Once the rice flowers pollinate, the grain filling and maturation phase beings.
This is when the grain of rice grows to its full size.
During this period, the grain increases in size and weight as the sugars are translocated from the leaf blades where the sugars are made and from the culms where they have accumulated, the grain changes color from green to gold or straw color at maturity and the leaves of the rice plant begin to senesce.[i]
Simply put, sugars made and stored in the leaves and stems during the growing season begin to move to the grain. The leaves and stems turn brown as the sugars move out of them, while the grain fills out in size and weight and changes color as it matures.
University of Arkansas Rice Research and Extension Center Professor Paul A. Counce explains the above photo as rice kernel development.
“What you are looking at is a group of developing rice kernels with the hull slightly separated. You can see the kernel easily,” he said. “This is the eventual brown rice kernel, which will be dehulled and milled to produce white rice. This stage of development has all the parts, but the inner part of the rice kernel is still developing and will continue to do so for five to 10 days before the green hull begins to turn yellow and then brown, and the kernel will begin to dry down.”
There are four stages in the ripening process: Milk stage, soft dough stage, hard dough stage and maturity.
The milk stage is the first step in the grain-ripening process. It’s characterized by developing starch grains in the kernel that are soft and the interior of the kernel is filled with white liquid resembling milk.
The soft dough stage is when the initial starch formation begins. The inside of the rice kernel is firm, yet still soft.
The hard dough stage is reached when the rice kernel is firm, but contains more than 20 percent moisture content. The photo below illustrates the hard dough stage, though the rice is not yet ready for harvest.
The rice is fully mature and ready for harvest when the kernel is very firm with a moisture content 20 percent or below.
Levee gates have maintained a steady flood on the rice plants for about two months. As the kernels enter their final stage of maturing, farmers begin preparing for harvest by removing the levee gates from the field to drain the water.
It usually takes two weeks from the time the rice fields are drained for the farmer to being harvest. As the fields dry, the rice kernels continue to harden until they reach full maturity. Counce said rice is drained and harvested during the R8 growth stage, which begins when at least one grain on the main stem of the panicle has a brown hull and ends when the final grain, which reached grain-filling, has turned brown.
“The rice crop develops with the top branches and outer part of the branches first and the lower branches and inner grains on the branches maturing later. In very few cases is rice drained or recommended for draining without some of the lower branches and inner grains being immature,” he said. “This is because, while these lower branches and inner branches of the panicles are maturing, the first maturing and highest quality grains are subject to rewetting, drying and fissuring. Thus, time of draining is always a balance between the need to dry the soil for harvest and allowing all good quality grain to mature.”
[i] Karen Moldenhauer, Charles E. Wilson, Jr., Paul Counce and Jarrod Hardke, “Rice Growth and Development,” Arkansas Rice Production Handbook (2012): 19.