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
This 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 In some regions, animals have been replaced by tractors.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.
The 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.
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:
- 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.
- The highest milling yield will be attained for hand threshed, sun dried rice at grain moisture content between 18-20%.
- Care must be taken not to overdry the crop if it is to be transported any distance before threshing as excessive shattering will occur.
- 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.