Jeff Coulter, Extension corn agronomist
Uniform emergence
Uniform emergence provides a foundation for maximizing corn yield. In a previous study in southwest Minnesota, corn plants that were one leaf stage behind their neighboring plants early in the season yielded about 80% of normal, while corn plants that were two leaf stages behind their neighboring plants early in the season yielded about 50% of normal (Ford and Hicks, 1992). For uniform emergence, corn needs ample and consistent (1) soil moisture in the seed zone, (2) seed-to-soil contact, and (3) soil temperature in the seed zone, along with lack of crust on the soil surface and lack of soil compaction near the seed. Of these factors, irregular soil moisture in the seed zone is the most common cause of uneven emergence. It can result from inconsistency in soil conditions, residue, and depth of seed placement.Soil moisture at tillage and planting
Avoid pre-plant tillage and planting when soils are wet, as this can negate potential advantages from earlier planting. Soil is suitable for tillage when soil in the zone of tillage and in the one-inch zone below the depth of tillage crumbles when squeezed. Tillage of wet soil causes clods and reduces seed-to-soil contact due to air pockets between seed and soil. A goal of planting should be to achieve excellent seed-to-soil contact, as this facilitates rapid and uniform absorption of moisture by seeds, leading to faster and more uniform emergence. Tillage when soils are wet can also create a compacted zone of soil just below the depth of tillage, which can restrict root development and the crop’s ability to take up water and nutrients.Planting when soils are too moist can also cause problems, especially for fine-textured soils. Planter disc openers cutting through wet soil can create compacted soil around the seed that is difficult for roots to penetrate after the soil has dried. Seed furrows can also open after planting in wet soils, resulting in poor seed-to-soil contact, uneven emergence, and a sub-optimal plant population.
Tillage intensity
If possible, try to avoid doing more pre-plant tillage than is necessary to optimize seed placement. Excessive pre-plant tillage reduces soil moisture, which can hinder the speed and uniformity of corn emergence. It can also lead to soil crusting if there are high-intensity rains, which can impede corn emergence. Excessive pre-plant tillage also increases the risk of soil wind erosion. Blowing sand can cut off or injure corn seedlings.Soil moisture conservation
A requirement for uniform corn emergence is adequate and consistent soil moisture in the seed zone. Therefore, tactics that conserve soil moisture are important for successful stand establishment, especially if soils are dry in the spring. Strategies for conserving soil moisture when using pre-pant tillage include:- Avoiding unnecessary tillage passes
- Avoiding a deeper depth of tillage than is needed
- Preparing seedbeds close to planting
- Having rolling baskets on the pre-plant tillage implement to firm the soil surface and thereby reduce evaporation of soil moisture after tillage
Soil temperature
For germination, corn seeds need to imbibe at least 30% of their weight in water and soil temperature needs to be at least 50°F. Risk of stand establishment problems are reduced if corn is planted when soil in the seed zone has reached or is near 50ᵒF and is expected to warm, since seeds can imbibe water even when soil temperature is less than 50ᵒF.Before planting, consider the expected weather and soil temperature during the 1 to 2 days after the intended planting date. This is because imbibitional chilling injury to corn can occur when soil temperature in the seed zone is near 40°F or lower within 1 to 2 days of planting and seeds imbibe cold water. Imbibitional chilling injury occurs because cellular tissues of seeds are less flexible under cold conditions and can rupture when seeds swell due to the uptake of water. However, if soil temperature drops to 40°F or lower beyond 2 days after corn planting, the risk of imbibitional chilling injury is greatly reduced compared to a similar drop in soil temperature within 1 to 2 days after planting.
Planting depth
The optimal planting depth for corn is 2 inches in most situations. Shallower seed placement increases risk of poor nodal root establishment. If soils are very dry, planting as deep as 2.5 inches to reach soil moisture is okay. Early in the planting process and when switching to fields with different soil and residue conditions, it is important to dig up planted seed to ensure consistent planting depth and good seed-to-soil contact. Make sure to do this at various locations across the planter width, as the planting depth from the wings of the planter is sometimes shallower than that of the center section of the planter.Planting date
Planting date is one of many factors that affect corn yield. However, other factors such as the uniformity of emergence, weather conditions during the two weeks before and two weeks after tasseling, hybrid selection, crop nutrition, weed and pest control, and crop rotation typically have a greater effect on yield than planting date.Across 26 planting date trials conducted across Minnesota from 2009 to 2016 by the University of Minnesota, corn grain yield was within 1% of the maximum when planting occurred between April 25 and May 12, and yield loss due to delayed planting did not occur until more than about 140 GDDs had accumulated since the first planting date (Table 1). However, in years when warm weather arrives earlier than normal and persists, planting in mid-April can produce similar yield if young corn plants are not significantly damaged by a freeze in May.
Table 1. Corn grain yield as affected by planting date across 26 planting date trials conducted across Minnesota from 2009 to 2016, along with growing degree days (GDD) accumulated since the first planting date in these trials.
| Planting date | Grain yield | GDD since 1st planting date |
|---|---|---|
| April 12-17 | 94-96 | 0 |
| April 18-24 | 97-98 | 13-18 |
| April 25-May12 | 99-100 | 31-137 |
| May 13-19 | 97-98 | 146-208 |
| May 20-25 | 94-96 | 219-281 |
| May 26-30 | 91-93 | 294-350 |
Planting rate
Optimal planting rates for grain corn in Minnesota vary somewhat with soil productivity and expected yield, and late-maturity hybrids typically have slightly lower optimum planting rates than early-maturity hybrids. Optimal planting rates also differ among hybrids, making it important to check with seed suppliers for hybrid-specific guidelines.Too high of a planting rate generally does not reduce corn yield but can reduce net economic return. For moderate- to high-yielding soils, optimal planting rates for grain corn are typically 34,000 to 36,000 seeds per acre but can be as high as 38,000 to 40,000 seeds per acre in some cases. On non-irrigated drought-prone soils where maximum yields are 130 to 160 bushels per acre, optimal planting rates are generally 30,000 to 33,000 seeds per acre but can be lower. On non-irrigated drought-prone soils where maximum yields are less than 130 bushels per acre, optimal planting rates are around 27,000 to 30,000 seeds per acre.
To verify optimum planting rates, consider using test blocks or strips that are planted at 3,000 to 4,000 seeds per acre higher and lower than the planned rate for each planting rate zone in a field, then measure the yields.
More information
Additional resources on corn production are available at Extension's Corn Production website.References
Ford, J.H., and D.R. Hicks. 1992. Corn growth and yield in uneven emerging stands. Journal of Production Agriculture 5:185–188.
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