Although it is not unusual that 5 to 10 percent of planted seeds fail to establish healthy plants, additional stand losses resulting from insects, frost, hail, flooding or poor seedbed conditions may call for a decision on whether or not to replant a field. The first rule in such a case is not to make a hasty decision. Corn plants can and often do outgrow leaf damage, especially when the growing point is protected beneath or at the soil surface (up until about the six-leaf collar stage). If new leaf growth appears within a few days after the injury, then the plant is likely to survive and produce normal yields.
When deciding whether to replant a field, assemble the following information: original planting date and plant stand, earliest possible replanting date and plant stand, and cost of seed and pest control for replanting. If the plant stand was not counted before damage occurred, providing that conditions for emergence were normal, estimate population by reducing the dropped seed rate by 10 percent. To estimate stand after injury, count the number of living plants in 1/1,000 of an acre (Table 4-12). Take counts as needed to get a good average―one count for every 2 to 3 acres.
Table 4-13 shows the effects of planting date and plant population on final grain yield. Grain yields for varying dates and populations are expressed as a percentage of the yield obtained at the optimum planting date and population.
When the necessary information on stands, planting, and replanting dates has been assembled, use Table 4-13 to locate the expected yield of the reduced plant stand by reading across from the original planting date to the plant stand after injury. Then, locate the expected replant yield by reading across from the expected replanting date to the stand that would be replanted. The difference be- tween these numbers is the percentage yield increase (or decrease) to be expected from replanting.
Here’s how Table 4-13 might be used to arrive at a replant decision. 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 4-13, the expected yield for the existing stand would be 79 percent 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 percent of the optimum. The difference expected from replanting is 81 minus 79, or 2 percentage points. At a yield level of 150 bushels per acre, this increase would amount to 3 bushels per acre, which would probably not justify replanting costs.
Keep in mind that replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season will be based mainly on plant stand and plant distribution. Later in the season as yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.
Table 4-13: University of Illinois Replant Chart Developed Under High Yielding Conditions (Adapted from Nafziger, 1995-96).
|Plants per Acre at Harvest|
|% of Optimum Yield|
Source: Nafziger, E. D. 1994. Corn planting date and plant population. J. Prod. Agric. 7:59-62.