Grain filling follows anthesis and refers to the period during which the kernel matures or ripens. Within a few hours of pollination, the embryo (rudimentary, undeveloped plant in a seed) and endosperm (area of starch and protein storage in the seed) begin to form and photosynthates (products of photosynthesis) are transported to the developing grain from leaves (primarily the flag leaf). In addition, starches, proteins, and other compounds previously produced and stored in leaves, stems, and roots are also transferred to the developing grain. The grain filling period is critical for producing high yields because kernel size and weight are determined during this stage. Yields will be reduced by any stress (high temperatures, low soil moisture, nutrient deficiencies, and diseases) occurring during grain fill.
Environmental factors affect the rate and duration of the grain filling period. The longer this filling period lasts, the greater is the probability for higher yields. If this period is shortened, yields will usually be lower. In Kentucky, the average length of the grain filling period is one month. The grain fill period can be as few as 25 days or less in high stress environments (hot and dry weather, heavy disease, and nutrient deficiencies) and may exceed 35 days in high yield, low stress environments (disease-free, high soil moisture, and moderate/cooler temperatures). The grain development stages are listed in Table 2-1 (Feekes 10.54 to 11.4; Zadoks 70 to 92). A brief description and comments of the grain filling and ripening stages follows below.
Watery ripe stage: Kernel length and width are established during this stage. The kernel rapidly increases in size but does not accumulate much dry matter. A clear fluid can be squeezed from the developing kernel.
Milk stage: During this stage there is a noticeable increase in solids of the liquid endosperm as nutrients in the plant are redistributed to the developing kernels. During the milk stage a white, milk-like fluid can be squeezed from the kernel when crushed between fingers. By the end of the milk stage, the embryo is fully formed.
Soft dough stage: The kernels are soft but dry. The water concentration of the kernel has decreased so that the material squeezed out of the kernel is no longer a liquid but has the consistency of meal or dough. The kernel rapidly accumulates starch and nutrients and by the end of this stage the green color begins to fade. Most of the kernel dry weight is accumulated in this stage.
Hard dough stage: The kernel has become firm and hard and is difficult to crush between fingers. It can be dented with a thumbnail. Kernel moisture content decreases from a level of 40 percent to 30 percent. At the end of the hard dough stage (Feekes 11.3; Zadoks 87-91), the kernel reaches its maximum dry weight and the wheat is said to be physiologically mature (no more weight is added to the grain). Physiological maturity often corresponds to kernel moisture content between 30 and 40 percent. Previous wheat swathing research at the University of Kentucky at various kernel moisture contents indicated physiological maturity occurred at a kernel moisture content of 38 to 42 percent (with no reduction in yield or test weight if cut at this stage). Harvesting can occur anytime after physiological maturity but often does not occur because of high kernel moisture.
Ripening stage: Kernel moisture content is still high, usually ranging from 25 to 35 percent, when wheat begins to ripen but decreases rapidly with good weather. The plant turns to a straw color and the kernel becomes very hard. The kernel becomes difficult to divide with a thumbnail, cannot be crushed between fingernails, and can no longer be dented by a thumbnail. Harvest can begin when the grain has reached a suitable moisture level (usually less than 20%).
Often harvest does not occur until grain moisture content is close to 15 percent, unless drying facilities are available. It is important for grain quality that the harvest begins as soon as possible. Test weight (and hence grain yield) may be reduced during the ripening process. Decreased test weight results from the alternate wetting (rains or heavy dews) and drying of the grain after the wheat has physiologically matured.
Herbek, James and Chad Lee. 2009. A Comprehensive Guide to Wheat Management in Kentucky. Section 2. Growth and Development. University of Kentucky Cooperative Extension. Online. http://www.uky.edu/Ag/GrainCrops/ID125Section2.html#StemElongation