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Various conditions can cause reduced corn stands. Before making any replant decisions, evaluate the existing stand for population, uniformity, and survival potential. Carefully consider the yield potential of the existing stand with the yield potential of a replanted stand, and the management practices needed to replant.
Evaluation of the existing stand - When evaluating corn stands, count only plants that have a good chance of survival. Observe the uniformity of the stand across the field to determine if replanting the entire field or portions of the field is justified. There may be areas in the field that have a low population and plants that will not survive, while the rest of the field has a good stand. Replanting only the area of the field with a poor stand may be a viable option. However, spotting in new seed without destroying the existing stand is not recommended.1
Estimating the corn stand can be done by counting the number of plants in a length of row equal to 1/1,000th or 1/100th of an acre based on row width (Table 1). Multiply the number of plants by 1,000 or 100 to get the plants per acre. Accuracy can be greater if you count plants in 1/100th of an acre. Repeat the process in several locations to get a good representation of the plant population. You will also need to estimate whether any of the plants may soon die, and not count these injured plants.
Check the stand for gaps and uniformity - Allow for some additional yield loss if gaps of one or more feet are commonly found between plants. Research shows that recurring gaps of 1 to 3 feet can decrease yield by an additional 2%, and by 5% if several gaps of 4 to 6 feet are within the row.2 Also check for uniformity of plant emergence. If there is a delay in emergence of less than 2 weeks between the early and late emerging plants, and the stand is good, replanting may not be economical. However, when 50% or more of the plants in the stand emerge 3 weeks later than the initial plant emergence, replanting could provide the opportunity to increase yield potential by about 10%.
Consider calendar date and yield potential - Compare the existing stand to the yield potential for replanted stand at a later planting date and target plant population. Replanting intentions should not be abandoned if planting is slightly delayed past optimum dates for the area. Irrigated corn can very easily produce optimum yields when planted later than the recommended dates. However, irrigation water availability also needs to be evaluated with later plantings, and if not available or limited, it may be more profitable to keep a marginal stand. Seeding rates can also be reduced for later plantings, because late corn generally develops more height and leaf area.
Deciding whether to replant - The cost of replanting a field is often the deciding factor. Costs can include seed, tillage, fuel, additional pesticides, and labor. The corn product and relative maturity needs to be determined. Management practices to protect against insects, like corn rootworm and European corn borer, also need to be considered. Later planted fields can be more affected by insects.
Remove existing stand after deciding to replant - If replanting becomes necessary, the existing stand needs to be removed for proper plant spacing and uniform crop maturity. Inter-seeding is not recommended because poor plant spacing can result in yield reductions caused by competition for light, nutrients, and water.
Tillage can be an option, but should be done at an adequate depth to properly control the original stand. Two tillage passes may be necessary, depending on the corn growth stage. Tillage can also decrease the efficacy of herbicide by placing it deeper than ideal for satisfactory weed control. Additionally, tillage prior to replanting may result in loss of soil moisture.
A more favorable option can be applying herbicides to remove the original stand, and then replanting without tillage. Roundup® brand agricultural herbicides can be used when the original stand of corn is not glyphosate-tolerant. When the corn is glyphosate-tolerant, herbicide control options are:
1 Nielsen, R.L. 2012. Corn replant decisions. Purdue University. www.agry.purdue.edu. 2 Abendroth, L. and Elmore, R. 2010. Replant checklist. Iowa State University Extension. http://www.agronext.iastate.edu. Web sites verified 5/11/15. 150512073811