Harvesting operations

harvesting-combine-harvester-003.jpgHarvesting rice consists of the basic operations cutting, threshing, and cleaning. These activities can be done in individual steps or in combination using a combine harvester.

There are also traditional harvesting system activities such as field drying and cut crop field stacking. However, traditional operations are not recommended due to increased incidence of rapid quality deterioration and increased losses.

Cutting

Cutting is the first operation in harvesting. Depending on the crop's condition, and availability of labor or machinery, cutting can be done either manually or mechanically.

harvesting cuttingManual system is the most common method of rice harvesting in Asia. This method includes cutting the rice crop by simple hand tools like the sickle (best for cutting 15−25 cm above ground level), and simple hand-held knives (best for cutting just below the panicle, e.g. ani ani in Indonesia).

When cutting crop with a sickle, always hold the stems with thumb pointing upwards, away from the sickle.

Advantages: Very effective in lodged crop conditions
Disadvantages: Labor intensive; Manual harvesting requires 40−80 person hours per ha. It will take an additional labor to manually collect and haul the harvested crop.

Open in new window: Manual cutting

harvesting reaperMechanical cutting uses reapers (machine that cuts and gathers), though this is not widely used in Asia. Reapers are hand-driven or mounted on the front of a tractor. Most reapers lay the crop in a windrow, which allows easy pick-up of the harvested crop. A reaper with a cutting-width of 1.5 m can operate at a rate of 2 to 4 ha per day. A leveled field is important when reaping as it is difficult to reap a lodged crop that is lying on the ground.

Advantages: Low labor requirements
Disadvantages: Difficult to reap lodged crop; Availability of machine may be an issue

Open in new window: Mechanical cutting

Threshing

Threshing is the process of separating the grain from the straw. It can be either done by hand, by using a treadle thresher or mechanized.

The common method for threshing by hand is separating the grain from the panicle by impact. This can be done by hand beating, treading, or by holding the crop against a rotating drum with spikes or rasp bars. Hand beating methods are normally used for threshing of rice that easily shatters.

Methods of manual threshing

Trampling

harvesting-animal tramping1This involves the use of bare feet or animals to thresh the crop. The crop is spread over a mat or canvass and workers trample with their own feet or use their animals. Animal treading or trampling is normally carried out at a designated location near the field or in the village. In some regions, animals have been replaced by tractors. After animal treading, the straw is separated from the grains and cleaning of the grain is done by winnowing, with or without the aid of an electric fan. Losses are high from broken and damaged grains.

Threshing rack

harvesting-threshing-rackThe farmer holds the crop by the sheaves and beats it against a slatted bamboo, wooden platform, or any other hard object such as a steel oil drum. This is very labor intensive.

Flail

The use of a flail or stick for thrashing the crop is not popular for rice.

Pedal thresher (recommended as best practice for manual threshing)

The pedal thresher or treadle thresher consists of threshing drum, base, transmission unit and a foot crank. When pedaled, the threshing drum rotates and rice can be threshed when panicles are applied against the threshing drum. Because small straws, chaff, and foreign matter drop along with the threshed grain, whole grains must be separated using a flail, sieve or by winnowing (see section on cleaning).

Guidelines for good manual threshing methods:

  1. For hand threshed crops, partial drying in the field for a couple of days may be necessary to lower the moisture content and make threshing easier.
  2. The highest milling yield will be attained for hand threshed, sun dried rice at grain moisture content between 18-20%.
  3. Care must be taken not to overdry the crop if it is to be transported any distance before threshing as excessive shattering will occur.
  4. While the crop may be dried before threshing, dried crop should not be subjected to rewetting. Rewetting causes grain fissures which lead to high broken grain when milled.

Open in new window: Manual threshing

The use of small stationary machine threshers commonly replaces manual threshing given the high labor requirements of manual threshing. Stationary threshing is generally done in the field, or near the field.

Many stationary threshers for paddy have peg-toothed threshing drums, however threshers fitted with wire-loop or rasp-bars are used as well. Most threshers are of the feed-in type (e.g. entire crop is fed through the thresher) which ensure high throughput. Hold-on threshers (only panicle is fed into the machine) generally have a lower capacity than feed-in threshers and are primarily used in areas where rice straw is bundled and stored for later use. Large stationary threshers are fitted with additional cleaning devices such as an oscillating screen, centrifugal blower, and windboard, and threshed grain can be handled without further cleaning.

In many regions, machine threshers are owned by individuals who offer custom operations to farmers. This requires that farmers schedule harvesting dates depending on the availability of the thresher. Many threshers are mounted on trailers or trucks which enables the operator to quickly move from field to field.

Principle of feed-in type axial-flow thresher

harvesting-axial-flowHarvested crop is loaded onto the tray and fed into the opening between the cylinder and the concave at one end of the machine. The pegs on the threshing cylinder hit the material separating the grain from the straw, and at the same time accelerating them around the cylinder. The majority of the grain is threshed during initial impact but further threshing is performed as the material moves axially until the straw is discharged at the opposite end. Threshed grain, including impurities such as leaves and short pieces of straw, pass through the openings in the concave and fall on the oscillating screen where large impurities are separated.

Machine threshing health and safety:

  1. Become familiar with the machine before the first operation. Read the manual that came with the machine
  2. Never leave the thresher unattended while the engine is running. Keep children away from the machine. Keep hands and feet away from moving parts
  3. Do not fill the tank while the engine is running
  4. Keep all flammable materials (including dry straw) away from the engine
  5. Do not oil, grease, or adjust the machine during operation. Wait until all moving parts have stopped before servicing
  6. Do not wear loose fitting clothing that may be picked up by moving parts
  7. While operating, keep all shields and guards in place
  8. Never extend hands or feet into the feed opening of the thresher
  9. Never operate your machine in a closed shed or garage. Exhaust fumes are dangerous to your health
  10. Do not operate machine with loose pegteeth, bolts and nuts. Loose pegteeth may be ejected at high speeds, causing injury to operators and damage to the thresher
  11. Never remove accumulated straw inside the machine during operation
  12. Keep a first-aid kit at hand
  13. Tie up long hair to prevent entangling
  14. Do not wear neckties or other garments that may be wrapped into moving parts of the machine

Open in new window: Machine threshing

Cleaning

Grain cleaning after harvest is important as it removes unwanted materials from the grain. Clean grain has a higher value than grain that is contaminated with straw, chaff, weed seeds, soil, rubbish, and other non-grain materials.

Grain cleaning will improve the drying, the storability of grain, reduce dockage at time of milling, and improve milling output and quality. Seed cleaning will reduce damage by disease, and improve yields.

Guidelines for cleaning grain and seed

  • Winnowing
    Lighter materials such as unfilled grains, chaff, weed seeds, and straw can be removed from the grain by using a blower, air fan, or by wind. Winnowing recovers only the heavier grains but other heavy particles like heavier weed seeds, off types, stones and dirt might still be included in the rice.
  • Screening/Sifting
    Smaller materials such as weed seeds, soil particles and stones can be removed by sieving the grain through a smaller sized screen (1.4 mm or less sieve opening).

Recommended winnowing practices:

  1. Place grain on a winnowing tray
  2. Place a net or mat on the ground
  3. Tilt the tray against the wind
  4. Pour grain slows at a height of about 1 m
  5. Wind will separate light from heavy grains
  6. Recover only the heavier grains
  7. Repeat the procedure if needed
  8. Use a fan or blower if there is not sufficient wind

Open in new window: Grain cleaning

  • Winnowing
    Lighter materials such as unfilled grains, chaff, weed seeds, and straw can be removed from the grain by using a blower, air fan, or by wind. Winnowing recovers only the heavier grains but other heavy particles like heavier weed seeds, off types, stones and dirt might still be included in the rice.
  • Screening/Sifting
    Smaller materials such as weed seeds, soil particles and stones can be removed by sieving the grain through a smaller sized screen (1.4 mm or less sieve opening).

Recommended winnowing practices:

  1. Place grain on a winnowing tray
  2. Place a net or mat on the ground
  3. Tilt the tray against the wind
  4. Pour grain slows at a height of about 1 m
  5. Wind will separate light from heavy grains
  6. Recover only the heavier grains
  7. Repeat the procedure if needed
  8. Use a fan or blower if there is not sufficient wind

Open in new window: Seed cleaning

Read: How to ensure seed quality

Combine harvesting

In India, China, Thailand, Vietnam and even in Cambodia the use of combine harvesters, or simply combines, for paddy rice is increasing rapidly. This is a development due to severe labor shortage and the resulting increase in harvesting cost, which makes the use of combines economically attractive.

Combines “combine” several operations into one: cutting the crop, feeding it into threshing mechanism, threshing, cleaning, and discharge of grain into a bulk wagon or directly into a bags. Straw is usually discharged behind the combine in a windrow.

Guidelines for good combine harvesting methods

  • Set the cutting height higher than with other harvesting methods. (How many centimeters from the panicle or the ground is recommended for setting the cutting height?)
  • Harvest at the correct time (20−25% moisture content or when 80% of the grains are straw colored and at least 20% of the grains at the panicle base have reached hard dough stage). The time interval for harvesting by combine harvester is often narrow: too early harvesting will result in a high percentage of chalky kernels, and too late harvesting will result in high shattering losses.
  • Set the threshing speed according to the machine directions based on the grain moisture content, volume of material entering into the combine, weeds, etc. Operating thresher speed either too fast or too slow reduces profit.
  • Fine tuning forward speed and header height is especially important to minimize field loss.

Combine threshing health and safety

  • Never attempt to lubricate, clean, adjust, or unplug harvesters when the machine or engine is running
  • Review the manuals that came with the harvester. Understand the controls before operating.
  • Do not allow anyone to climb onto the machine while it is in motion
  • Keep children at a safe distance from the machine
  • Do not operate when you are very tired. Change operators during the day
  • Always have a fire-extinguisher at hand on engine-operated equipment
  • Ensure that the fuel system has no leaks
  • While refueling, stop the engine and do not smoke
  • When operating around machinery, wear work clothes that fit snugly.

Read: Examples of combines common in Asia

 

Harvest is usually a busy time for farmers and contract service providers because most crop matures around the same time and, in addition, the fields have to be prepared for the next crop after harvesting. The potential for accidents is therefore high.

Operators of farm machinery should follow health safety precautions during harvesting.

Read: Health and safety precautions during harvesting