At the time I’m writing this column, most of the corn in Iowa has not been planted. However, every year in the last three decades, I’ve been asked about making a replant decision for a cornfield that does not look as good as the producer thinks it should. I hope no one needs this information this year, but the odds are good that someone does. I hope it is not you!

Regardless of the cause of a poor stand, the first step in determining if replanting is needed is to conduct a stand assessment and determine the yield potential of the existing stand, and then compare it to the yield potential of a later planting date that also has additional costs associated with replanting.

In the recently published Guide to Iowa Corn Planting from Iowa State University Extension. Following are steps from the guide, along with my own suggestions. Follow these stand assessment steps to estimate your yield potential:

Determine emerged stand counts. The guide says to count plants from a 1000th of an acre of row length from several well-represented areas of each field by following a W pattern. The more I am on the fence about the stand, the more sites I stop to count. I never count just one row at a time when I stop; I do at least four rows at each site.

Keeping track of each count in a notebook allows me to evaluate the numbers when done. In addition, I usually carry a spade with me to dig plants to evaluate health and root establishment. When I get close to an area I’m going to assess, I throw that spade to get a random place to start counting.

Identify stand uniformity. While determining stand counts, take note of uneven emergence. An ISU Integrated Crop Management news article last year summarized the plant-to-plant spacing part of this statement. It said if you have gaps of 2 to 3 feet, your yield expectation drops about 2%, and it drops about 5% with 4- to 6-foot gaps.

We also need to assess when these plants emerged relative to each other. If half or more of the plants emerged three weeks later than those that emerged first, replanting would increase yield by about 10%. If less than two weeks separated emergence on half or more of the stand, the replanting yield gain would only be about 5%. My point is, gaps and uneven emergence need to be considered separately.

Estimate yield potential. If plant populations are lower than expected, use the accompanying table to estimate yield potential. This is true for the existing stand, and for the stand you replant. Don’t forget to reduce the yield potential of the existing stand for gaps and uneven emergence.

Corn grain yield response to planting date and plant population using the APISM crop model and validated using field research trials from seven Iowa locations table

The table is part of the FACTS project at ISU, which is an ongoing project developed to forecast and evaluate in real-time soil-crop dynamics at specific ISU research fields (field scale) and at regional scale. The Agricultural Production Systems sIMulator, on which the project is based, is “internationally recognized as a highly advanced platform for modeling and simulation of agricultural systems.”

Additional considerations 

Don’t forget to reduce the value of the replanted crop by the cost of replanting. Be certain to talk with your crop insurance agent before you take action — there might be some dollars available to help offset that cost.

When making a decision to replant, you need to ask yourself how much of the field is actually affected. Can I just replant a part, or isn’t it worth the effort for a small area? I often suggest flagging the area you plan to replant instead of making that decision from the tractor seat. Replanting a small area may not be worth the effort, whereas planting a large block may reap economic benefits.

If you decide not to replant, but the stand was thinned, consideration should be made to control weeds in field areas where plant populations are low. Also, remember that fall frost risk and additional grain drying costs due to higher grain moisture at harvest are part of the equation. This can be addressed by switching to an earlier relative maturity corn hybrid.

When making a replant decision, you need to try to take the emotion out of the decision. I hope these hints help you make a research-based, not emotion-based decision.

DeJong is an ISU Extension field agronomist based at Le Mars in northwest Iowa. Contact him at jldejong@iastate.edu. 

Source: Joel DeJong, Wallaces Farmer