The optimal corn seeding rate for any situation will depend on the anticipated environment and how the hybrid responds to that environment. Thus, optimum seeding rate depends on the hybrid (genotype, G) and the interaction with the environment (E), in something that researchers want to term as the G x E interaction. Producers can look back to their corn crop from the previous growing season, or wait until the current growing season is nearly complete, and evaluate whether the population they used was adequate. Another factor that sometimes we neglected to mention is the effect of management practices (M component). Planting date, row spacing, crop rotations can also exert some influence on the yield response to the plant population factor.
Individual hybrids can respond differently, but the following guidelines may help in deciding if current seeding rates need to be adjusted. If more than about 5% of the plants are barren or if most ears have fewer than 250 kernels per ear, the population may be too high. If there are consistently more than 600 kernels per ear or if most plants have a second ear contributing significantly to grain yield, the population may be too low. Of course the growing conditions will influence ear number and ear size as well, so it is important to factor in the growing conditions for that season when interpreting these plant responses. In addition to the growing conditions, nutrient status can also exert some influence on the final number of grains per ear. For example, severe nitrogen (N) deficiency will have a high impact on the final number of grains, ear size and ear number.
Don’t be too concerned if a half-inch or so of the ear tip has no kernels. If kernels have formed to the tip of the ear, there may have been room in that field for more plants contributing to grain yield. Again, "tipping back" will vary with the G x E x M interaction. Potential ear size and potential number of kernel (1,000-1,200 per ear) are set before silking, but the actual final number of kernels is not determined until after pollination and early grain fill (lack of fertilization and early abortion of grain number).
Always keep the long-term weather conditions in mind. In a drought year, almost any population is too high for the available moisture in some areas. Although it’s not a good idea to make significant changes to seeding rates based only on what has happened recently, it is worthwhile taking into consideration how much moisture there is currently in the soil profile and the long-term forecasts for the upcoming growing season.
Making a decision on whether to keep seeding rates at your usual level or cutting back somewhat this year if the soil profile is drier than normal is a little like the famous line in the movie Dirty Harry: “How lucky do you feel?” If you think weather conditions will be more favorable for corn this year than the past years, stay about in the middle to upper part of the range of seeding rates in the table below. If you do not think growing conditions will improve enough to make up for dry subsoils, you might want to consider going toward the lower end of the range of recommended seeding rates, with the caveat that if growing conditions improve you will have limited your top-end yield potential.
Optimal seeding rates may need to be adjusted for irrigated corn if fertilizer or irrigation rates are sharply increased or decreased. For example, research at the Irrigation Experiment Field near Scandia has shown that if fertilizer rates are increased, seeding rates also have to be increased to realize the maximum yield benefit. Consult seed company recommendations to determine if seeding rates for specific hybrids should be at the lower or upper end of the recommended ranges for a given environment.
The recommended planting rates in the following tables attempt to factor in these types of questions for the typical corn growing environments found in Kansas. Adjust within the recommended ranges depending on the specific conditions you expect to face and the hybrid you plan to use.
The following recommend planting rates are from the K-State Corn Production Handbook.
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Suggested Dryland Corn Final Populations and Seeding Rates |
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Area |
Environment |
Final Plant Population (plants per acre) |
Seeding Rate* |
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Northeast |
100-150 bu/a potential |
22,000-25,000 |
26,000-29,500 |
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150+ potential |
24,000-28,000 |
28,000-33,000 |
|
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Southeast |
Short-season, upland, shallow soils |
20,000-22,000 |
23,500-26,000 |
|
|
Full-season bottomground |
24,000-26,000 |
28,000-30,500 |
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Northcentral |
All dryland environments |
20,000-22,500 |
23,500-26,500 |
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Southcentral |
All dryland environments |
18,000-22,000 |
21,000-26,000 |
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Northwest |
All dryland environments |
16,000-20,000 |
19,000-23,500 |
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Southwest |
All dryland environments |
14,000-20,000 |
16,500-23,500 |
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Suggested Irrigated Corn Final Populations and Seeding Rates |
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Environment |
Hybrid Maturity |
Final Plant Population (plants per acre) |
Seeding Rate* |
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Full irrigation |
Full-season |
28,000-34,000 |
33,000-40,000 |
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Shorter-season |
30,000-36,000 |
35,000-42,500 |
|
|
Limited irrigation |
All |
24,000-28,000 |
28,000-33,000 |
* Assumes high germination and that 85 percent of seeds produce plants. Seeding rates can be reduced if field germination is expected to be more than 85%.
For more information, see the K-State Corn Production Handbook, C-560: http://www.ksre.ksu.edu/library/crpsl2/c560.pdf
For the new corn genetics technology, the drought-tolerant (DT) hybrids have become available in recent years and questions about whether changes in seeding rates are needed with these new hybrids are becoming more frequent. A summary of information is in preparation as regarding the evaluation of DT vs. non-DT corn hybrids at different site-years around the state of Kansas (West, Northcentral, Eastern locations). From the evaluation performed in these site-years evaluating diverse seeding rates, hybrids, and water usage, differences in yield were observed when DT corn hybrids were compared with non-DT materials. Still, the most important point, as presented in the below figure, is that the yield response at plant-scale to plant population is similar for DT vs. non-DT corn hybrids (share equal slope). Thus, a change in plant population doesn’t seem to be needed with DT hybrids.
Figure 1. Plant-scale association between plant density or plant population (plants per square foot) and the plant yield (in bushels per 1,000 plants) [Adee, Balboa, Roozeboom, Schlegel, and Ciampitti].
New Research on Corn Seeding Rates
An intensive review of a large database from Dupont Pioneer (2000-2014 period) was utilized to synthesize yield response to plant population under varying yield environments (<100 bu/acre to >200 bu/acre). Overall, across the four different hybrids evaluated, yield response to plant population depended on the final yield environment (Fig. 2). In yield environments below 100 bu/acre, yield response to plant population was slightly negative. Yield response to plant population tended to be flat when yield environment ranged from 100 to 150 bu/acre; positive and quadratic with the yield environment improving from 150 to 180 bu/acre; and lastly, increasing almost linearly with increasing plant populations when the yield environment was more than200 bu/acre (Fig. 2).
Figure 2. Corn grain yield response to plant density in four different productivity environments, a) low yielding <100 bu/acre; b) medium yielding 100-150 bu/acre; c) high yielding 150-180 bu/acre; and d) very high yielding 190-210 bu/acre (Assefa et al., 2016, Crop Science Journal).
As a disclaimer, “agronomically” optimum plant population does not always coincides with the “economically” optimal plant population. Therefore, farmers should consider this aspect when deciding the final seeding rate for corn. In addition, as previously mentioned, final seeding rate depends on the environment, hybrid utilized, and production practices selected (e.g., planting date). Producers should consider looking at the previous crop to investigate if the seeding rate previously used in their different fields was adequate for their respective yield environments.
More information on the on-farm studies, seeding rates, and new corn hybrid technology will be summarized in future issues of the Agronomy e-Update. Stay tuned.
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist, KSUCROPS Lab
ciampitti@ksu.edu
Yared Assefa, Post-doctoral researcher, KSUCROPS Lab
yareda@ksu.edu
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