Table 1. Relative yield potential of corn by planting date and population. Note: Values are based on some earlier Iowa
research and modeling; 100% yield potential is estimated to occur with 35,000 plant population and early planting. This table is from the ISU Corn Field Guide (CSI 001 publication) on page 12.
Using information in this table, note that potential yield associated with the original planting date, and with the actual population, will give a fair estimate of the actual yield of the problem field. The table summarizes planting date and plant population (final stand) relationships. For example, if the original planting date was April 30, a population of 35,000 plants/acre is expected to provide maximum yield, based on this table. If the population is only 20,000 plants/acre, yield potential is still 89% of maximum. Compare the estimated yield potential of the existing stand with the anticipated yield for the replant stand, plus replant costs, herbicides, etc. to determine if replanting is likely to be profitable.
Consider destroying the old stand if replanting is necessary. Planting into well-established corn plants is like planting into an existing stand of weeds. The older, well established corn plants will successfully compete for light, water, and nutrients. Yield of the new stand that is planted into an existing stand of corn will likely be disappointing. If areas to be replanted are somewhat small and scattered, inter-planting may be the only option to increase harvest stand but there also may be less yield loss associated with these small areas than you might guess.
Don’t panic if a field is over-planted. If you accidentally plant a field at a higher-than-desired population, first determine the actual stand. Research suggests that the impact of plant stands up to 36,000 plants per acre on a silt loam-type soil is minimal. Plan to watch at harvest-time for standability issues.
The best way to correct an unacceptably high plant population is to destroy the old stand and start over. Attempts at thinning a stand to an acceptable level generally do not turn out well. If destroying the old stand and starting over is the option, consider using a herbicide-resistant hybrid (having a different trait than the original crop) for the replant crop. The original crop can then be destroyed chemically, eliminating the need for tillage, and thereby preserving soil moisture.
Image: Variability in Corn Size Because of Uneven Emergence (Source: GROWMARK, Inc.)