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).Researchers refer to this 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, sometimes overlooked, is the effect of management practices (M). Planting date, row spacing, and crop rotations can exert some influence on the yield response to the plant population factor.
Individual hybrids can respond differently to all these factors, 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, 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 kernels per ear. For example, severe nitrogen (N) deficiency will have an impact on the final number of kernels, 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 kernel numbers (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, among other factors, can influence the actual final number of kernels.
Always keep the long-term weather conditions in mind. The drought that affected much of Kansas in 2011 and 2012 made almost any population 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 happened recently, it is worthwhile taking into consideration how much moisture there is 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 two 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.
The latest Kansas Crop Progress and Condition report (released by Kansas Agricultural Statistics on March 9) lists near normal temperatures across much of the State. Overall, topsoil and subsoil moisture supplies were rated close to 50% as very short or short. The three western crop reporting districts (NW, WC, and SW) and the North Central district have >50% of topsoil and/or subsoil moisture conditions as very short or short in the report. The three eastern crop reporting districts (NE, C, and SE) are reporting >60% as adequate topsoil and subsoil moisture conditions.
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 table 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 |
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|
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/bookstore/pubs/c560.pdf
Drought-tolerant (DT) hybrids have arrived on the market in recent years. Questions about whether changes in seeding rates are needed when using these new hybrids are becoming more frequent. A summary of information is in preparation regarding the evaluation of DT vs. non-DT corn hybrids at different site-years around the state. From the evaluation performed the last 3 years in Kansas evaluating diverse seeding rates, hybrids, and water usage, differences in yield were observed when DT corn hybrids were compared with non-DT hybrids. Still, the most important point, as presented in the below figure, is that the yield response at the plant-scale to plant population is similar for DT vs. non-DT corn hybrids. Thus, a change in plant population doesn’t seem to be needed when this new corn hybrid technology is used.
Figure 1. Plant-scale association between plant density or plant population (plants per square foot) and the yield (in bushels per 1,000 plants) [Adee, Roozeboom, Schlegel, and Ciampitti].
On-Farm Corn Seeding Rate Studies: 2014 season – Central Kansas
During the last growing season, three on-farm research studies were established in collaboration with Tom Maxwell, Central Kansas District Extension Agent, and corn farmers in that district (Justin Knopf, Mark Pettijohn, and Karbers’ Farm). The experimental layout for those studies is presented below.
Field Variability: an example of the field variability was clearly reflected by the yield monitor information collected at harvest time for one location.
A summary of corn plant population response to all three on-farm locations allowed us to visualize the complex yield response to plant population and how essential it is to continue the on-farm research efforts to properly identify optimal corn plant population. Ultimately, this will result in better guidance to producers and key-stakeholders in the seeding rate decision-making process.
The three different colored dots in the graph above represent yields from the three different farms. Agronomically, the optimum population for all these on-farm corn population studies was different. Even when environmental conditions and planting times were very similar, optimal population ranged from 18,000 to 24,000 seeds per acre. The agronomically optimum population did not coincide with the economically optimum population, which was lower in most cases.
More information on the on-farm studies will be summarized in coming issues of the K-State Agronomy eUpdate. Stay tuned.
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
ciampitti@ksu.edu
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