Corn  response to population is continuously changing with seeding rates increasing about 280 plants/acre/year according to the National Agricultural Statistics Service  (2010). Plant populations in Ohio have increased 16% in the past 10 years (56% since the early 1970s). According to NASS, plant population for corn in Ohio averaged 28,200 plants/acre in 2010, which is lower than most Corn Belt states (e.g., Minnesota, 29,900; Iowa, 29,950; Illinois, 29,650; Indiana, 28,350 plants/acre).
Research has demonstrated that the superiority of modern hybrids is fully expressed only at higher plant populations (Duvick, 2005). When hybrids of different eras (from the 1930s through 2001) were planted at a very low population (4,000 plants/acre – a “low-stress” environment), there was little to no difference in the yields of hybrids from different eras. However, when established at a higher plant population (32,000 plants/acre) more typical of that required to maximize yields in recent years, hybrids exhibited progressively higher yields with each era of genetic improvement. The genetic improvements that have facilitated the positive interaction between hybrids and increasing seeding rates include increased yield stability across a range of environmental conditions, greater tolerance to drought (and higher plant populations), enhanced stalk and root strength, and resistance to diseases and insects.
The distribution of corn acreage with varying plant populations at harvest indicates major differences in seeding rates used for corn production in Ohio and greater adoption of higher seeding rates by some corn growers in recent years. Final stands on nearly 40% of the state’s corn acreage was 30,000 plants/acre or higher in 2010. Four years ago, final stands were 30,000 plants/acre or higher on only 14% of the corn acreage.
Most corn agronomists recommend adjusting seeding rates by using the yield potential of a site as a major criterion for determining the appropriate plant population. Seed companies recommend final stands as high as 35,000-36,000 plants/acre for some hybrids in high-yield environments. Average seeding rates of hybrids entered in the Ohio Corn Performance Test have increased from about 25,000 seeds/acre in the early 1970s to over 35,000 seeds/acre in 2010 (with final stands ranging from about 23,000 to 33,000). Ohio Corn Performance Test sites generally represent production environments with high yield potential.
Corn response evaluations
Since 2006, we have conducted evaluations of corn response to plant population across a range of environments (primarily involving varying climatic conditions). Tests were established at up to nine locations each year in fields with high yield potential (200+ bu/acre under favorable conditions). We considered final stands of 24, 30, 36 and 42,000 plants/acre. Results have varied over the past five years, with yields averaging 191, 191, 177, 228 and 201 bu./A for 2006, 2007, 2008, 2009 and 2010, respectively. In 2006, 2009 and 2010, results suggested that final stands of 36,000/acre or higher were required for optimal yields. In 2007 and 2008, there was no yield response to plant populations above 30,000 plants. Higher plant populations were usually associated with more stalk lodging (stalks broken below the ear). The most severe lodging occurred in 2008 when high winds associated with Hurricane Ike contributed to severe lodging (especially at 42,000 plants). Averaged across locations, lodging in 2008 was 52% at 42,000 plants and the extensive stalk damage may have accounted for the absence of yield increases above 30,000 plants. However, in 2007 stalk lodging was negligible at all populations yet there was no yield response above 30,000 plants. On average, increasing plant population from 24,000 plants to 42,000 plants/acre reduced grain moisture but effects were highly variable from year to year ranging from 0 in 2007 to 1.1% in 2009.
When yield potential is reduced due to stresses including to late planting, drought, root and stalk lodging, disease and insect injury and harvest delays, yield response to plant population is often limited. Under severe drought conditions, yield may even decrease at higher plant population due to increased barrenness and greater stalk lodging but this is relatively rare in hybrids developed in recent years.
Based on OSU studies to date, a seeding rate of 31,000-33,000 seeds/acre will be adequate for optimal yields in most production environments planted in late April and early May. For fields with low yield potential, seeding rates of 24,000-26,000 will probably be sufficient. For fields with very productive soils and high yield (180-bu./acre level and higher), seeding rates of 36,000-37,000 may be necessary. (These rate estimates are based on 10% mortality.) Planting rate or population can be cut to lower seed costs but this approach typically costs more than it saves. In the absence of major environmental stresses, most research suggests that planting a hybrid at suboptimal seeding rates is more likely to cause yield loss than planting above recommended rates (unless lodging becomes more severe at higher population levels). Because of differences in genetic backgrounds for various traits, especially stalk quality, seed company recommendations should be followed to adjust seeding rates for specific hybrids.