Optimal Corn Seeding Rate Recommendations

The optimal corn seeding rate depends on the hybrid (genotype, G) and the interaction with the environment (E), researchers termed 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 plant population they used was adequate. Another factor that sometimes we neglect to mention is the effect of management practices (M). Planting date, row spacing, and 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 plant 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 plant 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 influence 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 due to relative success of fertilization and degree of early abortion.

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 increase somewhat this year if the soil profile is wetter-than-normal is a little like the famous line in the movie Dirty Harry: “Do I feel lucky?” 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 warning 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.

Table 1. Suggested dryland corn final populations and seeding rates

Table 2. Suggested irrigated corn final populations and seeding rates

* 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%.

New research on corn seeding rates

An intensive review of a large database from Corteva Agriscience (2000-2014 period) was utilized to synthesize yield response to plant population under varying yield environments (<100 bu/acre to >200 bu/acre). Overall, yield response to plant population depended on the final yield environment (Figure 1). 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 than 200 bu/acre (Figure 1).

Figure 1. Corn grain yield response to plant density in four yield environments, a) <100; b) 100-150; c) 150-180; and d) > 180-210 bu/acre (Assefa, Ciampitti et al., 2016, Crop Science Journal).

As a disclaimer, “agronomically” optimum plant population does not always coincide with the “economically” optimal plant population. Lastly, final seeding rate depends on the environment, hybrid, and production practices (e.g., planting date, rotation, tillage).

The next step in our research study on yield response to plant density is to increase the level of prediction of agronomically and economically optimum plant density, based not only on the yield environment (or an average of past yield data), but also considering the available water at planting and the weather forecast for the growing season. Lastly, the corn yield response to plant density curves (solid lines) also present an uncertainty level (error, shaded areas) based on those main factors affecting yields (Figure 2). Including the risk level taking by farmers when selecting plant density is a relevant point of investigation in our future studies.

Figure 2. Corn grain yield response to plant density in four target yield environments (Lacasa, Ciampitti et al., 2020, Scientific Reports, https://www.nature.com/articles/s41598-020-72693-1). Solid lines are the main yield-to-plant density response curves for each yield environment and shaded grey areas represent the level of uncertainty on the predictions for each response model.