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When evaluating corn stands, count only 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. Spot-planting without destroying the existing stand is not recommended.3 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 determine plants per acre (ppa). Repeat the process in several locations in the field.
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. Use the same factors for both 30-inch twin- and single-row stand counts; however, the plants in both the twin rows should be counted.
After taking stand counts, consider the yield potential of the current stand compared to the yield potential of the targeted 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.
First, the existing stand should be removed. 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 diluting the herbicide throughout the tillage zone. 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 Roundup Ready® Corn 2 or corn with Roundup Ready® 2 Technology. If the original stand does contain Roundup Ready® Corn 2 or corn with 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 two to four leaves for the herbicides to be effective.
Complete kill may take up to 21 days after application. Liberty® herbicide can be used to remove corn that does not contain the LibertyLink® trait; however, control may be inconsistent.5 It is important to note that products with SmartStax® technology have tolerance to Liberty® herbicide in addition to Roundup® brand agricultural herbicides. For these reasons, the options shown in Figure 1 are recommended for more consistent control.
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 flowering.4 GDDs required to reach physiological maturity, or black layer, decreases approximately 6.8 GDDs per day of delayed planting after May 1.4
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. If replanting corn with insect-protected traits, contact your local agronomist or brand representative for assistance in selecting the most appropriate corn product. Later planted corn is more susceptible to second generation ECB. University research suggests significant advantages in overall yield potential and consistency of yield potential for corn with the corn borer-protection trait compared to their conventional counterparts, when planted in late May and early June.3
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.