Corn Hybrid Response to Plant Population

• Genetic improvement of corn hybrids for superior stress tolerance has contributed to increased yields by allowing hybrids to be planted at higher plant populations.
• Corn growers continue to steadily increase plant populations. Average planting rates are above 30,000 seeds/acre on most corn acres in the U.S. and Canada.
• Pioneer has conducted research studies for many years to measure how hybrids respond to plant population. During the last four years, studies were conducted across 165 environments in 17 states and three provinces.
• The earliest maturity hybrids in the study (CRM/100) showed a steeper population response curve than the later maturities. Past studies have also shown that very early hybrids require higher populations for optimum yields.
• Optimum economic planting rates ranged from 32,000 to 33,300 seeds/acre for hybrids later than 100 CRM. Earlier hybrids were most profitable at rates of 34,700 seeds/acre.
• In addition to plant population, hybrid standability can be affected by hybrid, crop management and environment. Results of Pioneer research suggest that addressing those factors may be more important than reducing plant population in order to improve hybrid standability.
• This Crop Insights reviews recent Pioneer research on plant population effects on yield and other traits.

The average yield of corn in the U.S. and Canada has tripled over the last half-century. Yield gains have resulted from improved hybrid genetics, more precise soil fertility practices, better weed control, and advances in other production methods. Among these factors, genetic improvements have contributed the most to yield gains, adding from 1.0 to 1.5 bu/acre each year. To accomplish these increases, corn breeders have selected for superior tolerance to drought and other stresses, and yield stability across diverse growing environments. A key result of enhanced stress tolerance is adaptability of hybrids to higher plant populations.

Changes in corn plant populations preferred by farmers have been dramatic (Figure 1). In just 10 years, the number of North America corn acres with seeding rates of at least 30,000 seeds/acre has more than doubled, increasing from about 25% of total acres in 1997 to nearly 55% today. Iowa and Minnesota now have average corn seeding rates greater than 30,000 seeds/acre across 80% or more of their corn acres. Seeding rates continue to climb in these and all major corn-producing states. In fact, nearly 1/4 of Iowa acres and 1/3 of Minnesota acres had seeding rates of 33,000 seeds/acre or higher in 2007 (data not shown).

Figure 1. Percent of corn acres with seeding rates of 30,000 seeds/acre or higher. Pioneer Hi-Bred Brand Concentration Survey, Market Research, Economics and Analysis.

Higher plant populations increase competition among individual plants for water, sunlight and soil nutrients. Although this may lower individual plant yield, it has increased yield per unit area by optimizing these yield components:

Pioneer routinely conducts research studies designed to measure genetic improvements in corn hybrids. One such study compared the plant population response of modern hybrids to that of older hybrids (Duvick, 1993). Hybrids from the 1930s to 1990s were planted to achieve populations of 4,000, 12,000, 22,000, and 32,000 plants/acre. The grain yield responses of these hybrids to low and high plant densities are shown in Figure 2.

Figure 2. Grain yield response to low and high plant populations for hybrids from four eras of plant breeding, two-year average (Duvick, 1993).

At very low plant populations (a low stress environment) there were no yield differences in hybrids from different decades. However, when seeded at current populations needed to maximize grain yield per acre, hybrids showed progressively higher yields with each era of genetic improvement. The newest hybrids were much better adapted to the higher stress levels and produced the highest yields.

To achieve these genetic gains, Pioneer plant breeders over the last 50 years have focused on yield stability across a range of environments, increased stalk and root strength, increased tolerance to drought stress and high plant population stress, and resistance to barrenness. These researchers report that several anatomical and physiological characteristics distinguish modern corn hybrids from older hybrids including:

• Higher leaf area index, which results in the plant intercepting more sunlight
• Higher rates of leaf photosynthesis
• Higher radiation use efficiency during grain filling

Breeders' efforts and resultant hybrid changes now allow producers to plant corn at higher plant populations where maximum grain yield potential can be realized.

Pioneer Agronomy Sciences researchers have studied plant population responses in multiple environments across U.S. and Canada for the past 20 years. The primary goal of these studies is to determine the optimum population of hybrids to maximize harvestable grain yield. With incremental improvements in hybrid stress tolerance, the highest population required to maximize yield has continued to increase.

Pioneer researchers now evaluate hybrids at plant populations as high as 42,000 plants/acre across multiple growing environments.

Continued improvements in stress tolerance have allowed corn hybrids to be grown at progressively higher populations for increased yields.

Grouping locations with similar yields is a useful way to analyze plant population results because it can indicate which populations are needed for the yield levels growers expect to achieve. Like previous Pioneer studies, the 2003 to 2007 trials across the U.S. and Canada show that corn hybrid response to plant population is affected by yield level at very low yields. However, at yield levels over 130 bu/acre, grain yield response to plant population is similar regardless of yield level (Figure 3).

Figure 3. Corn grain yield response to plant population by location yield level, 2004 to 2007 (n is the number of observations within a yield range.)

As the graph indicates, optimum yields occurred at harvest populations of 35-36,000 plants/acre when average yield levels at the test sight were 130 bu/acre or more. This plant density is higher than many producers are currently planting, but similar to the current plant populations often used for irrigated acres and contest fields.

The similarity of the top three curves showing population response in the yield range most commonly achieved in Corn Belt fields casts doubt on the effectiveness of variable-rate seeding to increase yields within this range (also see Doerge, 1999). However, for very low yielding areas of fields, variable-rate seeding should be beneficial, according to these results.

The population response curves above 130 bu/acre show that past the point of optimum population, corn hybrid yields level off rather than decrease. Even at lower yield levels below 130 bu/acre, yields did not drop drastically, but rather, declined gradually beyond the optimum population. This is in contrast to hybrids of 30 to 40 years ago, which were much more likely to incur barrenness and significant yield losses under high plant density stress. The application of these findings is that growers today can plant optimistically at higher populations needed to achieve top yields under good growing environments. This is because there is much less risk of significant yield reductions from over-planting if stress conditions develop.

At all yield levels, considerations for determining a recommended planting rate should include not only the expected yield level but also the economic return. Optimum economic seeding rate based on the cost of seed and the price of grain will be addressed later in this article.

To further clarify corn plant population response, results were grouped by hybrid maturity (Figure 4).

Figure 4. Grain yield response to plant population for corn hybrids from four maturity (CRM) ranges, 2004 to 2007.

This four-year data summary shows a similar response of hybrid maturities (CRMs) to plant population. However, there are slight differences in optimum plant populations by CRM. The earliest hybrid group (< 100 CRM) shows the steepest response curve with a population optimum of about 38,000 plants/acre. The middle two CRM groups have an optimum of around 35-36,000 plants/acre, while the latest hybrids are slightly lower, at about 34,500 plants/acre. Past studies have also shown that very early hybrids require higher populations for optimum yields. Researchers theorize that the disadvantages of smaller stature and lower leaf area index of early maturity hybrids are overcome by higher populations.

Increased stalk lodging is sometimes observed with high plant densities, as average stalk diameter is reduced. For this reason, Pioneer researchers conduct "late-season standability" trials across North America each year. In these plots, hybrid stands of 18,000 to 42,000 plants/acre are left in the field past normal harvest dates. This allows researchers to evaluate hybrid standability responses to plant population after exposure to adverse environmental conditions The results showed that in these high stalk-lodging environments percent stalk lodging increased approximately 0.5% for each 1000 plant/acre increase in population (Figure 5).

Figure 5. Late-harvest stalk lodging response of corn hybrids to plant population at locations with 10% or greater overall lodging. 35 environments, 2004 to 2007.

Standability at harvest is also affected by factors other than plant population, including hybrid, crop management practices and environment. Figure 5 demonstrates that decreasing plant population as much as 4,000 plants per acre would only reduce stalk lodging by about 2%. Therefore, careful selection of lodging resistant hybrids combined with proper hybrid placement, soil fertility and other crop management practices may be more important than reducing plant population in maximizing harvestable corn yield.

The net income response to corn seeding rate across different hybrid CRM ranges is shown in Figure 6. The net income/acre response to plant population is based on a corn grain price of $3.10/bu and a seed cost of $1.81/1000 seeds.

Figure 6. Four year (2004 to 2007) net income* per acre in response to corn plant populations ranging from 18,000 to 42,000 seeds/acre, from four CRM ranges.

*An input cost of $1.81/1000 seeds and corn grain price of $3.10/bu were used. A 5% increase in the optimum planting rate and subsequent seed cost was added to account for early season stand loss.

As the graph demonstrates, the response was similar for all four CRM zones tested. The optimum economic planting rates were approximately 34,700, 33,300, 32,500 and 32,000 plants per acre for the very early, early, medium and late hybrid maturity groups, respectively. Your Pioneer sales professional can demonstrate how economic seeding rates change with grain price and seed costs, using a decision aid.

Doerge, T. 1999. New opportunities in variable-rate seeding of corn. Crop Insights Vol. 9, No. 5. Pioneer Hi-Bred International, Inc., Johnston, Iowa.

Duvick, D. 1993. Personal communication.