Harvesting and Threshing
When harvesting is not done on time, grains may be lost due to damage by rats, birds, insects, shattering, and lodging of the crop in the field. Timely harvesting ensures optimum grain quality, higher maret price, and consumer acceptance. It also keeps seed dormancy within safe limits. With weakly dormant varieties or those without dormancy, the longer the grains are left in the field, the more likely it will germinate on the panicles, especially in the rainy season. It is important to understand when to harvest a crop of rice and to perform threshing, proper drying, milling and storage of grains because these postharvest activities will influence directly the market quality of rice (Gorrez 1983).
Determining when to harvest
Most improved rice varieties vary in their growing periodfrom 5 to 10 days. This variation may be influenced by daylength, temperature, season, available sunlight, and cultural management practices. However, the average growing period of a specific variety is always fixed and should serve as a guide in deciding when to harvest.
As harvest time approaches, the following should be observed:
1. The optimum time to harvest is between 30 and 35 days after heading, with moisture contents ranging from 25% to 27%, more or less.
2. Eighty percent of the grains in the panicles should be straw-colored, hard, and the grains in the lower portion are in the hard dough stage. Grains when ready for harvest are clear in appearance and firm. As stated earlier, water in the field should be gradually drained 7–15 days before harvest time or when the grains are in the hard dough stage. This will hasten maturity.
Hauling and stacking
In Asia, majority of the operations from harvesting to threshing are done manually. Losses can be high if handling of grains is not properly supervised. It is good practice to haul and stack the cut bundles to the site where threshing will take place to avoid too much handling. If threshing will be delayed, the harvested rice should be bundled and stacked in a dry place. Bundling will facilitate circulation of the air around and through the stack, thus preventing excessive heating but allowing continued drying in the stack.
Arrange panicle ends such that they are positioned in one direction. If circular piling or stacking is preferred, place the panicle ends toward the center. If cubicle stacking is made, pile only two bundles wide with panicles end to end or position them opposite each other. In both cases, the grains are better protected from rats, birds, and other farm animals. Put up a shade to protect the grains from rain.
Threshing
In separating grains from the panicles, the most common practices among farmers are hand beating, treading, animal trampling, or the use of mechanical tools such as a rotating drum with spikes or rasp bars.
Potential yield may be reduced through careless handling of the grains. Two methods of manual threshing to minimize losses are described.
Slotted bamboo platform
This is a square or rectangular shaped slotted floor platform made of bamboo and similar cheap materials raised about 2 feet above the ground. Three side walls are attached to the platform, which are constructed to catch flying grains when threshing takes place on the slotted floor. The bundle of panicles is held at one end opposite the panicles and in a swinging downward motion is struck hard against the slotted floor. The bundle is shaken after each beating to ensure that grains fall on the mat under the slotted
floor. Threshing is completed when all rice grains are separated from the panicle. With the new HYVs, it takes from three to five beatings to complete threshing per bundle.
Pedal thresher
This manually operated thresher is made up of a circular drum with spikes or peg tooth anchored on the drum surface. One person pedals the thresher with the drum rotating away from the operator, the bundle of panicles held at one end opposite the panicle is placed in contact with the rotating drum until it is clear of grains. The operation is conveniently carried out by two or more persons threshing alternately so that the drum is rotating continuously. There are portable and motor-operated axial flow threshers (Johnson et al 1970) with outputs of 300–400 kg/hour (or even higher) with a grain loss of only 2%.
Interrelated postharvest operations
In tropical Asia, mechanization of postharvest operations is limited. However, several new machines for harvesting, threshing, cleaning, and drying have been designed by IRRI and other national centers, offering opportunities to partially mechanize postharvest operations. The interdependence of all these activities means that each should be treated with an awareness of its effect on the other operations. For example, if harvesting is carried out at the wrong time or using the wrong method, it may be impossible to dry, mill, or store the resulting crop properly. This can result in poor quality milled rice (De Datta 1981, Gorrez 1989).