Timeliness of the harvest is a very important factor in determining grain quality and yield. The optimal stage to harvest grain is with 20-25% grain moisture content or when 80-85% of the grains are straw colored and the grains in the lower part of the panicle are in the hard dough stage. This occurs about 30 days after flowering.
A farmer carrying his harvest in Ninh Binh province, Vietnam.
The following table describes the consequences of harvesting too early, too late, or allowing variations in moisture content during harvesting.
| If | then |
| harvesting is too early, | there will be many immature grains that will reduce head rice yield and quality. Further, these immature rice kernels are very slender and chalky and will result in excessive amounts of bran and broken grains. |
| harvesting is too late, | many grains are lost due to shattering or drying-out and are cracked during threshing. Cracked grains break during milling. |
| moisture content is allowed to vary, | grain fissuring (cracking) can occur, along with spoilage through yellowing and the development of odors. |
Threshing should occur immediately after cutting. The longer cut panicles
remain in a stack, the higher their chance of turning yellow and discoloring.
Farmer threshing rice panicles in the field.
Threshing machine settings also affect the yield and quality of the harvest. The following table describes appropriate settings for most threshing machines.
| Settings | Value |
| Drum speed | Drum tip speeds for a peg tooth thresher should be 12-16m/sec, or approximately 600rpm. Higher speeds result in higher levels of grain damage while lower speeds increase the amount of grain retained in the panicle. |
| Fan speed | 800-850rpm |
| Sieve oscillation speed | 800-850rpm |
| Clearance between peg teeth and concave | ~25mm |