Corn Growth and Management Quick Guide (continued)
A-1173, June 1999
Reproductive Stages and Kernel Development
1. R1 Silking
The plant is about 55-66 days after emergence. This stage begins when silks are visible and pollination occurs. Pollination is when pollen grains contact the new, moist silks. A pollen grain grows down the silk and fertilizes the ovule in about 24 hours. Upon this fertilization, the ovule is a kernel. Silks grow about 1 to 1.5 inches per day. Normally, it takes two to three days for all silks on a single ear to emerge and be pollinated. Moisture stress or nutrient deficiency will result in poor pollination and seed set. The largest yield reduction occurs with stress at silking (early R1). Potassium uptake is essentially complete and nitrogen and phosphorus uptake is rapid in the plant. Nutrient content by leaf analysis is highly related to the final grain yield at this time. A response to previously applied fertilizer can be seen.
Figure 12. R1 ear and shank. (24KB color photo)
2. R2 Blister
This stage is about 12 days after silking. The kernels are white and shaped like a blister. The cob is close to full size. Silks darken and dry. Hot, dry conditions will cause the silks to darken even more. Irrigation, if needed, can insure adequate moisture for grain production. Kernels are in a steady and rapid period of seed-fill (this continues to R6). Starch is just beginning to accumulate in the watery endosperm (the kernels are about 85% moisture).
Figure 13. R2 ear and shank. (31KB color photo)
3. R3 Milk
This stage is about 20 days after silking. Kernels are beginning to yellow on the outside but contain a milky white inner fluid (starch accumulation—kernel is now at about 80% moisture). Most of the kernels have grown out from the surrounding cob material. The endosperm cell division in each seed is complete and growth will now be due to cell expansion and starch accumulation. Stress is not as severe at R3 as at R1; however, yield reduction can occur due to reduction in the number of kernels which ultimately develop and due to the final size and weight of the kernels. Very little root growth occurs after R3.
Figure 14. R3 ear and shank. (29KB color photo)
4. R4 Dough
This stage is about 26 days after silking. The kernel now has thickened to a pasty (doughy) consistency from the earlier milky state (starch has continued to accumulate and kernel moisture content has decreased). The embryo of the seed is growing while the kernels are just beginning to dry at the top (dent). Kernels have accumulated 50% of their dry weight and have about 70% moisture. Unfavorable environmental conditions or nutrient deficiencies can still result in unfilled kernels and "chaffy" ears.
Figure 15. R4 ear and shank. (26KB color photo)
5. R5 Dent
This stage is about 36 days after silking. Nearly all kernels are dented or denting. Drying kernels show a small, hard, white layer on top. A white line (known as the milk line or starch line) can be seen across the kernel shortly after denting (starch line indicates maturity—it will advance toward the kernel tip with maturity). Stress at this point can reduce kernel weight but not kernel number. A hard frost can stop dry matter accumulation and cause a premature black layer formation. Yields may also be influenced due to field losses from frost-damaged ears which are slower to dry and delay harvest (showing that hybrids should be selected which are mature at least 10 days to three weeks before the average first killing frost date). Kernels at this stage have about 55% moisture. At around 48 days after silking, all the kernels should be fully dented. The seed embryo is morphologically mature. Dry matter accumulation in the kernels will soon cease.
Figure 16. R5 ear and shank. (45KB color photo)
Corn Grain Frost Injury Yield Reduction
---------------------------------------------------------- Kernel Kernel Days after Days to Yield Development Moisture R1 PM* (R6) Reduction ---------------------------------------------------------- R4 - Dough 70% 26 31-37 35-50% R5 - Dent 50% 36 20-23 10-20% R5.5 - Mid-Dent 40% 48 10-12 4-5% R6 - PM* 30-35% 55 0 0% ---------------------------------------------------------- * PM is Physiological Maturity (black layer
6. R6 Physiological Maturity
This stage is about 55 days after mid-silk. All kernels have attained maximum dry weight. The starch line has advanced completely to the kernel tip and a brown or black layer is present (black layer progresses on the ear from the tip kernels to the basal kernels in about 10 days). Harvest for silage can be done now or slightly earlier, but grain harvest will require more drying. Husks and many of the leaves are no longer green, but the stalk may be green. At black layer the average kernel moisture is 30-35% (varying with hybrids and environmental conditions). At 20-26% moisture, grain harvested will still need artificial drying to safely store, thus more field drydown is often used (rate of field drydown varies with hybrid and environmental conditions). Shelled corn can be safely stored at 13-15% moisture.
Figure 17. R6 ear and shank. (49KB color photo)
Corn Moisture Content and Drydown---------------------------------- Approximate Stage Moisture ---------------------------------- R5 (dent) 50-55% R6 (PM) 30-35% Field Drydown from 30-25% 3/4% per day below 25% 1/4% per day ----------------------------------
Photography for this publication was provided through Iowa State University Photo Service, J. C. Herman, editor with technical assistance from S. J. Lupkes. Stage designation format was derived from the Iowa State Extension special report no. 48, "How a Corn Plant Develops" prepared by S.W. Ritchie, J.J. Hanway and G.O. Benson. Also, qualifications on production management suggestions were summarized from lectures of J.R. George at Iowa State University.
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A-1173, June 1999
