Corn yield trends continue to increase in Iowa and across the country. Yield has increased at about 2 bushels per acre per year in Iowa since the early 1970's. Research conducted at Iowa State University by the late Don Duvick, adjunct professor of agronomy, focused on comparing "era hybrids" to modern-day hybrids. Analysis of these hybrids has shown that approximately half of the annual yield increase we experience is due to plant breeding efforts and the other half is due to improved crop management practices.
So, why are we discussing seeding rates in relation to plant breeding efforts and crop management practices? It is because breeders have focused largely on increasing stress tolerance of individual corn plants, specifically by increasing their ability to withstand increased plant densities. A significant portion of the observed yield increase per year is directly correlated with increased plant populations. Yields are increasing largely due to having more plants on an acre that produce an ear, than in previous years.
Years ago, corn was planted with the check or hill-drop system that used seeding rates around 12,000 seeds per acre. In the 1960's, we planted approximately 16,000 seeds per acre. Over these past 50 years, seeding rates have increased annually. Iowa plant populations have increased about 425 plants per acre per year since 2001.
Are these higher populations crucial for obtaining high yields? Some suggest that the higher seeding rate trends are due to the advancement of hybrids that primarily perform best under higher populations. Yet, others suggest that producers are planting hybrids at seeding rates higher than recommended, thus forcing corn breeders to produce hybrids that can tolerate higher seeding rates. No matter which is ultimately true, it is clear that yield increases over the past 50 to 60 years have come not from increased grain per plant but rather from increased plants per acre. Increasing seeding rates has been paramount to increasing yields.
ISU plant population research conducted by Dale Farnham, former ISU Extension corn specialist, during 1997 to 2000, is contained in Figure 1. Yields were maximized with final plant populations near 32,000 plants per acre across most locations. Reducing the population by 1,000 plants per acre caused a greater yield loss than increasing it by 1,000 plants. Yields will be fairly stable at higher populations unless stress is severe during the growing season.
Figure 1. Corn yield response (percent of maximum yield) to plant population in Iowa, 1997-2000. (D. Farnham, Iowa State University)
Based on newer research (2006) we believe that the optimum seeding rate for corn is approximately 35,000 or greater seeds per acre. These results are only preliminary though and more research is needed to confirm the findings. Therefore, the following economic analysis will use ISU data from 1997-2000 (Figure 1) to understand how plant population and seed cost interact.
Corn seed prices are increasing an average of $1.30 per acre per year (see article Choosing corn hybrids). As seed prices increase, the yield advantage of planting more seed may become economically counterproductive. Seeding rates should be regarded as an investment; the seeding rate which results in the highest return should be selected. To show this relationship look at Figure 2. Remember that the figure is based off of final plant populations, therefore your seeding rate should be 5-10% greater to arrive near this final population.
Corn grain market price is set to $3.00 per bushel. An estimate of 10 percent stand loss was used to account for poor germination and emergence loss. Depending on the number of traits included, seed prices can vary significantly; thus, prices range from $1.00 to $2.50 per 1,000 seed. The stars mark the plant population that is the Most Profitable Plant Population (MPPP) for each seed price and yield level.
Steps to use Figure 2:
- Determine whether your yield goal is closer to 180 (blue lines) or 220 (red lines) bushels per acre.
- Determine your seed costs, possible choices are: $1.00 to $2.50 per 1000 seed. Each seed price option is a separate line in the figure.
- Find the MPPP (where the star is present). The black bars on each side of the star highlight a $5.00 range in return to seed.
Figure 2: Net income for different seed prices (per 1,000 seed).
We notice two things in Figure 2. First, as seed price increases, MPPP decreases. Therefore, with high seed costs, a lower population should be used. Second, the decrease in MPPP is more evident at the 180 bushel yield level than with the 220 bushel yield level. With the lowest seed price, a plant population of approximately 31,000 resulted in the highest MPPP for both yield levels. Yet, as seed price increases, a lower population becomes more advantageous especially in the lower yielding scenario.
Obtaining high yields is critical for staying ahead of persistently increasing input costs. Therefore, it is important to use proper seeding rates. As seed prices continue to increase, we must strive to use the most profitable plant population rather than focusing solely on attaining maximum yield.
Portions of this text, written by Roger Elmore and Lori Abendroth, originally appeared in the Integrated Crop Management extension newsletter on pages 82-83 of the IC-496 (6) - April 10, 2006 issue. Updated February 1, 2007 to reflect new market prices.