-Hall, Englewood Cliffs, New Jersey.

Dijkman M.J.1951. Hevea, The United State of America, p.p 91.

Eggum B O. 1970. The protein quality of cassava leaves. Br.J. Nutr. 24:761-768.

Gohl, B. 1981. Tropical Feed, FOA Animal Production and Health Series No.12, pp 254

Gomez G and Valdivieso M 1984. Cassava for animal feeding: Effect of variety and plant age on production of leaves and roots. Anim. Feed Sci. Technol. 11,49-55.

Gomez, G., 1979. Cassava as swine feed. World Animal Review-29: 13-20.

Howeler, R.H. 1991. Long-term effect of cassava cultivation on soil productivity. Field Crops Res., 26: 1-18.

Howeler, R.H. 1994. Integrated soil and crop management to prevent environmental degradation in cassava-based cropping systems in Asia. In: J. W. T. Bottema and D. R. Stoltz (Editors), Upland Agriculture in Asia. CGPRT Center, Bogor, Indonesia. pp. 195-224.

Jalaludin, S., 1977. Cassava as feedstuffs for livestock. In: Devendra, C: Hutagalung, R.I. (Eds) Proc. Symp. Feedstuffs for livestock in South East Asia. Pp. 158-159.

Johnson, R.M., Raymond, W.D., 1965. The chemical composition of same tropical food plants. IV. Manioc. Trop. Sci. 7: 109-115.

Kean Sophea and Preston T R 2001 Comparison of biodigester effluent and urea as fertilizer for water spinach vegetable. Livestock Research for Rural Development (13) 6: http://www.cipav.org.co/lrrd/lrrd13/6/Kean136.htm

Lancaster, P.A., Brooks, J.E. 1983. Cassava leaves as human food. Economic- Botany.37 (3): 331-348)

Le Ha Chau 1998a Biodigester effluent versus manure from pigs or cattle as fertilizer for production of cassava foliage (Manihot esculenta). Livestock Research for Rural Development (10) 3: http://cipav.org.co/lrrd/lrrd10/3/chau1

Le Ha Chau 1998b Biodigester effluent versus manure, from pigs or cattle, as fertilizer for duckweed (Lemna spp.). Livestock Research for Rural Development (10) 3: http://cipav.org.co/lrrd/lrrd10/3/chau2

Limon, R.L., 1992. Ensiling of cassava products and their use as animal feed. Root, tubers, plaintains and bananas. In: Animal feeding FAO 95. pp99-109.

MAFF, 1999. Annual Agricultural Statistics. Ministry of Agriculture Forestry and Fisheries in Cambodia 1999.

Maheswaran, A.1977. Prohibition and control of Pollution in the Rubber Industry: Promulgation of regulation.

Mohd. Tayeb Dolmat, Mohd. Zin Karim and Zaid ISA. 1979. Land-Disposal of Rubber Factory Effluent: Its effects on Soil properties and performance of Rubber and Oil Palm.

Morris D T and Stevenson C K 1997. Nitrogen fertilizer materials for field crops. OMAFRA, Ontario, Canada
www.gov.on.ca/OMAFRA/english/crops/facts/90-201.thm

Ngo Tien Dung 2001 Response of water spinach (Ipomoea aquatica) to fertilization with increasing concentrations of rubber tree latex wash water and biodigester effluent. http://www.mekarn.org/minipro/dung.htm

Peries. O.S and D.M.Fernando.1983. A Handbook of Rubber Culture and Processing, Rubber Research Institute of Sri Lanka, pp 341-345.

Oke O.L. 1978. Problems in the use of cassava as animal feed, Chemistry Department, University of life, Ile- (Nigeria). pp 345, 364, 365

Padullo, J.L. Jr. 1983. The effect of ipil-ipil (Leucaena leucocephala) planted at varying distances on the growth and yield of cassava in hilly land area. Undergrad. Thesis. VisCA, Baybay Leyte, Phill. 44 pp

Poungchompu, O., Wanapat, S., Polthanee, A., Wachirapakorn, C. and Wanapat, M. 2001. Effects of planting method and fertilization on cassava hay yield and chemical composition. In: Proceeding of the International Workshop “Current Research and Development on Use of Cassava as Animal Feed. Khon Kaen University, Thailand, July 23-24, 2001.

Preston T R 2001. Potential of cassava in integrated farming systems. In: Current research and development on use of cassava as animal feed (Editors: T R Preston, Brian Ogle and M Wanapat). Khon Kaern University: MEKARN, Sarec-Sida

Polthanee A, Wanapat S and Mangprom P. 1998. Row arrangement of peanut in cassava-peanut Intercropping: II Nutrient removal and nutrient balance in soil. Khon Kaen Agric.J.26 (3): 125- 131.

Polthanee A., Wanapat S, Wanapat M and Wachirapakorn C 2001 In: Proceedings of an International Workshop on Current Research and Development of cassava as animal feed, Khon Kaen University and the Mekong Basin Animal research Network (MEKARN), July 23-24, Kosa hotel, Khon Kaen, Thailand.

Palada M C and Crossman S M A 1999. Evaluation of Tropical leaf vegetable in the Virgin Islands. P. 388-393. In: J. Janick (editor), Perspectives on new crops and new uses. ASHS Press, Alexandria, VA. USA.
http// www.hort.purdue.edu/newcrop/proseedings1999/v4-388.html#convolvulaceae

Rogers, D.J. 1959.Cassava leaf protein. Econ. Bot.13: 261-263

Romanoff, S. and Lynam, J. 1992. Cassava and African food security: some ethnographic examples. Ecol. Food Nutr. 27: 29-41.

Rubatzky V 1991. Water Convolvulus. Chinese Water Spinach, Swamp Cabbage, Kang Kong. University of California Davis http:/www.island.wsu.edu/CROPS/WATERCON.htm

Ravindran V 1993. Cassava leaves as animal feed: potential and limitations. J. Sci. Food Agric. 61: 141-150.

Stephens J M 1994. Kangkong Ipomoea aquatica Forsk http://edis.ifas.ufl.edu/MV085

Tisdale, S.L., and Nelson, W.L. 1975. Soil Fertility and Fertilizers, 3^rd Edition. Macmillan, New York.

Vogt, H., 1966. The use of tapioca meal in poultry rations. World Poult. Sci. J. 22: 113-125.

Wanapat M., Polthanee A, Wachirapakorn C, Anekwit T and Mattarat S 2001. Crop-animal systems research network (CASREN). Progress report-Thailand, ILRI Paper, 20 pp.

Yeow. K.H. and Y. Ahmad Kamal 1983. The present status of Effluent Utilization in Malaysia.

Waste water from rubber processing as fertilizer for water spinach and forage cassava

Literature review

Background