Timely planting is a critical management practice for optimizing yields. Long-term studies conducted in Illinois show that the optimum planting date for corn is mid to late April (Figure 1). Yields declined gradually with planting delays in early May, reaching around 93% of the maximum by May 15. However, yield loss became more rapid thereafter, dropping to 84% of the maximum by May 31 and 78% by June 8. There were a few instances where planting before April 10 lowered yield potential, and a few trials where planting in late May or early June produced the same yield as planting in mid to late April.

Another important consideration when deciding when to start planting is the soil condition and forecasted weather for the next 7 to 10 days. Planting into wet soils or “mudding in” can increase the risk of sidewall compaction and poor root development, especially if the weather turns hot and dry after planting. This can result in reduced plant stands, delayed emergence, and restricted root growth, all of which can negatively affect yield.

Figure 2 below illustrates the response of corn yield to plant population in 17 sites across Illinois from 2016 to 2018. Each trial included four to six hybrids, with planted populations (which closely matched final stands) ranging from 18,000 to 48,000 plants per acre. Yields at the highest population of 48,000 were lower than at 36,000 plants per acre. A curve fitted to the data for each trial indicated that the optimum population—where the yield increase from the last seeds planted was just enough to cover the seed cost—ranged from under 30,000 plants (in a dry location) to slightly above 40,000. Achieving maximum yield required populations that were 2,000 to 6,000 higher than the optimum, but the yield increase didn’t justify the added seed expense. Interestingly, the optimum population was not correlated with yield level—sites with the highest yields didn’t necessarily require the highest populations.

Overall, planting at a rate of 36,000 plants per acre would have optimized the seeding rate across all trials, resulting in minimal income loss: a slight reduction in yield where optimum rates exceeded 36,000, and higher seed costs where optimum rates were below 36,000. If there were a reliable method to adjust seeding rates field by field, profits could have been slightly improved. However, these trials were conducted on similar soils at four or five sites over three years, and the responses to yield and population varied more by year than by location (soil productivity). This highlights the challenge of identifying a consistent basis for varying seeding rates, especially in fields with uniform soil types and productivity.

Ideal planting depth varies with soil and weather conditions. Emergence is more rapido from relatively shallow-planted corn, so early planting should not normally be as deep as later planting. For most situations, though, corn should be planted 1-1/2 to 1-1/4 inches deep. Later in the season and under dry conditions, planting as much as 2-1/2 inches into moisture may be advantageous, especially if the forecast is for continued dry weather. Depper planting tends to result in reduced stand due to crusting or wet soils and an increased chance of uneven emergence, which can cause yield losses.