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The harvest is the operation of gathering the useful part or parts of the plant.
It is a voluntary intervention by man, carried out at the time when all the nutrients have been developed and when the edible parts have reached the degree of maturity appropriate to the treatments to follow.
In general, the harvest takes place 10 or 15 days after the grain has reached physiological maturity.
At the time of maturity the grain has a specific moisture content and special physical characteristics.
For the harvest to take place at the most propitious time, account must be taken not only of the length of the growing cycles (which differ according to the varieties), but also of the degree of maturity of the grain.
The following table shows the degrees of moisture content considered appropriate for good harvest conditions and the characteristics permitting assurance of physiological maturity.
|Rice||22-28%||The panicles bend with their own weight, yellowed hulls, full grains, neither too ripe (cracked), nor too green.|
|Maize||23-28%||Cobs almost dry, hard and glassy kernels resistant to scoring with the thumbnail, black dot in the caryopsis.|
|Sorghum||20-25%||Dried stems and leaves, hard grains resistant to the thumbnail, glassiness depending on variety.|
|Beans||30-40%||Pods ripe and yellow, shells dried, skins of kernels easily detached.|
|Groundnuts||30-35%||Leaves yellow, shells dried, skins of kemels easily detached.|
|Sunflower||9-10%||Upper leaves dry and flower faded.|
The harvest may, nevertheless, take place at a time when, because of varying weather conditions and even though it has reached physiological maturity, the grain has a moisture content higher or lower than shown above.
Clearly, the higher the moisture content of the grain at harvest time, the greater the risks of losses from moulds, insects and germination. On the other hand, the longer the grain remains in the field (to further the drying of the product), the greater the risks of losses from spontaneous fall of grain, or from attacks by birds, rodents and other pests.
Harvesting can be done by hand, with simple farming implements, or by mechanized systems.
Choice of the degree of mechanization to adopt depends on the anticipated use of the machines.
Choice of the machine must be justified by the estimate of the areas to be harvested annually and by a cost-benefit analysis.
This cereal is harvested only after the grain has reached maturity, and two or three weeks after draining the rice-field.
Premature cutting of the rice keeps the grain from reaching maturity, and can cause serious losses in the quality of the product.
Furthermore, it is important to accomplish the harvest while the moisture content of the grain is acceptable. Excess grain moisture can create big problems during the ensuing treatments, by fostering alteration of the final characteristics of the rice.
On the other hand, too low a moisture content can cause the panicles to shatter at the time of cutting, leading to serious losses of product.
Harvesting by hand is done with a sickle or a scythe; the ears of rice are cut at about 2030 cm above the ground.
After cutting, the ears of rice are left to dry on the stubble for two or three days.
For guidance, about 80 to 160 man-hours per hectare are calculated as the average time required for manual harvesting of rice.
In some tropical regions, it is still the custom today to harvest only the panicles, using a knife; in this case, the ears of rice are cut 30-50 cm below the panicles.
By comparison with hand-harvesting with a sickle or scythe, this method requires about 175 percent more labour.
Manual harvesting of rice, which is still relatively common, especially in tropical areas, is being increasingly replaced by mechanical harvesting with combine-harvesters. Combine harvesters are machines that do the cutting, threshing and pre-cleaning of the rice in one operation.
Generally self-propelling, combine-harvesters have cutting apparatus, a threshing chamber composed of a revolving threshing drum (with teeth) and a stationary counter-thresher, and devices for cleaning the paddy.
Construction of these machines is chiefly based on two designs: Japanese and western. Depending on their construction features (width of cutter), the work capacity of these machines can vary from two to 15 hours per hectare (h/ha).
Some estimates indicate yields from these machines of 350 to 800 kg of paddy an hour (kg/h), with grain losses lower than 3 percent.
Western combine-harvesters are large machines whose cutting bars may be four or five metres long.
Depending on their construction features and the harvesting conditions (well-drained paddy-fields, sufficiently long plots, etc.) the work capacity of these machines can be as much as 1 to 1.5 h/ha, for an overall employment of labour of about 2.7-4.5 hours per hectare (h/ha).
Use of combine-harvesters offers an economic advantage for harvests from a minimum of 70 hectares a year upward.
In addition to combine-harvesters, side-delivery rakes (or wind-rowers) and binders are also used for harvesting rice.
Wind-rowers are machines that cut and unload paddy only laterally.
These machines, whose theoretical work capacity varies from 4 to 8 h/ha, have the disadvantage of requiring a big labour force (100 to 200 in/ha) for manual gathering and binding of the paddy.
Binders are machines that simultaneously cut, bind and unload paddy. Equipped with a cutting bar and a gathering and binding device, these machines do good work even in harvesting lodged paddy (20°-30º angle to the ground).
Depending on their construction features (adjustability of height, width of cutter), the work Capacity of these machines can vary from 5 to 20 h/ha, with grain losses lower than 2 percent.
In any case, mechanical harvesting of rice presents some problems.
For example, machines must frequently work on muddy ground offering poor traction. For this reason, combine-harvesters are generally equipped with tracks, rather than wheels, so that harvesting can be done even on very wet ground.
At the time of the harvest, the rice panicles do not stand up straight but are bent downward. In order to avoid excessive losses, the machines must be placed so that the ears are cut about 30 cm above the ground: this obviously necessitates coping with large quantities of straw.
The rice husks contain silica, which gives them a highly abrasive quality that provokes rapid wear on the moving parts of the machines.
Maize can be harvested when the process of nutrients uptake into the kernels (physiological maturity) is complete.
This cereal can be harvested by hand (harvesting ears) or mechanically (harvesting ears or grains).
When maize reaches physiological maturity, the moisture content of the grains can be as high as 37-38 percent.
For this reason, before proceeding to hand-harvest the ears, maize is often pre-dried standing in the field.
The techniques generally used for this operation are:
Field pre-drying techniques are fairly widespread, but they entail great risks of product loss, especially if the varieties grown are particularly sensitive to unfavourable weather (rain, humidity, etc.) and pests (insects, birds, rodents, etc.).
In addition, the time taken up by pre-drying in the fields decreases the possibilities for exploiting the land.
To harvest maize by hand, the ears are pulled from the stalk of the plant and no tool is used.
Indicatively, the average duration of a manual maize harvest varies from 120 to 200 man-hours ( 15-25 man-days) per hectare.
Shucking the ears, that is, the removal of the husks covering the ears, may be done by hand or by machine at the same time as the harvest.
If this operation is done by hand, it requires about 130 man-hours (about 16 man-days) per hectare.
Hand harvesting of maize is considered practicable for crops of under 12 hectares, if climate and availability of labour permit.
Mechanized harvesting of maize is done with corn-pickers, corn-shellers or combine-harvesters.
Still in use, but with decreasing frequency, are simple corn-snappers, which do only harvesting of ears.
Corn-pickers are machines that simultaneously harvest and shuck ears. They are therefore equipped with picking devices, shucking tables and loading gear. Generally coupled to a trailer for transport of the ears picked, one- or two-row corn-pickers can be tractor-drawn, carried, or self-propelled.
The work capacity of these machines varies from 1.6 to 3.45 h/ha, with 75 to 80 percent of the ears completely shucked, and total grain losses lower than 4.5 percent.
Two operators - a driver and a worker - are generally necessary to run these machines. he use of one-row corn-pickers is economically advantageous for harvesting a minimum of 2530 hectares a year; for two-row machines the minimum harvest should be 30-60 hectares.
Corn-shellers resemble corn-pickers but have a device for shelling and cleaning grains. These machines can thus simultaneously harvest, shuck and shell the ears and pre-clean the kernels.
Sometimes coupled to a trailer for transport of the harvested grains, one- or two-row corn-shellers can be tractor-drawn or carried.
The self-propelled type, capable of harvesting two, three, or four rows, is, however, the most widespread, since its performance is superior to that of the tractor-drawn models described above.
The work capacity of these machines is comparable to that of corn-pickers, with grain-losses lower than 3 or 4 percent.
Another machine that is capable of simultaneously harvesting, shucking, shelling, and cleaning maize is the combine-harvester.
This machine is derived from a combine-harvester for wheat on which modifications have been made to the cutting apparatus and the threshing device.
Although the work capacity of these machines depends on harvesting conditions and on the size and shape of the parcels, it can run from 0.8 to 1.2 h/ha for six-row machines processing the whole plant, and from 0.4 to 0.6 h/ha for six-row machines treating only the ears.
Overall grain losses, usually caused by the cutting device, are rarely higher than 3 percent.
Two operators are generally needed for these machines.
The use of combine-harvesters, like that of corn-shellers, offers an economic advantage for harvests of a minimum of 40-75 hectares a year.
Grain sorghum harvesting
Sorghum is harvested, by hand or machine, when the grain is very ripe.
Hand harvesting is done by cutting the panicles of grain or the sorghum plants with machetes or sickles.
When the ears of sorghum have been cut, they are allowed to pre-dry on the threshing-floor or in sheaves in the field.
Indicatively, the average duration of a manual sorghum harvest varies from 120 to 160 man-hours (15-20 man-days) per hectare.
When physiological maturity has been reached, sorghum can have a moisture content of about 35 percent. However, mechanized harvesting can be effective only when the moisture content is below 20-25 percent.
Mechanized harvesting is done with combine-harvesters, equipped for cutting and threshing.
These machines, whose work capacity varies from 0.8 to 1.5 h/ha, seem better adapted to the harvest of dwarf varieties, and preferably to those with sparse straw-development.
In order to obtain a dry product, beans are harvested by hand or machine when the pods very ripe but not yet open.
To harvest beans by hand, the plants are pulled up and allowed to pre-dry in the sun.
This operation is often done early in the morning, while the dampness of the night minimizes the risk that the grains will pop out of the pods.
In some countries, before the harvest, the plants are treated with chemical defoliants. This treatment is intended to hasten drying of the plants and reduce the quantity of plant matter to prevent its slowing up threshing operations.
Indicatively, the average time spent cutting the plants by sickle runs from 80 to 100 man-hours (10 to 12.5 man-days) per hectare.
Mechanized harvesting of beans is more common in countries with advanced technology, and is usually done with combine-harvesters designed for wheat but adapted for beans. These machines, whose work capacity varies from 0.9 to 1.1 h/ha, have the disadvantage of working efficiently only on rather extensive parcels of land that have been levelled and weeded, then planted with dwarf varieties whose plants stand erect and reach maturity simultaneously.
Groundnut harvesting, by hand or machine, basically consists of the following operations:
It is very difficult, however, to recognize the most propitious time to harvest groundnuts. This is because there is no particular physical manifestation or feature that defines with total certainty the time when the plant has reached its physiological maturity.
For the "Virginia" varieties, the best time for the harvest is indicated by the drying of the stems, the falling of the yellowed leaves, and the dark-brown colour of the insides of the shells.
On the other hand, in the "non-dormant" varieties, groundnuts are ready for harvesting when 2 percent of the plants show germination.
Too late a harvest will inevitably be paid for by product losses, since in pulling up the plants many shells will be left in the ground if it has already hardened.
If, on the contrary, the harvest is too early, drying operations are more difficult and more urgent, and the quality of the end-product is compromised.
Since rapid lowering of the moisture content to about 15 percent is essential, care must also be taken, after pulling up the plants, not to place or leave them in a pile while they are still green or wet with rain.
Hand harvesting is done with a tool that cuts the tap-root and lifts the plant.
Once pulled out, the plant is shaken and freed of the residues of earth still attached to the shells, then left on the ground to dry.
After two days at most, the groundnut plants are stacked or placed on drying racks to further the pre-drying, or drying, of the product.
Indicatively, manual pulling up and stacking of the plants takes about 80 to 150 working hours per hectare.
With draught traction, at harvest time, the groundnut plants are lifted by means of farm implements drawn by animals.
Two people are needed for draught traction harvesting: one to drive the team and the other to pull up the plants.
By using lifters instead of harvesting by hand, working time is usually reduced by about 20 hours per hectare.
Many machines are available for harvesting groundnuts.
The choice of machine depends on several factors: land features, varieties grown, local climatic conditions, proximity to drying installations and storage buildings, etc.
Depending on the machine, then, groundnuts can be harvested by doing one operation at a time or several of the previously described operations simultaneously.
Thus, there are lifters, lifter-shakers, and picker-threshers.
Harvesting by tractors fitted with lifters is done with a system of frame-mounted blades that cut the tap-roots and lift up the plants.
In addition to lifting, lifter-shakers uproot the plants and rid them of earth.
If they are equipped also to lay the plants in windrows, these machines are called lifter-shaker-windrowers.
Indicatively, mechanized pulling and windrowing of the plants requires about 4-6 working hours per hectare.
Picker-threshers usually finish the work of the above machines, by gathering up the windrows for threshing.
Sunflower harvesting, by hand or machine, takes place when the upper leaves have started to dry and the flowers are faded.
In some cases, especially when the fields are small, it is preferable to harvest the crop before maturity. This decreases the risk of losses from attacks by birds and other pests.
Manual harvesting is done by cutting off heads with a knife or clippers.
If, at harvest time, the moisture content of the seeds is higher than 15 percent, the heads should be pre-dried before threshing.
Mechanized harvesting of sunflowers is generally done with combine harvesters designed for wheat, but modified and equipped with a sunflower-picker head.
The work capacity of these machines is about 0.9 h/ha.
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