Published 2001 (Rev Sep 2007)
URL: http://www.kingcorn.org/news/timeless/GrainFill.html

Grain Fill Stages in Corn

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
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he grain fill period begins with successful pollination and initiation of kernel development, and ends approximately 60 days later when the kernels are physiologically mature. During grain fill, the developing kernels will be the primary sink for concurrent photosynthate produced by the corn plant. What this means is that the photosynthate demands of the developing kernels will take precedence over that of much of the rest of the plant. In essence, the plant will do all it can to "pump" dry matter into the kernels, sometimes at the expense of the health and maintenance of other plant parts including the roots and lower stalk.

A stress-free grain fill period can maximize the yield potential of a crop, while severe stress during grain fill can cause kernel abortion or lightweight grain and encourage the development of stalk rot. The health of the upper leaf canopy is particularly important for achieving maximum grain filling capacity.

Kernel development proceeds through several relatively distinct stages that were originally described by Ritchie et al. (1993). This article offers a pictorial tour of the grain filling period of a 109 CRM hybrid planted April 18, 2007 at the Purdue Agronomy Farm in west central Indiana.

Silking Stage (Growth Stage R1)

Some may argue whether silking should be labeled as a kernel growth stage, but nonetheless silk emergence is technically the first identifiable stage of the reproductive period. Silks remain receptive to pollen grain germination up to 10 days after silk emergence (Nielsen, 2007b). Silk receptivity decreases rapidly after 10 days if pollination has not yet occurred. Natural senescence of silk tissue over time results in collapsed tissue that restricts continued growth of the pollen tube. Silk emergence usually occurs in close synchrony with pollen shed (Nielsen, 2007c), so that duration of silk receptivity is normally not a concern. Failure of silks to emerge in the first place (for example, in response to silkballing or severe drought stress) does not bode well for successful pollination.

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Appearance of ear shoot at growth stage R1.	Appearance of silks at growth stage R1.	Appearance of cob and silks at growth stage R1.
Appearance of cob with silks removed at growth stage R1.	Closer view of cob and ovules with silks removed at growth stage R1.	Cross-section of cob at growth stage R1.

Kernel Blister Stage (Growth Stage R2)

About 10 to 14 days after silking, the developing kernels are whitish "blisters" on the cob and contain abundant clear fluid. The ear silks are mostly brown and drying rapidly. Some starch is beginning to accumulate in the endosperm. The radicle root, coleoptile, and first embryonic leaf have formed in the embryo by the blister stage. Severe stress can easily abort kernels at pre-blister and blister stages. Kernel moisture content is approximately 85 percent.

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Silks beginning to dry and turn brown at blister stage (growth stage R2); about 9 days after mid-silk.	Closer view of drying silks at growth stage R2.	Ear with husks removed showing kernels and spent silks at growth stage R2.
Closer view of cob and kernel "blisters" at growth stage R2.	Clear sugary fluid from developing kernel cut with knife at growth stage R2.	Depth of kernels in cross-section of cob at growth stage R2.

Kernel Milk Stage (R3)

About 18 to 22 days after silking, the kernels are mostly yellow and contain "milky" white fluid. The milk stage of development is the infamous "roasting ear" stage, that stage where you will find die-hard corn aficionados standing out in their field nibbling on these delectable morsels. Starch continues to accumulate in the endosperm. Endosperm cell division is nearly complete and continued growth is mostly due to cell expansion and starch accumulation. Severe stress can still abort kernels, although not as easily as at the blister stage. Kernel moisture content is approximately 80 percent.

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Silks dry and brown at milk stage (growth stage R3), husks still green; about 16 days after mid-silk.	Ear with husks removed showing kernels and spent silks at growth stage R3.	Kernel appearance at growth stage R3. Note the typical range in kernel development from butt (oldest) to tip (youngest) of ear.
Closer view of plump yellow kernels at growth stage R3.	Milky sugary fluid from developing kernel cut with knife at growth stage R3.	Depth of kernels in cross-section of cob at growth stage R3.

Kernel Dough Stage (R4)

About 24 to 28 days after silking, the kernel's milky inner fluid is changing to a "doughy" consistency as starch accumulation continues in the endosperm. The shelled cob is now light red or pink. By dough stage, four embryonic leaves have formed and the kernels have reached about 50 percent of their mature dry weight. Kernel moisture content is approximately 70 percent by R4. Kernel abortion is much less likely to occur once kernels have reached early dough stage, but severe stress can continue to affect eventual yield by reducing kernel weight.

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Silks dry and brown at dough stage (growth stage R4), husks still green; about 25 days after mid-silk.	Ear with husks removed showing kernels and spent silks at growth stage R4.	Kernel appearance at growth stage R4. Approximately 44 kernels per row and about 6 non-pollinated ovules at tip.
Closer view of kernels at growth stage R4.	Depth of kernels in cross-section of cob at growth stage R4. Note the pinkish color developing in the cob tissues.	Cross-section of R4 kernel reflecting the conversion of sugary fluids present at R3 in the endosperm to the solid (doughy) starch present at R4.

Kernel Dent Stage (R5)

About 35 to 42 days after silking, all or nearly all of the kernels are denting near their crowns. The fifth (and last) embryonic leaf and lateral seminal roots form just prior to the dent stage. A distinct horizontal line appears near the dent end of the kernel and slowly progresses to the tip end of the kernel over the next 3 weeks or so. This line is called the "milk line" and marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernels. Severe stress can continue to limit kernel dry weight accumulation. Kernel moisture content at the beginning of the dent stage is approximately 55 percent.

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Husk appearance at early R5 (dent); about 29 days after mid-silk.	Kernel appearance at early R5. Approximately 36 kernels per row and about 7 non-pollinated ovules at tip.	Closer view of kernels at early R5 that are beginning to dent.
Depth of kernels in cross-section of cob at early R5. Kernel milkline not yet visible. Pinkish cob tissue clearly visible.	Cross-section of early R5 kernel.

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Husk leaves beginning to dry at mid R5 (dent); about 38 days after mid-silk.	Kernel appearance at mid R5. Approximately 43 kernels per row and about 4 non-pollinated ovules at tip.	Closer view of kernels at mid R5 that are fully dented.
Depth of kernels in cross-section of cob at mid R5. Kernel milkline visible and 1/4 to 1/3 down the face of the kernel.	Cross-section of mid R5 kernel showing the early stages of discoloration of the placental cell layers.

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Husk leaves nearly fully dry at late R5 (dent); about 56 days after mid-silk.	Kernel appearance at late R5. Approximately 42 kernels per row and about 2 non-pollinated ovules at tip.	Closer view of kernels at late R5 that are fully dented.
Depth of kernels in cross-section of cob at late R5. Kernel milkline has nearly disappeared into cob glume tissue.	Cross-section of late R5 kernel showing the discolored placental cell layers that have noticeably thinned.

Physiological Maturity (R6)

About 55 to 65 days after silking, kernel dry weight usually reaches its maximum and kernels are said to be physiologically mature and safe from frost. Physiological maturity occurs shortly after the kernel milk line disappears and just before the kernel black layer forms at the tip of the kernels. Severe stress after physiological maturity has little effect on grain yield, unless the integrity of the stalk or ear is compromised (e.g., damage from European corn borer or stalk rots). Kernel moisture content at physiological maturity averages 30 percent, but can vary from 25 to 40 percent grain moisture.

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Ear and kernel appearance at growth stage R6 (physiological maturity); about 63 days after mid-silk.	Closer view of R6 kernels.	Depth of kernels in cross-section of cob at growth stage R6. Kernel milkline has disappeared into cob glume tissue.
Cross-section of kernel at growth stage R6 depicting the kernel black layer (dead & collapsed placental cells near the tip of the kernel).	Kernel w/ tip removed to show the kernel black layer at growth stage R6; dead & collapsed placental cells near the tip of the kernel.

Harvest Maturity

While not strictly a stage of grain development, harvest maturity is often defined as that grain moisture content where harvest can occur with minimal kernel damage and mechanical harvest loss. Harvest maturity is usually considered to be near 25 percent grain moisture.

Related References

Nielsen, R.L. (Bob). 2004. Yield Loss Potential During Grain Fill. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/articles.04/GrainFillStress-0705.html. (URL verified 7/4/07).

Nielsen, R.L. (Bob). 2007a. Kernel Set Scuttlebutt. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/KernelSet.html. (URL verified 9/3/07).

Nielsen, R.L. (Bob). 2007b. Silk Emergence. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/Silks.html (URL verified 7/2/07).

Nielsen, R.L. (Bob). 2007c. Tassel Emergence & Pollen Shed. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/Tassels.html (URL verified 7/2/07).

Ritchie, S.W., J.J. Hanway, and G.O. Benson. 1993. How a Corn Plant Develops. Iowa State Univ. Sp. Rpt. No. 48. [On-Line]. Available at http://www.extension.iastate.edu/pages/hancock/agriculture/corn/corn_develop/CornPlantStages.html. (URL verified 7/2/07).