Rice harvesting and threshing can simply be described as those methods used in harvesting and threshing rice. Some of the methods used in harvesting and threshing rice include the following;

1. Manual Harvesting

In many countries rice ears are cut by hand. A special knife is frequently used in South East 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 stocking.

2. Mechanized Harvesting

During past decades the mechanisation of rice harvesting has rapidly evolved. It was 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.

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.

Rice 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.

1. Traditional Threshing Method

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 man hour.

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%.

2. Mechanized Threshing

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.

The capacities of the models from European manufacturers (e.g., 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. 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, 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.

3. 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 especially 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’ organisations 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.