When corn is produced for grain in Ohio, recommended plant populations at harvest (or final stand) can range from 24,000 to 34,000+ plants per acre, depending on the hybrid and production environment. Yield response to plant population is influenced by several factors including environmental conditions, the hybrid, and the end use of the corn crop. To account for effects of the production environment, plant population adjustments should be made on a field-by-field basis using the average yield potential of a site over a three- to five-year period as a key criterion for determining the appropriate plant population. When determining the realistic yield potential for a site over a five-year period, it may be appropriate to ignore the highest and lowest yields, which may have occurred during years that were unusually favorable or unfavorable for corn performance.
Hybrids differ in their response to plant population with some exhibiting stalk lodging at the upper end of the plant population range. Seed companies specify a range in final stands for the various corn hybrids they market. Because of differences in genetic backgrounds for various traits, especially stalk quality, these seed company recommendations should be considered when adjusting seeding rates for specific hybrids.
Based on OSU studies, a plant population of 31,000 to 32,000 seeds per acre will optimize yields in most Ohio production environments. For fields with low yield potential, final stands of 24,000 to 26,000 seeds per acre will probably be sufficient. For fields with very productive soils and exceptionally high yield potential, final stands greater than 34,000+ seeds per acre may be necessary. Seeding rates can be cut to lower seed costs, but this approach typically costs more than it saves. In the absence of major environmental stresses, most research suggests that planting a hybrid at suboptimal seeding rates is more likely to cause yield loss than planting above recommended rates (unless lodging becomes more severe at higher population levels).
Plant populations recommended for corn silage are great- er than those for grain. According to recent Pennsylvania State University research, optimum plant populations for silage are about 2,000 to 4,000 plants per acre greater for silage than for grain. Higher plant populations can increase silage yields but may reduce forage energy content.
If a grower plans to rely extensively on field drying that can delay harvest, there may be little benefit from using high plant populations much above 30,000 plants per acre. A recent OSU study evaluated effects of plant population (24,000 to 42,000 plants per acre) and harvest dates (early/mid October, November, and December) on the agronomic performance of four hybrids differing in maturity and stalk quality (Table 4-10). Although the hybrids exhibited similar yield potential when harvested early (early/mid October), differences in yield became evident with harvest delays, which could be attributed to differences in stalk quality. Yield differences among plant population were generally small on the first harvest date, but with harvest delays, major yield losses occurred at the higher plant populations, especially 42,000 plants per acre, due to increased stalk lodging. Grain moisture averaged about 24 percent on the first harvest date, 18 percent on the second harvest, and 17.5 percent on the third harvest date. After the first harvest in early/mid October, stalk lodging increased to as much as 80 percent for certain hybrids at high plant populations, resulting in yield losses of nearly 50 percent by mid-December.
Results from trials conducted at OSU and other universities indicate that higher seeding rates do not necessarily require higher nitrogen rates. In Ohio State University research, two different cropping rotations (corn after soybeans and corn after corn) and two seeding rates (30,000 and 40,000 seeds per acre) were evaluated across a range of nitrogen rates. In five out of eight site-years, seeding rate had no impact on fertilizer nitrogen response (the optimum nitrogen rate was similar regardless of seeding rate). When there were differences in optimum nitrogen rates, it was not because the higher seeding rate required more nitrogen. Only one out of the eight site- years (for corn after corn) revealed that the higher seeding rate required more nitrogen.
Table 4-10. Harvest Date and Plant Population Effects on Grain Yield, Moisture and Stalk Lodging.
|Harvest Population (plants/ac)|
|Grain Moisture, %|
|Stalk Lodging, %|
Final stands are always less than the number of seeds planted per acre. Cold, wet soil conditions, insects, diseases, cultivation, and other adversities will reduce germination and emergence. Generally, you can expect up to five to 10 percent fewer plants at harvest than seeds planted. To compensate for these losses, you need to plant more seed than the desired population at harvest.
To calculate your own planting rate, consider the following formula:
Planting Rate= (Desired Population per Acre)/(Germination x Expected Survival)
Germination is the percent seed germination shown on the seed tag (converted to decimal form). Expected survival is the percent of seedlings and plants that you expect to reach harvest maturity under normal conditions (converted to decimal form). If you are planting very early when the soil will likely remain cool for several days following planting, you may want to increase seeding rate by 5 percent. A similar approach should be followed when planting no-till, especially in heavy corn residues.
Target stand at harvest – 30,000 plants per acre
Seed tag indicates 95% seed germination
Assume 97% survival (3% plant mortality)
Planting rate = 30,000 / (0.95 x 0.97) = 32556 seeds per acre
According to the formula, you should consider a planting rate of approximately 32,600 seeds per acre to achieve the desired final stand of 30,000 plants per acre.
Uneven plant spacing and emergence reduces yield potential. The impact of uneven emergence is usually greater than that of uneven spacing. Seed should be spaced as uniformly as possible within the row to ensure maximum yields and optimal crop performance―regardless of plant population and planting date. Corn plants next to a gap in the row may produce a larger ear or additional ears (if the hybrid has a prolific tendency), compensating for missing plants. These plants, however, cannot make up for plants spaced so closely together in the row that they compete for sunlight, water, and nutrients. Crowding―especially when with uneven emergence―can result in barren plants or ears too small to be harvested (nubbins), as well as stalk lodging and ear disease problems. Although uniformity of stand cannot be measured easily, studies have indicated that reduced plant stands will yield better if plants are spaced uniformly than if there are large gaps in the row. As a general guideline, yields are reduced an additional 5 percent if there are gaps of 4 to 6 feet in the row and an additional 2 percent for gaps of 1 to 3 feet. Studies at Purdue University suggest that corn growers could improve grain yield from 4 to 12 bushels per acre if with- in-row spacing were improved to the best possible uniformity (depending on the unevenness of the initial spacing variability).
The most effective way to improve planter accuracy is
to keep planting speed within the range specified in the planter’s manual. Following are additional considerations for improving seed placement uniformity:
- Match the seed grade with the planter plate.
- Check planters with finger pickups for wear on the back plate and brush (use a feeler gauge to check tension on the fingers, then tighten them correctly).
- Check for wear on double-disc openers and seed tubes.
- Make sure the sprocket settings on the planter transmission are correct.
- Check for worn chains, stiff chain links and improper tire pressure.
- Make sure seed drop tubes are clean and clear of any obstructions.
- Clean seed tube sensors if a planter monitor is being used.
- Make sure coulters and disc openers are aligned.
- Match the air pressure to the weight of the seed being planted.
Uneven emergence affects crop performance because competition from larger, early emerging plants decreases the yield from smaller, later emerging plants. The primary causes of delayed seedling emergence in corn include shallow planting depths, poor seed to soil contact resulting from cloddy soils, inability of no-till coulters to slice cleanly through surface residues, worn disc openers, and maladjusted closing wheels. Other causes include soil moisture and temperature variability within the seed depth zone, soil crusting prior to emergence, occurrence of certain types of herbicide injury, and variable insect and/or soil-borne disease pressure.
Based on research at the University of Illinois and the University of Wisconsin, if the delay in emergence is less than two weeks, replanting increases yields less than 5 percent, regardless of the pattern of unevenness. However, if one-half or more of the plants in the stand emerge three weeks late or later, then replanting may increase yields up to 10 percent. To decide whether to replant in this situation, growers should compare the expected economic return of the increased yield with both their replanting costs and the risk of emergence problems with the replanted stand.
Use Tables 4-11 and 4-12 to determine the number of kernels dropped or the plant population per acre.
Table 4-11: Kernel Spacings Within the Row at Planting Rates (Kernels/ac) and Row Spacings.
|Row Spacing (in.)|
|Final Stand/ac (10% Loss)||15||20||22||28||30||36||38||40|
|Inches Between Kernels|
Table 4-12: Length of Row Required for 1/1000 Acre at Various Row Widths.1
|Row Width (in.)||Length of Row for 1/1000 ac|
|15||34 ft. 8 in.|
|20||26 ft. 2 in.|
|28||18 ft. 8 in.|
|30||17 ft. 5 in.|
|36||14 ft. 6 in.|
|38||13 ft. 9 in.|
|40||13 ft. 1 in.|
|42||12 ft. 5 in.|
1Example: For 30-inch rows, count the number of kernels dropped or the number of plants in 17 feet, 5 inches and multiply by 1000. If there are 21 in the 17 feet, 5 inch row, the population is 21,000 per acre.
For twin rows, measure from the center of the twin rows to the center of the next set of twin rows to determine the effective row width. Count the plants in both of the twin rows on each side of that center. Example: If twin rows are planted 6 inches apart planted every 30 inches, the effective row spacing is 30 inches (There are rows 3 inches to each side of that 30 inch center). You need 17 feet, 5 inches of row in 30 inch rows. Measure off 17.5 feet of row and count the plants in both of the twin rows that are on each side of the 30 inch center.