Yield response to plant population for corn in Indiana
For the 46 trials characterized as experiencing a normal range of growing conditions (moderate to low stress), there was no relationship between yield level and optimum plant population even though optimum yields varied from 121 to 245 bpa (Fig. 1). This result suggests, that if spatial variability for productivity within a specific field was similar to this range, there may be minimal value to variable rate seeding.
Severe Stress Conditions. The average optimum PLANT population for the 12 severely stressed (mostly drought) trials was 25,500 ppa or about 6,000 fewer ppa than those trials that experienced more “typical” growing conditions. At 95% stand establishment rate, the 25,500 PLANT population would equal a SEEDING rate of about 26,800 spa. Among the individual trials, optimum PLANT populations ranged from 20,400 to 33,000 ppa and optimum yields ranged from 71 to 168 bpa (Fig. 2).
Hybrid Differences. Thirteen of the 58 trials included targeted comparisons of paired hybrids characterized as “less” or “more” responsive to plant populations with the objective to document whether or not such advertised hybrid population “ratings” resulted in significantly different optimum plant populations. Of the 13 such trials, only 3 resulted in significant hybrid differences in yield response to population (Fig. 3). Of these, optimum populations for the “less” responsive hybrids were higher than for the “more” responsive hybrids in two of the trials (opposite of expectation), but the differences in optimum population were only about 1,000 ppa (Fig. 3). In the third trial that exhibited significant hybrid differences for optimum population, maximum yield for the “more” responsive hybrid did indeed occur approximately 4,600 ppa higher than for the “less” responsive hybrid, (Fig. 3). However, the latter pair of hybrids was also evaluated at three other locations and they did not respond differently to population (data not shown).
Effect of Nitrogen (N) Fertilizer Rate. Seven of the 58 trials included both the farmer’s normal N rate and a rate from 50 to 75 lbs higher than his normal rate with the objective of determining whether yield response to plant population might be influenced by N availability. However, optimum plant populations were not influenced by N rate in any of these 7 field trials (data not shown).
Economics. This summary has focused on “agronomic” optimum populations that result in maximum grain yield. Clearly, your market price for grain and the cost of seed influence the estimation of an economically optimum seeding rate. Table 1 provides such estimates based simply on a range of grain prices and seed costs, using the average grain yield response in the 46 trials that represent “typical” growing conditions.