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 of that hybrid with the environment (E), in something that researchers call 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 neglect to consider as much as we should is the effect of management practices (M component). Planting date, row spacing, and crop rotations can also exert some influence on the yield response to plant populations.
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 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 significant 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 kernels (usually in the range of 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 due to the potential for lack of fertilization or early abortion of grain numbers.
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 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.
Recommended planting rates
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* |
|
Northeast |
100-150 bu/a potential |
22,000-25,000 |
26,000-29,500 |
|
150+ potential |
24,000-28,000 |
28,000-33,000 |
|
|
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 |
|
Northcentral |
All dryland environments |
20,000-22,500 |
23,500-26,500 |
|
Southcentral |
All dryland environments |
18,000-22,000 |
21,000-26,000 |
|
Northwest |
All dryland environments |
16,000-20,000 |
19,000-23,500 |
|
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 |
|
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
Drought-tolerant hybrids
As a final note, for the new corn genetics technology, the drought-tolerant (DT) hybrids have arrived on the market recently and 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 as regarding the evaluation of DT vs. non-DT corn hybrids at different site-years around the state of Kansas (western, north central, and 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 when this new corn hybrid technology is employed.
Figure 1. Plant-scale association between plant density or plant population (plants per square foot) and the plant yield (in bu per 1,000 plants) [Adee, Balboa, Roozeboom, Schlegel, and Ciampitti].
On-farm corn seeding rate studies: 2015 season – Central Kansas
During the last growing season, four on-farm research studies were established in collaboration with Tom Maxwell (Central Kansas District Agriculture Extension Agent) and corn farmers around the area (Justin Knopf, Mark Pettijohn, Dwight Conley and Matt Everhart, and Karbers’ Farm). The experimental layout for those studies is presented in the below scheme.
Field Variability: An example of the field variability was clearly reflected by yield monitor information collected at harvest time at two locations. Each of the strips in the maps represents a different plant population. Taking a close look we can see different response to population across the field within the same strip. This shows the need for more detailed studies that take in account different potential yields within the field. A combination of Precision Ag and proper assessment from an agronomist with geostatistics skills can help farmers better understand the complex nature of these interactions and to be more efficient when they select the optimal plant population, considering the entire field variability. The Extension Crop Production team is willing to collaborate with farmers who want to start doing strip trials across the field to generate on-farm data.
If you are interested please see: http://bit.ly/KSUONFARMPROJECT
In addition, if you want to share yield monitor data with us, please don’t hesitate to reach us out and we can discuss how to coordinate and organize a meeting for discussing your needs and providing an interpretation of your field variability for improving the use of the on-farm Precision Ag tools (e.g. yield monitor).
A summary of corn plant population response to all four on-farm locations allowed us to visualize the complexity yield response to plant population and how essential is to continue the on-farm research efforts for properly identifying optimal corn plant population and providing a better guidance to producers and key-stakeholders in the seeding rate decision making process.
The agronomically optimum populations for all these on-farm corn population studies was different, even when environmental conditions and planting time was very similar. The economically optimal population ranged from 20,000 to 24,000 seeds per acre. The agronomically optimum did not coincide with economically optimum population, with the economic optimum population being lower in most of the situations evaluated.
More information on the on-farm studies will be summarized in upcoming issues of the Agronomy eUpdate. Stay tuned.
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
ciampitti@ksu.edu
Guillermo Balboa, PhD Fulbright Scholar, KSUCROPS Production Lab
balboa@ksu.edu
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