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Harvest constitutes a major operation among agricultural activities. Considered for a long time as the last step in production, it must rather be approached as the first one in the postproduction system, because of its influence on subsequent processing and preservation of the products.
Harvesting methods differ according to the part of the plant to be used. As regards forage crops, the whole plant is cut, but for underground crops (eg, groundnuts, roots and tubers), the crop is lifted while the soil sticking to it is removed. With cereals, the crop is first cut either as a whole or partially (ears), and then threshed and cleaned to separate the grain from the ears and straw.
In the latter case two main alternatives exist: separate harvesting and threshing, or combined harvesting and threshing.
In developing countries the first alternative is generally the most widely applied. Although harvesting and threshing are still frequently done by hand, their mechanization has begun to develop during recent years, especially where the crop is produced not for self-consumption but rather for commercial purpose. Nevertheless, such mechanization has not developed everywhere to the same extent but according to the type of crop concerned, because labour requirements remain high for handling the produce before threshing.
In industrialized countries, attempts have been made since the beginning of the 20th century to devise machines which would both harvest and thresh grain, so as to reduce the labour requirements involved. Combine harvesters ('combines') which can cut, convey, separate and thresh the grain were the product of this development work. They are in widespread use, and have been used already on large grain production schemes in a number of developing countries.
Rice Harvesting and Threshing
(i) Harvesting methods
In many countries rice ears are cut by hand. A special knife is frequently used in SouthEast Asia ("ani-ani"), Latin America ("cuchillo") and Africa. For instance, in the Casamance region of Senegal rice is cut stem by stem with a knife, 10 cm below the panicle so as to leave straw in the field in amounts large enough to produce grazing for cattle. Nevertheless such practice is labour intensive.
To harvest denser varieties (500 stems/sq metre instead of 100) a sickle is used mainly on a generally wetter produce. But work times remain high: 100 to 200 man-hours per ha for cutting and stooking.
During past decades the mechanization of rice harvesting has rapidly evolved. It first developed in Japan, then in Europe and has now reached many tropical countries.
The first machines used were simple animal-drawn (horses in Europe, oxen in the tropics) or tractor-driven mowing machines fitted with a cutter bar. The improvements made on this equipment have first resulted in the development of swathers (Figure 4.1. Swather.). These drop the crop in a continuous windrow to the side of the machine making it easy to pick up the panicles and manually tie them into bundles. The next step forward has been the reaper that forms unbound sheaves; and finally the reaper/binder which has a tying device to produce sheaves bound with a twine. However the supply, cost and quality of the twine are the main problems associated with the use of such equipment.
The output of these machines varies between 4 and 10 hours per hectare, which is slow. However, they may be usefully introduced into tropical rice growing areas, where hand harvesting results in great labour problems. In temperate countries they have been gradually replaced by combine harvesters.
(ii) Threshing methods
After being harvested paddy bunches may be stacked on the plot. This in-field storage method results in a pre-drying of the rice ears before threshing, the purpose of which is to separate seeds from panicles.
The traditional threshing of rice is generally made by hand: bunches of panicles are beaten against a hard element (eg, a wooden bar, bamboo table or stone) or with a flail. The outputs are 10g to 30kg of grain per man-hour according to the variety of rice and the method applied. Grain losses amount to 1-2%, or up to 4% when threshing is performed excessively late; some unthreshed grains can also be lost around the threshing area.
In many countries in Asia and Africa, and in Madagascar, the crop is threshed by being trodden underfoot (by humans or animals); the output is 30kg to 50kg of grain per manhour. The same method, but using a vehicle (tractor or lorry) is also commonly applied. The vehicle is driven in circles over the paddy bunches as these are thrown on to the threshing area (15m to 20m in diameter around the stack). The output is a few hundred kg per hour. This method results in some losses due to the grain being broken or buried in the earth.
In south-east Asia, total losses induced by traditional harvesting and threshing methods are estimated between 5 and 15%.
From a historical viewpoint, threshing operations were mechanized earlier than harvesting methods, and were studied throughout the 18th century.
Two main types of stationary threshing machines have been developed.
The machines of Western design are known as 'through-flow' threshers because stalks and ears pass through the machine. They consist of a threshing device with pegs, teeth or loops, and (in more complex models) a cleaning-winnowing mechanism based upon shakers, sieves and centrifugal fan (Figure 4.2. 'Through-flow' Thresher.). The capacities of the models from European manufacturers (eg, Alvan Blanch, Vicon, Borga) or tropical countries (Brazil, India, etc.) range from 500 to 2000kg per hour.
In the 70s, IRRI developed an axial flow thresher which has been widely manufactured at local level. Such is the case in Thailand where several thousands of these units have been put into use. They are generally mounted on lorries and belong to contractors working about 500 hours per year.
More recently, a Dutch company (Votex) has developed a small mobile thresher provided with either one or two threshers (Figure 4.3). The machine has been widely adopted in many rice growing areas. The simple design and work rates of these machines (about 500kg per hour) seem to meet the requirements of rural communities.
The 'hold-on' thresher of Japanese design (Figure 4.4. 'Hold on' thresher - Japanese design.), is so-called because the bundles are held by a chain conveyor which carries them and presents only the panicles to the threshing cylinder, keeping the straw out. According to the condition of the crop, work rates can range between 300kg and 700kg per hour (Iseki model). The main disadvantage of these machines is their fragility.
(iii) Combined harvesting and threshing methods
Combine-harvesters, as the name implies, combine the actions of reaping and threshing. Either the 'through-flow' or the 'hold-on' principle of threshing may be employed, but the reaping action is basically the same. The main difference is that combine-harvesters of the Western ('through-flow') type are equipped with a wide cutting bar (4-5m) while the working width of the Japanese ('hold-on') units is small (1m). According to the type of machine used, and specially to their working width, capacities range from 2 to 15 hours per hectare.
Such machines are being increasingly used in some tropical countries. In the Senegal river delta region, private contractors or farmers' organizations have recently acquired combine harvesters, mainly of the Western type (Massey Ferguson, Laverda, etc.). So, almost 40% of the Delta surface area is harvested with a pool of about 50 units. Between 200 and 300 hectares of winter rice are mechanically harvested. In this region the popularity of combine harvesters is high despite their poor suitability for some small-sized fields.
In Brazil, several manufacturers have adapted machines to rice growing conditions by substituting tracks for wheels; some machines are simple mobile threshers equipped with cutter bars.
In Thailand, local manufacturers have recently transformed the IRRI thresher into a combine harvester so as to reduce the labour requirement. The unit can harvest 5ha per day and seems to have been rapidly adopted.
Because of their size, conventional harvesters and combine harvesters prove unsuitable for many rice growing areas with small family farm holdings. In response to this problem, research services, during the last ten years, have developed small-sized machines for harvesting the panicles without cutting the straw. Such machines are known as strippers.
In the UK, the Silsoe Research Institute (SRI) has developed a rotor equipped with special teeth for strip-harvesting spikes or panicles. IRRI recently adopted this technology and has developed a 10hp self-propelled 'stripper gatherer' with a capacity of about 0.1ha per hour. However, the harvested grain has to be threshed and cleaned in a separate thresher. Since harvesting unthreshed produce results in frequent stoppages for emptying the machine, this constitutes the main drawback to the progress of the prototype.
In France, CIRAD-SAR has designed and developed a machine which strips panicles from the plants and threshes them in only one pass (Figure 4.5). The stripper has been specially designed for harvesting paddy rice on small plots. The essential component is a wire looped in line with the direction of movement of the machine, which is mounted on a three-wheeled carriage and powered by a 9hp engine. With a 30 cm working width the stripper capacity is about 1 ha per day.
Maize Harvesting and Threshing
(i) Harvesting methods
In village farming systems the crop is often harvested by hand, and cobs are stored in traditional structures. Quite often, the crop is left standing in the field long after the cobs have matured, so that the cobs may lose moisture and store more safely after harvest.
During this period the crop can suffer infestation by moulds and insects and be attacked by birds and rodents. To reduce such risks, an old practice (called "el doblado") is sometimes applied in South and Central America. This involves hand-bending the ears in the standing crop without removing them from the stalks. It helps mainly to prevent rainwater from entering the cobs, and also limits bird attacks; but, because of the high labour requirements involved, the practice is gradually falling into disuse.
Manual harvesting of maize does not require any specific tool; it simply involves removing the cob from the standing stalk. The work time averages 25 to 30 days per ha. Traditionally, maize cobs are commonly stored in their unhusked form. To improve their drying, it is often recommended to remove the husks from the cobs. Maize husking is usually a manual task carried out by groups of women. Some machine manufacturers (e.g. Bourgoin in France) have developed stationary maize huskers, such as the "Tonga" unit.
The first mechanized harvester to detach ears of maize from the standing stalks, the 'corn snapper', was built in North America in the middle of the 19th century. This was followed by the development of 'corn pickers', which incorporated a mechanism for removing the husks from the harvested ears. The first animal-drawn maize pickers were replaced by tractor-drawn units (l or 2 rows) and then tractor-mounted units (1 row). Finally came the development of self-propelled units capable of harvesting from up to 4 rows. A specific feature in maize harvesters is the header which leaves the stalks standing as it removes the ears.
The rates of work can vary from 2 hours per hectare with a 3-row self-propelled harvester to 5 hours per hectare with a tractor-drawn or -mounted single row unit. Generally speaking, harvest losses range from 3% to 5%, but they may be up to 10%-15% under adverse conditions. Depending on the situation, a single-row harvester can be employed effectively on up to 20 hectares or more; but the use of a multi-row machine demands several tens of hectares to be economically effective.
Specially designed for harvesting maize as grain, the corn-sheller was initially a cornhusker in which the husking mechanism was replaced by a threshing one (usually of the axial type). Corn-shellers are self-propelled machines of the 3 to 6-row type with capacities of 1 to 2 hours per hectare (Figure 4.6. Maize sheller.). The surface areas harvested during a 180-hour campaign range between 100ha (with a 3-row unit) and 200ha (with a 6-row one).
Another alternative consists of equipping a conventional combine with a number of headers corresponding to the machine horsepower. However, although widely used, such a method requires many adjustments to the threshing and cleaning mechanisms.
(ii) Threshing methods
Shelling and threshing
Traditional maize shelling is carried out as a manual operation: maize kernels are separated from the cob by pressing on the grains with the thumbs. According to the operator's ability the work rate is about 10kg per hour. Outputs up to 20kg per hour can be achieved with hand-held tools (wooden or slotted metal cylinders). To increase output, small disk shellers such as those marketed by many manufacturers can be recommended (Figure 4.7. Maize hand shellers.). These are hand-driven or powered machines which commonly require 2 operators to obtain 150kg to 300kg per hour. Another threshing method, sometimes applied in tropical countries, involves putting cobs in bags and beating them with sticks; outputs achieved prove attractive but bags deteriorate rapidly.
Nowadays many small maize shellers, equipped with a rotating cylinder of the peg or bar type, are available on the market. Their output ranges between 500 and 2000kg per hour, and they may be driven from a tractor power take off or have their own engine; power requirements vary between 5 and 15hp according to the equipment involved. For instance the French Bourgoin "Bamba" model (Figure 4.8. "Bamba" motorized maize sheller.) seems well-suited to rural areas in developing countries because of its simple design, easy handling and versatility (maize, millet sorghum, etc.).
Millet and Sorghum Harvesting and Threshing
(i) Manual harvesting
In Africa, and especially in the Sudano-Sahelian area, these cereals constitute the staple food in the human diet. They are harvested almost exclusively by hand, with a knife (Figure 4.9. Knife ("ngobane") for harvesting millet.) after unroofing or bending the taller stems to reach the spikes. Harvesting and removal from the field takes 10 to 20 days per hectare, according to yields. Harvested ears are stored in traditional granaries while the straw is used as feed for cattle or for other purposes (e.g. thatching).
(ii) Gradual mechanization of threshing
Women separate the grain from the ears with a mortar and pestle, as it is needed for consumption or for marketing purpose (Figure 4.10). The threshed grain is cleaned by tossing it in the air using gourds or shallow baskets.
This traditional method is arduous and slow (10kg per woman-day). Consequently, research has been conducted for some years on how to mechanize it.
The mechanical threshing of sorghum ears does not raise any special problems: conventional grain threshers can be used with some modifications; such as adjustment of the cylinder speed, size of the slots in the cleaning screens, etc. On the other hand, the dense arrangement of spikelets on the rachis and the shape of millet ears (especially pearl millet), make their mechanical threshing excessively difficult.
The first millet and sorghum threshers were developed in Senegal in the 1960-70s: the Siscoma BS 1000 and the Marot DAK II. Giving relatively high outputs (about 1000kg per hour) they have been intended for village farmers' groups, cooperatives or private contractors going from village to village to work on big threshing layouts. The multipurpose "Bamba" thresher, better suited to rural communities, has a capacity of about 300kg per hour. The
Senegalese pool of millet and sorghum threshers currently amounts to 120-150 units.
As regards mechanized harvesting at family level, some hand-operated threshers (Champenois) were developed and tested experimentally but they did not prove very successful. CIRAD is currently working on the design of powered millet threshers of low capacities (50 to 100kg per hour).
Threshing operations leave all kinds of trash mixed with the grain; they comprise both vegetable (e.g. foreign seeds or kernels, chaff, stalk, empty grains, etc.) and mineral materials (e.g. earth, stones, sand, metal particles, etc.), and can adversely affect subsequent storage and processing conditions. The cleaning operation aims at removing as much trash as possible from the threshed grain.
The simplest traditional cleaning method is winnowing, which uses the wind to remove light elements from the grain (Figure 4. 11).
(i) Mechanized cleaning
The most rustic equipment is the winnower (Figure 4.12. Cereals winnower.): a fan-originated current of air passes through several superposed reciprocating sieves or screens. This type of machine was widely used in the past for on-farm cleaning of seed in Europe. It can be either manually powered or motorised; capacities range from a few hundred kilogrammes to several tonnes per hour.
In Europe, with the use of combine harvesters and the development of centralized gathering, cereal winnowers have been progressively replaced by seed cleaners in the big storage centres. These machines, also equipped with a system of vibrating sieves, are generally capable of very high outputs (several tens of tonnes per hour).
In developing countries, mechanizing the cleaning operation at village level has seldom been felt as a necessity, because of the lack of quality standards in grain trading. However, because of the current trend towards privatization of marketing networks, the demand for cleaning machines will probably increase. The local manufacture and popularization of simple and easily portable equipment, such as winnowers or screen graders suited to cereal crops, need to be encouraged. CIRAD/SAR has recently developed cleaning machines of the rotary type with outputs of a few hundred kilogrammes per hour.
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