Replanting Decisions for Soybeans

Planting soybeans has been significantly delayed in the Midsouth in 2019, and many of the acres that have been planted at this date have poor or unhealthy stands. Thus, some producers are likely facing a decision about whether or not to keep marginal soybean stands. This is a decision that should be based on factors such as condition of the marginal stand, availability of seed of preferred varieties, the lateness of a replanted crop, and the costs associated with destroying the marginal stand and replanting.

Results from a study entitled Think Twice Before Replanting Soybeans that was conducted in Wisconsin in 2012 and 2013 are provided below. This report, authored by Adam Gaspar, Shawn Conley, and John Gaska, offers helpful guidelines to make this decision.

The first order of business in the decision-making process is to accurately determine the stand from the initial planting that failed to emerge completely. This should be done by actually counting the plants in a prescribed fashion to estimate the number that are present rather than just making a visual estimate.

The number of plants remaining in a failed or damaged stand can be estimated by counting the number of plants in a length of row that is based on the row spacing used (see Table 1). This should be done at least five times in individual areas of a field that show a consistently lower stand, or five or more times in a whole field that has a consistent stand failure over its entirety. Calculate the average from these counts and then multiply that number by 1,000 to get plants per acre. Use the figures in the below tables for this operation.

The results from this 2-year study determined that the highest soybean yields were obtained from initial stands that were 100 thousand plants/acre or greater. This was confirmed when they used seeding rates of 40, 60, and 80 thousand seeds/acre with no replanting, and yields were 10, 5, and 4 bu/acre less than the maximum yield, respectively. Replanting where initial stands were less than 100 thousand plants/acre significantly increased yield, but not to levels achieved where initial stands were greater than 100 thousand plants/acre and no replanting occurred.

Thus, the authors concluded that their results indicated the threshold for replanting soybeans is 100 thousand plants/acre. Remember, these results were obtained in a part of the US where planting date is usually not as critical for yield potential as it is in the Midsouth.

This study went a step further than just determining the replanting threshold. This involved two scenarios.

1.  Filling in the initial stand by replanting between the rows of the failed stand, and

2.  Destroying the failed stand and replanting the entire site.

From this portion of the study, they concluded that filling in below-threshold stands using the process in no. 1 is the best replanting method to ensure the highest yield as long as the replant seeding rate is high enough to increase the final stand to over 100 thousand plants/acre. Using the method in no. 2 greatly limited yield potential because of the later planting date.

They also concluded that using a seed treatment had no effect on replant decisions. However, they did state that using a seed treatment (especially fungicide/insecticide treatments) with the initial planting may help avoid replanting because of the likelihood that plant stands will be enhanced in the initial planting.

This study did not evaluate the economics of any of the replant decisions, or the crop insurance implications that may affect those decisions. For a detailed review of this research, click on the above link to the article.

These results are from a study that was conducted in the Northern US Corn Belt. However, it is likely that indeterminate soybean varieties that are replanted in the Midsouth will follow the same general trends as those in these results. This is especially so for the later planting effect that will result from replanting a failed stand that is destroyed.

Dr. Trent Irby recently provided results from research (p. 18-21) that was conducted as part of the MCES SMART program. A summary of those results follow.

•   Experiments were conducted at two East Miss. locations in 2016 and 2017. An indeterminate MG IV variety was planted at a rate of 130,000 seed/acre in 38-in.-wide rows..

•   Initial planting dates ranged from Apr. 21 to May 12. All replants occurred withing 3-4 weeks of the initial planting dates.

•   All plantings were irrigated as needed.

•   Results indicate that producers should keep a stand that is 50% or more of the plant population in a stand with normal emergence. In these studies, this translated to keeping stands from initial plantings that are > 60,000 plants/acre. This is evidence of the importance of early planting to achieve maximum soybean yield potential.

•   If initial stands are < 50% of a plant population in a stand with normal emergence, best yield results are obtained if the initial stand is completely destroyed prior to replanting.

Click here for results from an MSPB-funded study that provide guidance on addressing reduced stands in twin-row soybean plantings.

If replanting is decided, there may be a need to reconsider the maturity group (MG) of the variety or varieties that will be planted. Results from recent research indicate that a MG IV variety will perform well when planted during a wide window that will go through the end of May. However, producers should use the SOYMAP resource to guide them in this decision (Click here for details about how to use SOYMAP and links to the accessory resources for this tool). This tool allows a producer to enter the planting date and latitude of the location that will be planted to find the best MG for that planting date at the designated location. It may be that the variety or varieties already on hand to be planted will still be the best choice, but this tool will provide guidance for determining if that is the case (click here for a Planting Date White Paper on this website).

All of the above information should be used as a guide to a replanting decision since there are many variables that will affect that decision. The most important things to remember are:

•   The initial stand must be accurately assessed to determine the number and condition of emerged plants. Remember, uniform distribution of healthy plants in a reduced stand is the key.

•   Any replanting of a less-than-desired stand from an initial planting should take into account the lateness of the replant and how that may affect yield potential even with an optimum plant population in the replant.

•   Results from numerous other studies indicate that a healthy and uniform soybean stand of 80,000 plants/acre is sufficient for maximum soybean yield potential, so this plant population is a good benchmark to use to determine stand sufficiency in most cases.

•   Replanting soybeans without recompense will add to the total cost of the crop and thus will reduce realized net return regardless of yield realized from the replanting.

•   Extra attention should be paid to protecting a thin stand in a retained initial planting to ensure that any abiotic (e.g. weather) or biotic (e.g. insects, diseases) stresses that may occur are minimized. In other words, a retained initial planting with a minimum plant population cannot afford to lose more plants.

A final note. Replanting a failed soybean stand in the Midsouth is never preferred because of the associated extra cost and the potentially lower yield from the replant. Therefore, it is imperative that a producer accurately determine that destroying and replanting a failed initial stand is the best option based upon points in the above discussion.

Composed by Larry G. Heatherly, May 2019, larryheatherly@bellsouth.net