In this article, the different methods of harvesting some tropical crops are discussed. Harvesting of crops generally constitutes the concluding stage of land cultivation.
It includes several steps such as cutting in the case of cereals like maize, sorghum, and millet, gathering the harvest, delivering for processing, transporting the processed material for storage or sale, and storage.
The primary stage in crop harvesting consists of two steps: the removal of the bulk plant material (i.e., picking the fruits and berries, cutting the grains and grasses, digging the tubers, and pulling the flax) and subsequent processing.
The harvesting method used depends on the crop species, biological characteristics, climatic conditions, and the technical equipment available.
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Importance of Proper Harvesting in Tropical Crop Production
Harvesting is an important component of crop production practices. The overall yield of a particular crop depends to some extent on the efficiency of the chosen harvesting method (e.g., manual or mechanical).
Harvesting is carried out when a crop has reached physiological maturity or at the point of senescence, when leaves begin to change colour from green to yellow and drop (as seen in cereal grains, legumes, roots and tubers, and bulbs), or when fruits show signs of ripening. Harvesting too early may reduce the total yield due to immaturity.
Harvesting at the correct stage of maturity greatly reduces crop spoilage. Where produce is to be held for any length of time, it is preferable to harvest at a mature but unripe stage.
Harvesting of Cereal Crops in the Tropics
In cereal grains, unripe crops contain high moisture levels and deteriorate more rapidly than mature ones. If matured grain crops are left in the field too long, alternating rain and sunshine, as well as dew at night and hot weather during the day, can adversely affect the grains.
Repeated wetting and drying cause certain grains, such as long-grained rice paddy, to crack and shed in the field. Grain legumes may split their pods, causing seeds to drop on the soil.
Insect infestation, particularly from field-to-storage pests, is common during this period. Harvest timing is critical for achieving maximum yields with minimal loss.
Some matured grains like rice are harvested when they still contain some moisture, while others such as maize, sorghum, and millet are left in the field to dry, provided the weather remains hot and sunny.
All harvested grains must be free from cracks, insects, and diseases to ensure good storage quality. Timely planting is crucial, especially for grains, so that maturity coincides with the approaching dry season.
In rice, heads are harvested with a sickle, dried outside for a few days, and then stacked. Cracking may occur due to rapid field drying. Sorghum and millet stems are cut low, bundled, and field-dried.
Millet heads are cut about 15 cm below the stem, dried, and stored. In some areas, millet heads are bundled with the stalks tied and the heads exposed, then rolled for transport. Maize is harvested by plucking ears from the plant once they are sufficiently dried.
Although grains may appear dry at harvest, they often contain residual moisture and must be properly dried before storage.
Harvested grains are placed in ventilated containers or structures to allow airflow, reducing heat and moisture build-up, discouraging mould growth, and preventing insect infestation that compromises grain quality.
Manual Harvesting Methods for Rice
In many countries, rice ears are harvested by hand. A special knife is often used. In Nigeria, for example, rice is cut stem by stem with a knife, 10 cm below the panicle to leave sufficient straw in the field for cattle grazing.
This method, however, is labour-intensive. Denser rice varieties (500 stems/m²) are harvested with sickles, especially when produce is wetter. Work time remains high, between 100 to 200 man-hours per hectare for cutting and stocking.
Mechanized Harvesting of Rice
Initial machines were animal-drawn (horses in Europe, oxen in the tropics) or tractor-driven mowing machines with a cutter bar. These evolved into swathers that lay the crop in windrows for easier manual bundling.
Next came reapers that form unbound sheaves and then reaper/binders with twine-tying devices. Challenges include twine supply, cost, and quality.
These machines yield 4 to 10 hectares per hour, relatively slow, but useful in tropical rice areas with labour shortages. In temperate regions, they have been replaced by combine harvesters.
Traditional Threshing Methods for Rice
After harvesting, paddy bunches may be stacked in-field to pre-dry before threshing, which separates seeds from panicles.
Traditional threshing is generally done by hand, beating panicles against a hard surface (wooden bar, bamboo table, or stone) or using a flail. Outputs range from 10g to 30kg per man-hour, depending on rice variety and method. Grain losses are 1-2%, rising to 4% if threshing is delayed. Some grains remain unthreshed or are lost near the threshing area.
In parts of Asia and Africa, threshing is done by humans or animals treading on the crop. This method yields 30kg to 50kg per man-hour.
Vehicles (tractors or lorries) are sometimes driven over paddy bunches on a 15m–20m threshing area, producing several hundred kg per hour. This can cause grain breakage or loss in the soil.
Mechanized Threshing of Rice
Threshing mechanization predates harvesting mechanization, studied since the 18th century. Two main types of stationary threshers exist. Western-style ‘through-flow’ threshers pass stalks and ears through the machine.
These machines use pegs, teeth, or loops for threshing, and more advanced models include shakers, sieves, and centrifugal fans for cleaning. Capacities range from 500 to 2000kg/hour, used in Europe and tropical countries like Nigeria, Brazil, and India.
In the 1970s, IRRI developed the axial-flow thresher, now widely used in countries like Thailand. These are often truck-mounted and used for about 500 hours per year.
A Dutch company (Votex) created a mobile thresher with one or two threshing units, adopted in many rice-growing regions. Its simple design and 500kg/hour rate suit rural needs.
Japanese-designed ‘hold-on’ threshers use a chain conveyor to present only panicles to the threshing cylinder, excluding the straw.
Depending on crop condition, output ranges from 300kg to 700kg/hour (e.g., Iseki model). Their main drawback is fragility.
Combined Harvesting and Threshing Systems
Combine-harvesters perform both reaping and threshing. They use either the ‘through-flow’ or ‘hold-on’ threshing principle, though reaping is similar across models.
Western combines (‘through-flow’) use wide cutting bars (4–5m), while Japanese combines (‘hold-on’) have narrower widths (1m). Depending on machine type and width, capacities range from 2 to 15 hectares per day.
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Manual Harvesting of Maize
In traditional village farming systems, maize is harvested by hand and the cobs are stored in traditional structures. Farmers often allow the crop to stand in the field after maturity so the cobs lose more moisture, improving their storability.
However, this delay increases the risk of mould, insect infestation, and attacks by birds and rodents.
To reduce these risks, a method known as “el doblado” is sometimes practised in parts of South and Central America. It involves bending the ears while still on the stalks to prevent rainwater from entering and to reduce bird damage. Despite its benefits, the high labour demand is causing its gradual decline.
Manual harvesting does not require specialized tools—it involves detaching the cob directly from the stalk. On average, harvesting by hand takes about 25 to 30 days per hectare.
Traditionally, cobs are stored with husks, but removing the husks before storage improves drying. Husking is commonly done manually, often by women in groups. Some mechanical huskers like the “Tonga” unit by Bourgoin (France) offer a stationary alternative.
Mechanized Harvesting of Maize
The ‘corn snapper’, introduced in 19th-century North America, was the first machine to detach ears from maize stalks. This innovation was followed by corn pickers, which also removed husks.
Early versions were animal-drawn, later replaced by tractor-drawn and tractor-mounted units, and eventually by self-propelled machines harvesting up to four rows simultaneously.
Work rates vary significantly:
i. 3-row self-propelled harvester: 2 hours per hectare
ii. Single-row tractor-mounted unit: 5 hours per hectare
Harvest losses generally range from 3% to 5%, increasing to 10%-15% under poor conditions. While a single-row machine is suitable for up to 20 hectares, multi-row machines require larger fields to be economically viable.
For grain maize, corn-shellers replaced husking mechanisms with axial threshers. These machines can cover between 100 hectares (3-row) and 200 hectares (6-row) during a 180-hour season.
Alternatively, conventional combine harvesters can be adapted with maize headers, though they need considerable adjustments to threshing and cleaning systems.
Harvesting of Millet and Sorghum
1. Manual Harvesting
In African farming systems, millet and sorghum are vital cereals, mostly harvested manually. Farmers use knives to cut the plants, often bending taller stems for ease.
It takes about 10 to 20 days per hectare, depending on yield. The ears are stored traditionally, while the straw serves as animal feed or for other uses like roofing.
2. Gradual Mechanization of Threshing
Traditionally, women thresh millet and sorghum using a mortar and pestle, with a daily output of 10kg per woman. Threshed grain is cleaned manually using gourds or shallow baskets for winnowing.
Mechanical threshing is viable for sorghum using modified grain threshers (adjusted speed, screen sizes, etc.). However, millet, especially pearl millet, presents more challenges due to dense spikelets and ear shape, making it hard to mechanize.
Grain Cleaning
1. Traditional Cleaning
Threshing leaves behind debris like foreign seeds, chaff, empty grains, and stones. Winnowing, using natural wind, is the simplest traditional method to remove lighter trash from grain.
2. Mechanized Cleaning
Winnowers, once common in European farms, could be manual or motorized with capacities of several tonnes per hour. In modern systems, centralized seed cleaners with vibrating sieves are now widely used.
In developing regions, mechanized cleaning has not been prioritized due to low grain quality standards. However, with privatization of marketing networks, demand is expected to grow. Promoting locally made, portable cleaning machines like winnowers and screen graders is essential.
Harvesting of Tree Crops
Tree crops are harvested at the stage of full maturity for maximum flavour and shelf life. Fruits intended for long-distance transport are harvested before full ripeness to withstand storage and handling. Immediate post-harvest weight loss and quality reduction make careful harvesting and transportation vital.
2. Methods of Harvesting Citrus
Citrus fruits can be harvested using mechanical pickers that use rotating disks to detach fruits. These machines are tractor-powered and designed to move between tree rows.
The rotating disks are supported by adjustable arms that reach into the tree canopy from top to bottom, ensuring a thorough harvest.
Methods of Harvesting Coffee
Coffee cherries are harvested when ripe using any of the following methods:
i. Selective Picking: Only ripe cherries are picked by hand.
ii. Stripping: All cherries, ripe and unripe, are stripped off the branch.
iii. Mechanical Harvesting: Machines are used to shake or strip cherries from the branches.
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