Planting soybeans has been and still is underway in the Midsouth, and 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 availability of seed of preferred varieties, the lateness of the 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 determine the stand from the initial planting that failed to emerge completely, or the number of healthy plants that remain after a severe weather event that caused significant injury to just-emerged seedlings. This should be done by actually counting the plants 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 the threshold for replanting soybeans is 100 thousand plants/acre.

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.

I realize that these results are from a study that was conducted in the Northern US Corn Belt. However, it is my opinion that indeterminate soybean varieties that are replanted in the Midsouth should 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.

Composed by Larry G. Heatherly, May 2015, larryheatherly@bellsouth.net