The optimum plant population range for corn varies depending on the yield potential of the soil, the hybrid, and the intended use of the crop. For many Pennsylvania soils, a harvest population of 28,000 to 30,000 plants per acre is necessary to achieve maximum economic grain yields. On the best soils, where yields of 175 bushels/acre and higher are common, plant populations of 30,000 to 35,000 plants per acre are needed to maximize grain yields. On droughty or poorly drained soils, with grain yield potentials in the 100 bu/A category (see Table 1.1-1), optimum plant populations are lower, in the 24,000 to 28,000 range.
Hybrids vary in their tolerance of high populations, and some may experience some lodging at the upper end of these ranges. Consequently, you should review the seed company plant population recommendations for each hybrid and adjust accordingly. Also consider populations at the lower end of these ranges in fields where gray leaf spot is common and there is an increased risk of lodging.
For silage production, higher plant populations are warranted. Generally, optimum populations appear to be about 2,000 to 4,000 plants per acre higher for silage than grain. Thus, on most soils, 30,000 to 34,000 plants per acre appear to optimize both yield and forage quality. Higher plant populations can increase yields, but the energy content of the forage appears to decrease slightly. In a recent two-year test conducted at Rock Springs, increasing plant populations from 30,000 to 36,000 plants per acre increased yield by 0.7 tons/acre (35 percent DM) and decreased whole plant digestibility by 0.7 percentage units. The forage quality response to increased plant populations may also vary with hybrids, particularly with leafy hybrids, so be sure to review the seed company planting recommendation for hybrids grown for silage as well.
An extremely useful measurement is the length of row equal to 1⁄1,000 acre. This row length is dependent on the row width (Table 1.4-10). Yield, population, harvest loss, and other measurements can be taken in this row length and multiplied by 1,000 to convert them to a per-acre basis. For example, if you count 24 plants in a 1⁄1,000-acre row length, you can estimate the population on a per-acre basis to be 24 by 1,000, or 24,000 plants per acre. For improved accuracy, use the average of several 1⁄1,000-acre row lengths for these estimates.