Reproductive Corn Growth Stages and Scouting Tips
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R1 Stage
This stage begins when silk is visible outside the husks. Pollination occurs when these moist silks catch falling pollen grains. Pollen takes about 24 hours to move down the silk to the ovule where fertilization occurs. The ovule becomes a kernel. Generally, all silks on an ear are pollinated in 2 to 3 days. The silks grow 1.0 to 1.5 inches each day until fertilized. The R1 kernel is almost engulfed in cob materials and is white on the outside. The inner material is clear with little fluid present.
R2 Stage
Begins 10 - 14 days after silking.
R2 kernels are white on the outside and resemble a blister. The endosperm and its now-abundant inner fluid are clear. The embryo is still developing, but it now contains a developing miniature corn plant. Much of the kernel has grown out from the surrounding cob materials. The cob is close to full size. Silks are darkening and beginning to dry out. Starch has just begun to accumulate in the watery endosperm. Kernels are beginning to accumulate dry matter. Seed-fill is beginning.
Tips*
- Nitrogen and phosphorus are accumulating rapidly and relocating from vegetative to reproductive parts of the plant
- The kernels are about 85% moisture and will dry down from this point.
GDUs Required** to reach R1 growth stage depends on maturity and is approximately:
| CRM | GDUs |
|---|---|
| 80 | 1,305 |
| 90 | 1,470 |
| 100 | 1,635 |
| 110 | 1,800 |
| 120 | 1,965 |
| Scout for these Common Pests and Diseases | Timing* |
|---|---|
| Common Rust | V12 through R4.*** |
| Corn Rootworm | Larval feeding is most extensive in early through mid-July in most regions of the Corn Belt. |
| European Corn Borer | As the true stalk develops and European corn borer larvae increase in size, larvae can be found tunneling into the stalk. |
| Eyespot | V6 through R4. Disease primarily found in northern environments. |
| Goss's Wilt | VE through R6. Disease primarily found in Nebraska and portions of other western states. |
| Gray Leaf Spot | VT through R4.*** |
| Northern Leaf Blight | VT through R4.*** |
| Southern Leaf Blight | VT through R4.*** |
| Stewart's Wilt | Disease is initiated with corn flea beetle feeding from VE to V5. Leaf symptoms appear after R1. |
| Western Bean Cutworm | Initiate scouting based on local pheromone trap captures of adults. If trap data are unavailable, begin scouting at VT. |
R3 Stage
Begins 18 - 22 days after silking.
The R3 kernel is yellow outside, while the inner fluid is now milky white due to accumulating starch. The embryo is growing rapidly. Most of the R3 kernel has grown out from the surrounding cob. Silks are brown and dry or becoming dry.
Tips*
- The kernels, well into their rapid rate of dry matter accumulation, are about 80% moisture. Cell division within the endosperm is essentially complete, so growth is mostly due to cell expansion and starch-fill.
- Final yield depends on the number of kernels that develop and the final size or weight of the kernels.
- Stress can still impact yield by reducing both factors.
GDUs Required** to reach R1 growth stage depends on maturity and is approximately:
| CRM | GDUs |
|---|---|
| 80 | 1,405 |
| 90 | 1,580 |
| 100 | 1,760 |
| 110 | 1,940 |
| 120 | 2,120 |
| Scout for these Common Pests and Diseases | Timing* |
|---|---|
| Anthracnose Top Dieback | R3 through R5. |
| Common Rust | V12 through R4.*** |
| Corn Rootworm | Larval feeding is most extensive in early through mid-July in most regions of the Corn Belt. |
| European Corn Borer | As the true stalk develops and European corn borer larvae increase in size, larvae can be found tunneling into the stalk. |
| Eyespot | V6 through R4. Disease primarily found in northern environments. |
| Goss's Wilt | VE through R6. Disease primarily found in Nebraska and portions of other western states. |
| Gray Leaf Spot | VT through R4.*** |
| Northern Leaf Blight | VT through R4.*** |
| Southern Leaf Blight | VT through R4.*** |
| Stewart's Wilt | Disease is initiated with corn flea beetle feeding from VE to V5. Leaf symptoms appear after R1. |
| Western Bean Cutworm | Initiate scouting based on local pheromone trap captures of adults. If trap data are unavailable, begin scouting at VT. |
R4 Stage
Begins 24 - 28 days after silking.
Continued starch accumulation in the endosperm causes the milky inner fluid to thicken to a pasty consistency. Usually 4 embryonic leaves have formed as the embryo has grown dramatically from the R3 stage. The shelled cob is a light red to pink. Toward the middle of R4, the embryo will stretch across more than half of the width of the kernel side. Just before R5, kernels along the length of the ear begin to dent or dry. The fifth (last) embryonic leaf and the lateral seminal roots have formed. If this seed is planted, these 5 embryonic leaves will appear the following season after germination & VE.
Tips*
- The embryo continues to develop very rapidly.
- Kernels are about 70% moisture and have accumulated about half their mature dryweight.
GDUs Required** to reach R1 growth stage depends on maturity and is approximately:
| CRM | GDUs |
|---|---|
| 80 | 1,570 |
| 90 | 1,775 |
| 100 | 1,980 |
| 110 | 2,180 |
| 120 | 2,385 |
| Scout for these Common Pests and Diseases | Timing* |
|---|---|
| Anthracnose Top Dieback | R3 through R5. |
| Common Rust | V12 through R4.*** |
| Corn Rootworm | Larval feeding is most extensive in early through mid-July in most regions of the Corn Belt. |
| European Corn Borer | As the true stalk develops and European corn borer larvae increase in size, larvae can be found tunneling into the stalk. |
| Eyespot | V6 through R4. Disease primarily found in northern environments. |
| Goss's Wilt | VE through R6. Disease primarily found in Nebraska and portions of other western states. |
| Gray Leaf Spot | VT through R4.*** |
| Northern Leaf Blight | VT through R4.*** |
| Southern Leaf Blight | VT through R4.*** |
| Stewart's Wilt | Disease is initiated with corn flea beetle feeding from VE to V5. Leaf symptoms appear after R1. |
| Western Bean Cutworm | Initiate scouting based on local pheromone trap captures of adults. If trap data are unavailable, begin scouting at VT. |
R5 Stage
Begins 35 - 42 days after silking.
At R5, all or nearly all kernels are dented or denting. The shelled cob is dark red. The kernels are drying down from the top, where a small hard layer of starch is forming. This starch layer appears shortly after denting as a line across the back of the kernel (the non-embryo side). With maturity, the hard starch layer and line will advance toward the cob. Accumulated starch is hard above the line but still soft below the line.
Tips*
- Stress at this stage will reduce yields by reducing kernel weight.
- At the beginning of R5, kernels have about 55% moisture content.
GDUs Required** to reach R1 growth stage depends on maturity and is approximately:
| CRM | GDUs |
|---|---|
| 80 | 1,735 |
| 90 | 1,960 |
| 100 | 2,185 |
| 110 | 2,410 |
| 120 | 2,640 |
| Scout for these Common Pests and Diseases | Timing* |
|---|---|
| Anthracnose Top Dieback | R3 through R5. |
| Anthracnose Stalk Rot | R5 through R6. |
| Diplodia Ear Rot | R5 through R6. |
| Diplodia Stalk Rot | R5 through R6. |
| European Corn Borer | Second-generation egg masses are laid in late-July through August. |
| Fusarium Ear Rot | R5 through R6. |
| Fusarium Stalk Rot | R5 through R6. |
| Gibberella Ear Rot | R5 through R6. |
| Gibberella Stalk Rot | R5 through R6. |
| Stewart's Wilt | Disease is initiated with corn flea beetle feeding from VE to V5. Leaf symptoms appear after R1. |
| Western Bean Cutworm | Scout ear tips for feeding. |
R6 Stage
Begins 55 - 65 days after silking.
By the R6 stage, kernels have attained their maximum dry weight or dry matter accumulation. The hard starch layer has advanced completely to the cob. A black or brown abscission layer forms, moving progressively from the tip ear kernels to the basal kernels of the ear. It's a good indication of physiological maturity and signals the end of kernel growth. The husks and many leaves are no longer green, although the stalk may be.
Tips*
- A hard early frost before the R6 stage may halt dry matter accumulation and cause premature black layer formation. This could reduce yields by causing delays in harvest (frost-damaged corn is slower to dry.)
- To reduce potential frost problems, choose a hybrid that matures about 3 weeks before the average date of the first killing frost.
- Kernel moisture averages 30 to 35%, but this can vary considerably between hybrids and environmental conditions. Safe storage requires 13 to 15% moisture.
GDUs Required** to reach R1 growth stage depends on maturity and is approximately:
| CRM | GDUs |
|---|---|
| 80 | 1,895 |
| 90 | 2,145 |
| 100 | 2,390 |
| 110 | 2,640 |
| 120 | 2,890 |
| Scout for these Common Pests and Diseases | Timing* |
|---|---|
| Anthracnose Stalk Rot | R5 through R6. |
| Diplodia Ear Rot | R5 through R6. |
| Diplodia Stalk Rot | R5 through R6. |
| Fusarium Ear Rot | R5 through R6. |
| Fusarium Stalk Rot | R5 through R6. |
| Gibberella Ear Rot | R5 through R6. |
| Gibberella Stalk Rot | R5 through R6. |
| Stewart's Wilt | Disease is initiated with corn flea beetle feeding from VE to V5. Leaf symptoms appear after R1. |
* Information from Pioneer and Iowa State University Extension.
** Hybrid growth is influenced by the growing environment. Individual results may vary.
*** Scout for these diseases earlier in seed production fields.
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