Corn Replanting Decisions in Southern Illinois

Cool temperatures and wet conditions can put stress on corn germination and emerging seedlings. This may cause some concern about reduced corn stands. Before deciding to replant, evaluate the stand for population and uniformity. Next, compare yield potential of the existing stand with yield potential of the replant, as well as insurance or government program restrictions. Finally, if the decision is made to replant, consider various management practices discussed in this ALERT to help optimize yield potential.

 

Evaluating the Existing Stand

 
When evaluating corn stands, only count plants that have a good chance of survival. Observe the uniformity of the stand across the field to decide whether replanting the entire field or portions of the field is justified. Spotting in without destroying the existing stand is not recommended3. There may be low areas in fields that will not survive, while the rest of the field will have a good stand. Replanting only the drowned out areas may be a viable option. Carefully evaluate the field for replant options.
 

One way to evaluate corn stands is to count the number of plants in a length of row equal to 1/1,000th of an acre based on row width (Table 1). Multiply the number of plants by 1,000 to get the plants per acre (ppa). Repeat the process in several locations in the field.

 

Row Length Chart A more accurate method is to count 150 plants and measure the distance from start to finish with a measuring wheel. Divide the number of feet traveled into the appropriate factor in Table 2 to determine plant population. For example, if you walked 94 feet while counting 150 plants in 30-inch rows, the population is 2,613,600 ÷ 94 = 27,804. Because a longer row length is counted, the samples are more representative and fewer locations are required.

 

Utilize the same factors for both 30-inch twin- and single-row stand counts; however, the plants in both the twin rows should be counted.

 

Stand Count Evaluation FactorsDeciding Whether to Replant

 

 

 
After taking stand counts, consider the yield potential of the current stand, compared to the yield potential of the target replanting date and population, and the costs associated with replanting. Table 3 illustrates percent of maximum grain yield expected from various planting dates and final plant populations, based on uniform stands, however it represents northern Illinois environments. Table 4, from the University of Missouri, has been included to represent several of the more challenging yield environments found in southern Illinois, although it does not include a planting date component. These tables are to be used as guidelines and are only one component to consider when deciding if replanting is appropriate. Please contact your local seed dealer or agronomist for more specific recommendations.
 

 

 
Percent of Maximum Yield Expected
 

When the Decision is Made to Replant

 

 

 
First you must remove the existing stand. Tillage is an option in many cases, and should be done at an adequate depth to properly control the original stand. Depending on the growth stage of the original stand, two tillage passes may be necessary. If a pre-emergence herbicide has been applied, tillage may decrease the efficacy of the herbicide by placing it deeper than ideal for satisfactory weed control. Additionally, tillage prior to replanting may result in loss of soil moisture. In drier areas, and where soil conditions allow, a more favorable option might be applying a herbicide to remove the original stand and then replanting without tillage. Roundup® brand agricultural herbicides are also an option when the original stand does not contain the Roundup Ready® Corn 2 trait or Roundup Ready® 2 Technology. If the original stand does contain the Roundup Ready® Corn 2 trait or Roundup Ready® 2 Technology, the tank mix options in Figure 1 can provide control of the original stand. The original plants must exhibit growing green tissue and 2 to 4 leaves for the herbicides to be effective. Complete kill may take up to 21 days after application.
 

Tank Mix Options
Second, determine which relative maturity (RM) to use when replanting. As planting occurs after May 1, corn requires approximately 1.6 fewer growing degree days (GDDs) per day of delayed planting to reach flowering4. GDDs required to reach physiological maturity, or black layer, decreases approximately 6.8 GDDs per day of delayed planting after May 14. In southern Illinois, typically corn can be planted through the end of May without switching to an earlier RM. Since fewer GDDs are required to mature corn and since they are adapted well to southern Illinois, several of the 108 to 112 day hybrids are good options to plant through the first two weeks of June. After the middle of June is the time to start considering options such as switching to an earlier RM corn that might not be as well adapted to this geography or planting soybeans. However, final stands of 20,000 ppa can produce acceptable yields compared to replanting full stands in June, especially south of the glacial moraine.

 

Third, decide on a management practice to protect against corn rootworm and other soil insects, as well as European Corn Borer (ECB). Most soil insecticides cannot legally be applied twice in the same growing season in the same location in a field. Growers can replant over the old row and expect some control from the first application of soil insecticide. Another option is to use a different soil insecticide when replanting. A third option is to replant with corn containing the Genuity® family of traits. Depending on region and insect pests present, Genuity® SmartStax® RIB Complete™ and Genuity® VT Double PRO® RIB Complete™ corn blends, and Genuity® VT Triple PRO® Corn offer multiple modes of action for protection against a broad spectrum of insects. Later planted corn is more susceptible to second generation ECB. Research from The Ohio State University suggested significant advantages in overall yield and consistency of yield for corn with the corn borer-protection trait compared to their conventional counterparts, when planted in late May and early June3.

 

Finally, later planted corn has a greater chance of being exposed to heat and drought stress during pollination. This risk can be managed by selecting corn with heat and drought tolerance and early flowering.