One of the reasons that the topic of seeding rates is a popular one in coffee shops, internet chat rooms, the farm press and crop seminars is that variable rate seeding technology is becoming more and more commonplace today as a standard accessory on modern corn planters. Another factor that spurs the interest in corn seeding rates is the not uncommon belief that today’s modern hybrids will respond dramatically to aggressively high plant populations. This belief is fueled by the harvest populations often associated with national corn yield contest winning entries, coffeeshop scuttlebutt, and encouragement from seed company sales folk.
Corn plant populations have been steadily increasing in Indiana for the past 25 years at approximately 300 plants per acre per year (Nielsen, 2013). In 2012, the estimated average plant population statewide was just over 29,000 plants per acre (ppa). Considering stand establishment success ranging from 90% to 95%, this means that the average statewide seeding rate is probably between 30,000 and 32,000 seeds per acre (spa).
Statewide increases in plant population have occurred as growers have shifted from quite low seeding rates to intermediate and higher seeding rates. In 1998, nearly 46% of Indiana's corn acres were estimated to have final stands less than 25,000 ppa and only 5% with final stands greater than 30,000 (Nielsen, 2013). Whereas in 2012, only 14% of Indiana's acres were reported to be less than 25,000 ppa and 50% of the acres were reported to be greater than 30,000 ppa. Among the changes that have allowed growers to steadily increase plant populations has been the genetic improvement in overall stress tolerance that has resulted in a) ear size and kernel weight becoming less sensitive to the stress of thicker stands of corn and b) improved late-season stalk health.
Field-Scale Seeding Rate Trials
I began a more focused effort several years ago to evaluate yield response to seeding rates in field-scale trials at Purdue Ag. Centers and with growers around the state. As of the end of the 2012 growing season, I have completed 43 field-scale seeding rate trials and believe that I now have a critical mass of results to begin developing seeding rate guidelines for Indiana growers.
The severe drought of 2012 offers an interesting insight into the relative risk of higher seeding rates for crops under severe stress. Consequently, the results of the 43 trials are presented separately in terms of the 31 trials from 2001 through 2011 and the 12 trials conducted in the droughty 2012 growing season.
The relationship between yield and plant population is shown in Fig’s 1 and 2 in terms of “percent of maximum yield” rather than actual bushels per acre. This facilitates the interpretation of results from multiple trials with differing overall yield levels.
Figure 1 represents the yield response to plant population for the 31 trials conducted prior to the drought year of 2012. Surprisingly, the visual effect of plant population on grain yield is not very dramatic. In fact, the yield response is so flat that one cannot confidently calculate an optimum plant population using the entire dataset. However, optimum plant populations can be calculated for each individual trial and the average of those individually calculated optimum plant populations is 31,150 ppa. This average optimum plant population would be equivalent to seeding rates of approximately 32,500 to 34,600 spa depending on your typical stand establishment success rate.
Calculated in the same manner, the average optimum plant population among the 12 trials conducted in the drought year of 2012 (Fig. 2) was lower, at 28,000 ppa or seeding rates of approximately 29,500 to 31,000 spa. The yield penalty for higher plant populations under drought conditions is more evident, but varied among the locations. In fact, the highest yield occurred at the lowest plant population (approximately 21,000 ppa) at three of the 12 locations. The effect of severe drought stress on optimum plant population is also evident when the optimum plant populations for each trial are graphed against the optimum yields at those locations (Fig. 3).
More importantly, though, Fig. 3 also illustrates that there is little relationship between optimum populations and yield levels ranging from the low 100’s to the low 200’s (bu/ac). This is relevant information for growers who are contemplating the merits, or lack thereof, of variable rate seeding for corn in Indiana because the results suggest there would be little value in varying seeding rates in fields with yield potentials within that range.
Results from 43 field-scale trials around Indiana since 2001 suggest that optimum plant populations for corn grown under typical yield levels and growing conditions are in the neighborhood of 31,150 ppa or seeding rates between 32,500 and 34,600 spa. The results further suggest that corn grown under moderate to severe drought stress conditions may perform best at plant populations no higher than 28,000 ppa and perhaps as low as 21,000 ppa under truly severe growing conditions (actual drought, non-irrigated center pivot corners, non-irrigated sandy fields with minimal rainfall, etc.).
On-Farm Field-Scale Seeding Rate Trials
The 43 research trials that provide the basis for the guidelines offered in this report are scattered nicely throughout the state and represent a fair range of growing conditions. However, there is a need for even more such trials to better “capture” the effects of different growing conditions on yield response to plant population. Field-scale on-farm seeding rate trials are simple to conduct, especially if your planter is equipped with GPS-enabled variable rate controls. These trials can be customized to include two or more nitrogen fertilizer rates or multiple hybrids. Such trials can also be used to evaluate yield response to plant population in different areas or “zones” within fields to help address the question about the relative merits of variable rate seeding. If you would to participate in one or more on-farm seeding rate trials with corn, please download the protocol for this at the following URL and contact me for additional information.
The field-scale, often on-farm, research summarized in this update has been supported, in part, by funds from the Indiana Corn Marketing Council, the Purdue Mary S. Rice Farm Fund, and the Indiana Certified Crop Adviser Program. The collaboration of the participating farmers, crop consultants, retail agronomists, county Extension Educators, and Purdue Ag. Center personnel is gratefully acknowledged.
Nielsen, RL (Bob). 2013. Thoughts on Seeding Rates for Corn. Corny News Network, Purdue Extension. Online at http://www.kingcorn.org/news/timeless/SeedingRateThoughts.html[accessed Feb 2013].