Past university research indicates that optimal plant populations for early (mid to late April) and late planted (late May to early June) corn are similar. Based on results of these studies, most Extension agronomists recommend that final plant populations should not be changed as planting date is delayed.
If planting is delayed until early June, some Ohio data suggests that certain hybrids are more susceptible to stalk lodging at high populations. In delayed planting situations, use the optimal seeding rates for the yield potential of each field. Recommended seeding rates for early planting dates are often 10% higher than the desired harvest population. However, soil temperatures are usually warmer in late planted fields, and as a result germination and emergence should be more rapid and uniform.
So, as planting is delayed, seeding rates may be lowered (decreased to 3 to 5% higher than the desired harvest population) in anticipation of a higher percentage of seed emerging.
Localized ponding and protracted saturated soil conditions have adversely affected corn in many fields across Ohio. Heavy rains have also resulted in soil crusting which is contributing to reduced emergence. Producers confronted with poor stands due to these problems may be considering replanting their fields.
Replant decisions in corn should be based on strong evidence that the returns to replanting will not only cover replant costs but also net enough to make it worth the effort. Don’t make a final assessment on the extent of damage and stand loss too quickly. The following are some guidelines to consider when making a replant decision.
If the crop damage assessment indicates that a replant decision is called for, some specific information will be needed, including:
Original target plant population/Intended plant stand
Plant stand after damage
Uniformity of plant stand after damage
Original planting date
Possible replanting date
Likely replanting pest control and seed costs
To estimate after-damage plant population per acre, count the number of viable plants in a length of row that equals 1/1000 of an acre and multiply by 1000. Make several counts in different rows in different parts of the field. Six to eight counts per 20 acres should be sufficient. Table 4-12 in the OSU Agronomy Guide shows row lengths required to equal 1/1000 acre when corn is planted at various row widths.
A major consideration in making a replant decision is the potential yield at the new planting date and possibly different planting rate; this can vary depending on the hybrid used, soil fertility and moisture availability. Table 4-15 in the OSU Agronomy Guide is a chart developed by Dr. Emerson Nafziger at the University of Illinois that show effects of planting date and plant population on final grain yield for the central Corn Belt. Dr. Bob Nielsen has modified this table to provide estimates of potential yield losses for planting dates in early June (on-line here.)
Grain yields for varying dates and populations in both tables are expressed as a percentage of the yield obtained at the optimum planting date and population.
Here’s how these tables might be used to arrive at a replant decision (Table 4-15 from the OSU Agronomy Guide will be used in this example). Let’s assume that a farmer planted on May 9 at a seeding rate sufficient to attain a harvest population of 30,000 plants per acre. The farmer determined on May 28 that his stand was reduced to 15,000 plants per acre as a result of saturated soil conditions and ponding. According to Table 4-15, the expected yield for the existing stand would be 79% of the optimum. If the corn crop was planted the next day on May 29 and produced a full stand of 30,000 plants per acre, the expected yield would be 81% of the optimum. The difference expected from replanting is 81 minus 79, or 2 percentage points. At a yield level of 175 bushels per acre, this increase would amount to a gain of about 3.5 bu per acre.
It’s also important to note plant distribution within the row. Remember that values in replant charts like Table 4-15 from the OSU Agronomy Guide are based on a uniform distribution of plants within the row. Add a 5% yield loss penalty if the field assessment reveals several gaps of 4.