Pioneer has been conducting plant population studies with corn hybrids for over three decades. Research studies have been conducted at over 320 locations throughout the U.S. and Canada in the last 6 years (Figure 4). Pioneer researchers target representative environments based on maturity zone, expected yield (high or low), specific stresses, and other unique location characteristics. Over the past several years, Pioneer has also conducted plant population research focused specifically on lower-yielding water-limited environments (Figure 5).
Figure 4. Pioneer plant population test locations in North America, 2013-2018.
Figure 5. Pioneer water-limited plant population research locations in North America, 2013-2018.
Additionally, hundreds of on-farm Pioneer agronomy seeding rate trials are conducted each year comparing multiple corn products at up to four seeding rates at each location. These trials have considerable value for local observation, evaluation and refinement of plant population agronomic response. Farmers can use the multi-year and multi-location results to identify the best potential planting rates specific to their hybrid, location, and management practices.
Field Productivity Level
In general, corn hybrid response to plant population follows a quadratic response model in which yield increases with greater plant population up to an optimum point, beyond which yield declines. Pioneer research has shown that yield response to plant population depends on the yield environment. An analysis of 15 years of plant population response data showed that in low yielding environments (below 100 bu/acre), maximum yield was attained at a plant population level of 24,000 plants/acre. In very high yield environments (above 200 bu/acre), yield response to plant population continued to increase even at 40,000 plants/acre.
Economic Optimum Seeding Rate
As yields increase with each increment of higher seeding rate, a point is reached where the yield benefit from the next addition of seed no longer exceeds the cost of the seed. That point is the optimum economic seeding rate. By definition, it is the seeding rate that generates the most income when seed cost and grain price are factored in. The economic optimum seeding rate will always be less than the seeding rate at which yield is maximized.
Results from recent Pioneer plant population research show that the economic optimum seeding rate increased from approximately 30,000 seeds/acre at the 150 bu/acre yield level to around 37,000 seeds/acre at the 240 bu/acre yield level (Figure 7).
Figure 7. Corn yield response to population and optimum economic seeding rate by location yield level (7-yr average of all hybrids tested).
Averaged across all hybrids tested. Economic optimums based on a corn rain price of $3.50/bu and a seed cost of $3.00 per 1,000 seeds; assumes 5% overplant to achieve target population.
At water-limited locations where yield levels were lower, economic optimum seeding rate varied from less than 22,000 seeds/acre for locations yielding 90 bu/acre to around 24,000 seeds/acre for yields of 150 bu/acre (Figure 8).
Figure 8. Corn yield response to population and optimum economic seeding rate by location yield level at water-limited sites, 7-yr average.
Averaged across all hybrids tested. Economic optimums based on a corn grain price of $3.50/bu and a seed cost of $3.00 per 1,000 seeds; assumes 5% overplant to achieve target population.
Research has generally shown a higher optimum plant population for shorter comparative relative maturity (CRM) hybrids. Some researchers theorize that the disadvantages of smaller stature and lower leaf area index of early maturity hybrids are alleviated by higher populations. Increasing leaf area index may be required for highest yields in northern areas with limited light availability during late ear-fill stages.
An analysis of 15 years of Pioneer plant population research data showed that corn yield was generally lower and optimum population was greater with hybrids of shorter CRM. Long (106-115 CRM) and very long (>115 CRM) maturity hybrids generally reached their maximum yield within a very narrow plant population range of 34,000 to 35,000 plants/acre. On the opposite CRM range, very early to medium (<78 CRM to 105 CRM) maturity hybrids typically achieved maximum yield at plant populations ranging from 36,000 to 39,000 plants/acre.