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Major agricultural crop residues in Indonesia and their potential as raw materials for bioconversion


S. Saono and D. Sastrapradja

National Biological Institute, Indonesian Institute of Sciences, Jakarta, Indonesia


Introduction
Present status of agricultural residues
The prospects for intensifying and expanding the use of agricultural residues as raw materials for bioconversion
Major constraints hindering the use of agricultural residues as raw materials for bioconversion
Conclusions
Bibliography


Introduction


Agricultural production increased significantly during the first and second five-year development plans (Repelita I and 11) in Indonesia. This increase was due more to improvements in the technology of plantgrowing (i.e., the application of mineral fertilizers, the introduction of high-yielding varieties, the adoption of strict plant protection, and the introduction of new planting techniques) than to improvements in post-harvest technology (i.e., handling, storage, and processing of the harvest). As a consequence of this lopsided attention, a significant portion of the harvest has not been efficiently utilized. Because these under-used residues contain considerable amounts of carbohydrates, proteins, lipids, vitamins, and minerals, it is imperative that ways be found to use them. Otherwise they will be wasted and their potential benefits lost.

This paper presents a brief overview on the present status of agricultural residues, their utilization, the prospect for intensifying and expanding their uses as raw materials for bioconversion, and the constraints to be overcome in order to achieve that objective.

In this paper, the following definitions are used:


Present status of agricultural residues


Rice

Rice (Oriza saliva) is the staple food of Indonesia. It is widely cultivated in the country on wet as well as on dry lands, particularly on the six major islands and island groups: Java-Madura, Sumatra, Sulawesi, Bali-Nusa Tenggara, Kalimantan, and Maluku-lrian Jaya, in that order of importance. Its cultivation and processing are labour-intensive and require a large capital input. Therefore, it is not surprising that more than 60 per cent of the total paddy produced in 1976 (approximately 30 million tons) originated from Java-Madura, the most densely populated as well as the most developed and fertile islands in Indonesia.

Because of the magnitude of the harvest and the fact that the method of harvesting and post-harvest processing is largely manual, paddy production generates a huge amount of residues. These consist of straw or stalks, husks or hulls, and bran. The total amount of straw generated in 1976 was about 37.5 million tons; of husks, approximately 6.8 million tons; and of bran, about 3 million tons.

The bulk of the straw is left rotting in the field, sometimes after burning, or is ploughed back into the soil as a conditioner and organic fertilizer. A large proportion of the straw is also used as cattle bedding and feed. As a feed it contains approximately 66 per cent dry matter, 3.4 per cent crude protein, 0.73 per cent digestible protein, and 38.1 per cent total digestible nutrients. A significant quantity of the straw also serves as raw material for board and paper manufacturing. Straw is an excellent substrate for the cultivation of paddy straw mushrooms (Volvariella volvacea). This is widely practiced in West Central Java as a home industry. Under favourable conditions, a yield of 6 per cent is quite usual.

Some of the rice husks are used for fuel. Some of the ash generated is used as washing powder, but the bulk is wasted. Most of the husks around large rice mills, however, are also wasted. In some areas, significant quantities of husks serve as a component of cattle feed. The husk has a quality comparable to that of straw because it contains 91 per cent dry matter, with 3.8 per cent crude protein,1.2 per cent digestible protein, and 19.55 per cent total digestible nutrients.

Although a significant amount of the bran is used as a component of cattle and chicken feed, a larger proportion is not used as it should be. As feed, it contains about 90 per cent dry matter, 10.3 per cent crude protein, and 6.78 per cent digestible protein.

Maize

Maize (Zea mays) is the second staple for many Indonesians, particularly those in Madura, Nusa Tenggara, and Sulawesi. It is mainly cultivated on dry lands in a multiple cropping system. Compared to rice, its cultivation and processing demand less labour and capital input. Major producing islands are in the following order of importance: Java-Madura, Sulawesi, Bali-Nusa Tenggara, Sumatra, Maluku-lrian Jaya, and Kalimantan. Total production in 1976 amounted to more than 2.5 million tons, the bulk of which (about 71 per cent) came from Java-Madura.

Most harvesting and post-harvest processing of maize are still manual operations that generate a large amount of residues, including stalks, husks, skins, cobs, pressed cake, and bran. It is estimated that stalk production per hectare is between two and four tons. Thus, the total quantity of stalks generated in 1976 was between 4 and 8 million tons. The husks, skins, and cobs produced as the result of separating the grains amounted to 1.7,0.34, and 2 million tons,.

Most of the stalks, husks, and skins are left to rot or ploughed back into the soil as a conditioner and an organic fertilizer. A significant proportion of the stalks - particularly the fresh, young ones - are used as green fodder. Stalks have relatively good feed nutritive values, containing about 21 per cent dry matter,1.7 per cent crude protein,1.1 per cent digestible protein, and 16.3 per cent total digestible nutrients. Dried stalks are often used as fuel.

Some of the husks and skin trimmings are also fed to cattle. In fact, they have better feed nutritive values than those of fresh stalks, because they contain higher digestible protein (18.1 vs.1.1 per cent) and higher total digestible nutrients (67.2 vs.16.3 per cent).

A by-product of the oil extraction process is presscake. With its content of 18.1 per cent digestible protein and 74.1 per cent total digestible nutrients, it has a feed nutritive value comparable to that of the whole grain.

Cassava

Cassava (Manihot utilissima) is an important food crop, particularly in places where the soil is relatively poor or in times of food scarcity. It is also an important export commodity in the form of pellets and chips. The plant is widely cultivated in the country either as a monoculture or in a multiple-cropping system. It does not need special attention or large capital input. Major producing islands are in the following order of importance: Java-Madura, Bali-Nusa Tenggara, Sumatra, Sulawesi, Kalimantan, and Maluku-lrian Jaya. Total production in 1976 was around 12 million tons, 73 per cent of which came from Java-Madura.

Since only the roots are collected, harvesting cassava yields large quantities of residue in the form of woody stems, leaves, and soft plant parts. If a mature cassava plant consists of 50 per cent roofs, 30 per cent woody stems, and 20 per cent soft plant parts and leaves, the total residues generated at harvest time in 1976 alone amounted to 7.2 million tons of woody stems and 4.8 million tons of soft parts and leaves.

The roots are usually further processed into pellets, chips, or tapioca flour. The processing generates residues as peels and tapioca flour wastes. The total quantity of peels produced in 1976 was approximately 2.4 million tons. Since only about 10 per cent of total cassava production is processed into flour, the flour wastes generated in 1976 were approximately 0.27 million tons.

Most of the woody stems are generally burned as fuel, while the soft plant parts and leaves - particularly those of non-toxic varieties - are fed to cattle or left on the soil as a conditioner and fertilizer. A significant amount is also used as green fodder, which has good feed values, containing 26 per cent dry matter, 23.1 per cent crude protein, and 13.86 per cent digestible crude protein.

Significant proportions of the peels are fed to cattle, and contain about 5.3 per cent crude protein, 20.9 per cent crude fibre, 1.6 per cent ether extracts, 65.7 per cent nitrogen-free extract, and 6.1 per cent ash. Most of the peels, however, are not used.

The bulk of the flour waste is used as a component of feedstuff. It contains about 90.3 per cent dry matter, 5.6 per cent crude protein, and 3.73 per cent digestible protein. A small amount is also used as a component for the preparation of traditional fermented foods such as oncom (fermented peanut presscake) and tempeh.

Soybeans

Soybeans (Glycine max) are one of the important crops of Indonesia, serving as the raw material of many soybean-based traditional fermented and non-fermented foods, such as tempeh, kecap, taoco, and tahu. It is usually planted in monoculture in rotation with rice. In places with low rainfall, multiple cropping of soybeans with other crops is commonly practiced. Compared to rice, soybean cultivation requires less attention and capital input. Total production in 1976 was about 500,000 tons. The largest share came from Java-Madura, followed successively by Bali-Nusa Tenggara, Sumatra, Sulawesi, Kalimantan, and Maluku - Irian Jaya.

Harvesting and separating the seeds from the pods generate residues in the form of straw and shells. Total straw production in 1976 was about 550,000 tons, while that of shells was approximately 50,000 tons. Significant amounts of residues are produced as the result of processing soybeans into foods. In tempeh preparation, for example, about 28 per cent of the raw material is wasted. Approximately the same proportion (27.4 per cent) is wasted in tabu (soy curd) production. If, in 1973 alone, tempeh and tahu production amounted to 18,000 tons, then the total residues generated amounted to approximately 5,000 tons. No information is available regarding the proportion of residues generated in kecap preparation, but, considering that in 1973 more than 5.5 million litres of kecap were produced, there is no question that the amount of residues is considerable.

The dry straw and shells are usually burned as fuel or left on the field and ploughed back into the soil as a conditioner and fertilizer. The young plant parts and leaves, however, are fed to cattle as green fodder. They have good nutritive value, containing 27.9 per cent dry matter, 24.7 per cent crude protein, and 12.53 per cent digestible crude protein.

No information is available on the use of the bulk of tempeh residues, but in some parts of Java a small proportion of solid residue is used as one of the raw materials for the preparation of second-class tempeh and kecap In the case of tahu residues, the solid portion is usually fed to pigs as a feed component. It contains 22.84 per cent protein, 7.27 per cent fat, 65.42 per cent carbon, and 4.47 per cent ash. The liquid portion is usually discharged into the sewers, causing a considerable pollution problem.

Groundnuts

Groundnuts (Arachis hypogaea) are the second important crop among the pulses in Indonesia. They are widely used as an ingredient of many Indonesian foods and also as a source of vegetable oil. As in the case of soybeans, they are usually planted in monoculture in rotation with rice, while in places with low rainfall they are part of a multiple-crop system. Their cultivation requires about the same amount of labour and capital input needed for soybean production. Total production in 1976 amounted to 300,000 tons, the largest proportion of which (74.54 per cent) came from Java-Madura. Other producing islands are in the following order of importance: Sumatra, Sulawesi, Bali-Nusa Tenggara, Kalimantan, and Maluku-lrian Jaya.

The residues from groundnut production are mainly generated at harvest and during separation of the seeds from the pods, i.e., stems and shells. In 1976 alone a total of about 330,000 tons of stems and 9,900 tons of shells resulted from harvesting and processing. Moreover, oil-extraction processing of the seeds also generates residues in the form of presscake. If about 50 per cent of the total production is used for oil production, then the cake generated in 1976 amounted to about 120,000 tons.

The stems and leaves are mostly fed to cattle as green fodder or as plant meal. In both forms they have relatively good nutritive values. Green fodder contains 17.6 per cent dry matter, 19.9 per cent crude protein, and 13.08 per cent digestible crude protein, whereas plant meal contains 88 per cent dry matter and 16.6 per cent crude protein. Stems that are not suitable for feed are usually left on the field and ploughed into the soil as a conditioner and fertilizer. The dry shells are commonly burned as fuel.

The bulk of the presscake is utilized as an ingredient of feed. It contains 90.15 per cent dry matter, 44.9 per cent crude protein, and 41.32 per cent digestible protein. A significant amount of high-quality presscake constitutes the main ingredient in oncom (fermented groundnut presscake) preparation.

Coconuts

Indonesia is the second largest producer of coconuts (Cocos nucifera) in the world after the Philippines, and copra is one of the more important export commodities. Coconuts grow extensively in the country, covering an area of not less than 2 million hectares. Major producing islands are Sulawesi, Maluku-lrian Jaya, and Bali-Nusa Tenggara. Total production in 1976 was about 3,483 million nuts, or the equivalent of about 6.6 million tons of unhusked nuts and 1.4 million tons of copra.

Coconut is a general-purpose fruit, as practically all parts of it are usable. Husks, coir dust, and shells are the residues after the shelled nut has been obtained. In the preparation of copra from shelled nuts, additional residue is generated in the form of liquid Finally, as the result of oil extraction from copra, coconut presscake is obtained. The amounts of the respective residues in 1976 were approximately as follows: husks, 2.2 million tons, consisting of 1.5 million tons of coir dust and 0.7 million tons of fibre and outer rind; shells, 0.4 million tons; liquid, 0.7 million tons; presscake, 0.4 million tons.

Most of the husks are wasted, but a significant quantity are used for mats and matting, floor coverings, brushes, strong ropes, etc. The shells usually serve as fuel for drying copra. In recent years there has been some interest in using shells to produce activated charcoal. Practically all of the liquid is wasted, and in many cope-producing centres it causes a serious pollution problem. In some places in Java the presscake is consumed as a side dish, but the majority serves as a feed ingredient. It has relatively good feed nutritive values, containing 91 per cent dry matter, 21.3 per cent crude protein, and 18.2 per cent digestible crude protein.

Oil Palm

Oil palm (Elaeis guineensis) is an important export commodity for Indonesia. It is mostly planted on large plantations in Sumatra. In 1976 alone it covered 179,000 hectares, with a total production of about 433,000 tons of oil.

Harvesting the fruit and the oil-extracting process generate considerable amounts of residues as empty stalks, fibres, pulp (pericarps), shells, and cake. The amounts of the respective residues generated in 1976 were approximately: empty stalks, 0.6 million tons; fibre, 0.2 million tons; pulp, 0.8 million tons; shells, 0.3 million tons; cake, 0.6 million tons. Most of the empty stalks, pulp, fibre, and shells are burned as fuel in the factory. The cake generally serves as an ingredient in cattle feed.

Sugar Cane

Although Indonesia is no longer an exporter of cane sugar, sugar cane (Saccharum officinarum) still is an important crop in the country. It is mostly planted on large plantations in Java. In recent years, however, new plantations have been started in Sumatra and Sulawesi, while sugar cane planting by small-holders has also been expanding. In 1976 the planted area covered about 0.2 million hectares. Total refined sugar production in the same year was about 1.3 million tons, with 1 million tons produced by the estates and 0.3 million tons by small-holders.

Cane sugar production generates a large amount of residues, both at the time of harvesting and during the sugar-extracting process. At harvest the main residues are cane tops, while those associated with sugar extraction are molasses, bagasse, and pith. Residues in 1976 amounted to about 3.4 million tons of cane tops, 0.6 million tons of molasses, 3.3 million tons of bagasse, and 1.1 million tons of pith.

Fresh cane tops are commonly used as green fodder. They contain 2.7 per cent digestible protein and 45.7 per cent total digestible nutrients. The dry tops and leaves, however, are left on the soil and (sometimes after burning) ploughed back into it as a conditioner and fertilizer. Large amounts of the bagasse and pith are used as fuels in the factory. They are also important as raw materials for the production of board and paper, but the amount used is not large.

Most of the molasses is exported. Only a small proportion of it is fermented into alcohol domestically, and a still smaller proportion serves as a component of cattle feed. Molasses has a relatively good feed value, containing 74.7 per cent dry matter, 6.5 per cent crude protein, and 3.7 per cent digestible protein.

Rubber

Indonesia is one of the major rubber producers in the world. The plant (Hevea brasiliensis) is widely cultivated in Sumatra, Kalimantan, and Java, on big plantations as well as in small-holdings. In 1976 the area planted was about 2.3 million hectares, producing aporoximately 0.8 million tons of dry rubber.

Latex contains about 4 per cent non-rubber constituents, most of which are in the serum fraction. During latex processing, the serum fraction is washed away. These non-rubber components are protein (1 per cent w/w), lipids (1 per cent w/w), carbohydrates - mostly quebrachitol (1 per cent w/w), inorganic salts - largely phosphates, K, Mg, Na, Ca, Fe, and Cu (0.5 per cent w/w), among others. Except for the lipids, most of the carbohydrates, inorganic salts, and 67 per cent of the latex proteins are in the serum. Thus, approximately 3.5 per cent of the latex is washed away during rubber processing. Assuming dry rubber production in 1976 was about 0.8 million tons (the equivalent of about 2.5 million tons of latex), the non-rubber constituents wasted would have amounted to 17,000 tons of proteins, 25,000 tons of carbohydrates, and 12,000 tons of inorganic salts. They are usually discharged into open streams, thus causing a serious pollution problem.

Coffee

Coffee (Coffea robusta) is one of the most important export commodities of Indonesia. Throughout Asia, only India's coffee production exceeds that of Indonesia. It is cultivated particularly in the highlands of Java and Sumatra. In 1976 the planted area was about 0.4 million hectares, producing about 185,000 tons of dry beans.

Coffee processing generates residues in the form of pulp or husk, hull, and bean. These represent approximately 78, 6, and 16 per cent of the berry. Thus, the residues generated in 1976 amounted to about 309,000 tons of fresh pulp and 24,000 tons of fresh hulls.

No information is available on the use of these residues, but it has been reported that the pulp has a relatively good feed value, containing 91.8 per cent dry matter, 9.2 per cent protein, 3.4 per cent digestible crude protein, and 42.2 per cent total digestible nutrients.

Forestry Products

The importance of forestry products as export commodities is exceeded only by petroleum. In 1977, wood exports totalled more than 15.8 million tons, with a value of US$951.2 million. Tropical forest covers much of Indonesia. In 1976 it covered more than 122 million hectares, producing approximately 16.3 million m of wood. This consisted of 14.6 million m of logs and 1.7 million m of boards. More than 65 per cent of the total came from Kalimantan, about 22.9 per cent from Sumatra, and the remaining 12.1 per cent from the other islands.

Logging and wood processing generate a considerable number of residues in the form of logging wastes, defective logs, sawdust, edgings, slabs, and trimmings. In 1976, logging trees with a diameter larger than 30 cm generated more than 3.1 million m logging wastes and 1.7 million m defective logs. Processing the logs in saw mills generated more than 0.37 million m sawdust, 0.9 million m edgings and slabs, and 0.61 million m trimmings. Practically all of the logging debris is left in the forests. This is also the case with the bulk of wood-processing residues, except for an insignificant amount that is burned as fuel.

Livestock

The important domesticated animals in Indonesia include cattle, buffaloes, horses, goats, sheep, pigs, and poultry (chickens and ducks). The total livestock population in 1976 was about 108 million. More than 58 per cent of the animals were in Java-Madura, 21 per cent were in Sumatra, and the remaining 21 per cent were on the other islands.

No information is available on the residues of slaughterhouses in Indonesia, but there are data on the manure produced by these animals. The total annual production of fresh manure is approximately as follows (in kilograms per head): cattle, 5,400; horses, 2,700; pigs, 1,500; sheep and goats, 500; and poultry, 13.

Except for human wastes, most manures are used as organic fertilizers. Only recently has there been an increased interest in using manure as a raw material for biogas production. This practice is still very limited, however.

 

The prospects for intensifying and expanding the use of agricultural residues as raw materials for bioconversion


Traditional Fermented Foods

Traditional fermented foods such as tempeh, oncom, tape, kecap, and taoco are widely consumed in Indonesia. With the exception of oncom and tape, most of these products are based on whole soybeans. Tape is fermented cassava or glutinous rice, whereas oncom is a fermented peanut/groundnut presscake or solid residue of tahu.

Besides improving the nutritive value, attractiveness, and taste of the raw material, microbial fermentation, in some cases, also extends the shelf-life and reduces the toxic substances present in the basic ingredients.

Owing to the high price of the raw materials commonly used in traditional food fermentation and the abundance of agricultural residues in Indonesia, it would apparently be more economical to utilize certain residues as substitutes for some of the raw materials. These residues may be used singly (e.g., solid waste of tabu for black oncom preparation), or in combination (e.g., a combination of peanut presscake, solid waste of tahu, and solid waste of tapioca flour for red oncom preparation). A number of residues that deserve to be studied further as raw materials for traditional fermented foods are: peanut presscake, coconut presscake, cononut liquid, and the solid residues of tahu, tapioca flour, and tempeh.

As most traditional fermented foods are manufactured on a small scale as home industries using relatively simple techniques and non-sterile conditions, the quality of the end-product often varies from batch to batch or from time to time. Thus, appropriate microbial cultures as well as reliable and appropriate techniques should be developed if a product with a uniform quality is desired.

Animal Feed

As outlined in the preceding section, fresh residues from a number of crops (e.g., maize, cassava, soybeans, groundnuts, and sugar cane) have relatively good feed values.

Because of their seasonal availability there are times of surplus or deficit in supplies of these residues. It is desirable, of course, to have enough available year-round. This probably can be realized by conserving excess fresh residues at harvest time as silage. By this means, not only could residues be kept much longer but their feed value would also be improved. Silage of fresh maize stalks and leaves, for instance, contains higher levels of crude protein (1.9 vs. 1.7 per cent) and total digestible nutrients (18.3 vs. 16.3 per cent) than the fresh material, while its digestible protein content remains the same.

Making silage from agricultural residues has not been extensively practiced in Indonesia. However, considering the abundance and varieties of residues, as well as the fact that most of the livestock is raised on Java and Madura, the time has come to pay more attention to this important aspect of animal husbandry.

Of the many residues, the following deserve particular attention: rice straw, fresh maize stems and leaves, young cassava stems and fresh leaves, fresh soybean stems and leaves, fresh groundnut stems and leaves, sugar cane tops, and fresh coffee pulp. Because these residues are widely dispersed, the silage-making technique should be simple and appropriate for implementation by small farmers.

The presscakes resulting from oil extraction of groundnuts, coconuts, and oil palm seeds also deserve further study as animal feed supplements.

Mushroom Cultivation on Agricultural Residues

Volvariella volvacea is probably the most suitable type of mushroom for cultivation in Indonesia. It has been grown here for a long time, largely on paddy straw, and a yield of 6 per cent is not unusual. Studies that have been conducted in many Asian countries show that V. volvacea grows well on:

In order to maximize the use of available agricultural residues, the most appropriate technique and substrate for a particular environment should be selected. Of the many residues, attention should be given especially to paddy straw, oil palm pericarp, dry maize stems and leaves, sugar cane bagasse, dry soybean shells, dry peanut shells, coconut husks, and sawdust.

Biogas Practically any agricultural residues can be utilized as raw materials for biogas production under suitable conditions provided that the C/N ratio is correct. Most of the principles and technical aspects of biogas production are known. In fact, a number of prototypes of biogas generators were developed in Indonesia and are operating in some places on Java. The main factors hampering the wider use of biogas at present are economic rather than technical.

Organic Fertilizer

As discussed earlier, the bulk of many agricultural residues is left on the field or ploughed back into the soil as a conditioner and fertilizer. In certain soils, the incorporation of fresh residues, particularly those with a high C/N ratio, may result in severe nitrogen immobilization. Therefore, it would be much safer if residues such as rice straw, dry maize stems, sugar cane bagasse, and others with a high C/N ratio were composted first, before being incorporated into the soil as fertilizer.

Most of the principles and technical aspects for composting agricultural residues are known. However, some technical problems in conducting efficient small-scale composting under tropical conditions remain to be solved, such as the minimum size of the pile, the rate of aerating it, the size of the particles, the amount of watering, the effect of residue composition, simple criteria for determining the maturity of the compost, etc.

Owing to the scattered distribution and seasonal availability of the residues, small-scale composting will probably be more economical than a large-scale operation. For example, a number of composting plants have been built in Medan and Surabaya, but there seem to be many obstacles to operating them economically.

Single-Cell Protein (SCP)

Although it probably will not be economical to carry out large-scale SCP production under present conditions in Indonesia, the abundance of raw materials such as molasses, rubber serum, coconut liquid, and liquid waste from cassava flour extraction could justify the initiation of research on a laboratory scale. Some problems that deserve to be studied are the selection of active thermo-tolerant strains of microorganisms, optimal conditions for fermenting the different residues, the appropriate scale of production, the technique for separating the end-product, etc.

Should SCP production eventually be realized in Indonesia, the plants should preferably be located near the source of the raw material. Thus, if molasses is used as the substrate, the plant should be located near or at a sugar refinery. The production scale should also be adjusted to existing facilities, and the micro-organisms should be thermo-tolerant in order to minimize energy consumption for cooling the fermentors. In this way, production costs could be kept as low as possible.

 

Major constraints hindering the use of agricultural residues as raw materials for bioconversion


Constraints vary with the residues, but, for the most part, they include the lack of economic incentives, assured markets, capital, and managerial initiative. The technology is usually available, but because of the seasonal availability and dispersed distribution of most of the residues, some adjustments will be required to process smaller quantities of residues under tropical conditions. Thus, there is a need for conducting research to adapt technologies for the most appropriate small-scale processing of residues.

 

Conclusions


Agricultural crop residues are abundant both in kind and quantity. Currently, most of them are wasted on the fields or ploughed back into the soil as conditioners and organic fertilizers. Because of their relatively good feed values, some are also used as animal feed. A much smaller quantity, through bioconversion, serves as raw material for some traditional fermented foods, as well as substrates for mushroom cultivation. However, considerable amounts of copra and rubber-processing residues, e.g., coconut liquid and rubber serum, are still discharged into the environment, causing a serious pollution problem. Because of the lack of economic incentives, capital, managerial initiatives, and, in some cases, suitable technology, progress in wider use of residues is much slower than it would be if there were no such constraints.

 

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