Corn plant populations and yield goals.

Continued annual increases in corn grain yields have been observed in this country since the introduction of hybrid corn in the 1940's. While better agronomic practices, including weed control and fertilization methods have contributed greatly to increased yields, genetic improvement has been demonstrated to have had the largest effect. Most of the increase in corn genetic yield potential has been attributed to enhanced stress tolerance. This is exemplified by the fact that current hybrids have increased ability to tolerate crowding. In many cases, grain yields on a per acre basis will increase with increasing plant population up to a point of diminishing returns. Ever more often, high yields result from high plant populations. However, higher plant populations increase competition among individual plants for water, sunlight and nutrients. This effect may lower individual plant yield but increase yield per unit area. It may also prove highly detrimental to yield if one or more of the necessary growth factors are in very short supply. This is often the case for moisture in Virginia. Considering this point, corn plant population should be based on the amount of available water that a particular soil can supply. This is also directly related to a yield goal for the soil type or field. Current seeding rate recommendations for Virginia are:

Yield potential, bu/A	Seeds planted, seeds/A
<90	18000-20000
90-120	20000-22000
>120	22000-26000+
"Stretch" hybrids*	18000-20000

*"Stretch" hybrids have the ability to compensate for increased yield potential in-season.

Plant >26,000 seeds only on soils w/ 4 plus inches of available moisture storage capacity in the surface 3 feet, and average yields over the last five years of >125 bu/A.

A recent review of over 400 corn yield trials conducted in Virginia from 1994-2004 was evaluated for plant population effects on corn grain yield. Figure 1 shows the results of this effort. Average grain yield across many of these sites is higher than would normally be expected under whole-field conditions and probably results from the fact that the data come from small uniform areas within fields that are planted to elite genetics with special attention to management. It is important to note that very poor yields occur at all populations, but that a higher percentage of poor yields occur at populations above 26000 plants/A as compared to populations of 22000 plants or less/A. This is likely a direct reflection of increased competition among plants for limiting resources. Also important to note is that few very yields above 150 bu/A occur with populations below 22,000 seeds/A. You must have the plants to produce the high yields on soils that will support them.

Figure 1. Plant population and yields from 407 yield trials conducted in Virginia, 1994-2004.(Virginia Hybrid Evaluations, On-farm research tests, and company plots for Dekalb and Pioneer Hi-Bred.).

Seeding rate/population considerations:

• Understand your specific soil and climatic conditions and make adjustments to plant population based on the risk of severe soil moisture deficiency.
• Plan to drop 5 - 10% more seeds than your target population to allow for germination of less than 100% and seedling plant losses.
• Plant at the proper depth and date for rapid emergence and establishment of strong root systems.
• Some hybrids tolerate high populations better than others. Check specific recommendations for the hybrid you intend to plant. In general, shorter and earlier hybrids are more responsive to higher populations
• Lodging occurrence and severity often increase with increasing population.
• High populations need high levels of fertility. Be certain fertilility is adequate.
• Ensure consistent plant spacing and seeding at that desired planting rate through planter calibration and maintenance.

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