Planting Soybeans Behind the Flood

It is now June 13, 2021. The recent inundative flooding of a large acreage in the Arkansas and Mississippi Delta left soybean producers with a dilemma.

For those who had planted soybeans and lost an established crop, a completely different production scenario now exists. For those who had not planted, intended planting date will be delayed, and this too will change intended management practices.

Two basic questions come into play regarding planting soybeans in this situation. 1) Can soybeans be planted behind the flood with a reasonable expectation of harvesting a profitable crop? 2) Will the remaining growing season length be adequate for soybean development to R7 or physiological maturity?

The answer to the first question is a qualified YES, especially with high commodity prices. However, yield potential will be greatly reduced to estimated maximums of 35 bu/acre in nonirrigated plantings and 50 bu/acre in irrigated plantings. These estimated maximums assume a high level of management and average or above-average rainfall for the remainder of the growing season. Of course, yields could be lower or higher than these estimates depending on level of management and weather for the remainder of the growing season.

To answer the second question, determine the estimated length of growing season that can be expected based on projected planting dates behind the flood coupled with estimated first fall frost date.

Data in the below table provide an estimate of growing season length at six Mississippi locations based on planting dates ranging from June 20 to August 10 and the estimated date of the first fall frost (36 deg.). Keep in mind that a first fall freeze (32 deg.) is estimated to occur 10 days to 2 weeks later than the estimated first frost dates shown for each location. Also keep in mind that, even though a killing frost may not occur on the dates shown in the table, temperatures at this level will likely signal the end of any appreciable plant activity.

The following narrative is based on using the 50% first frost date (an earlier date than the one shown is expected to occur in 5 years out of 10) at these locations as the projected end of a growing season. Stage R7 or physiological maturity is the reproductive stage beyond which a frost will have little or no effect on yield.

•    For a June 20 planting date, the estimated growing season length exceeds 128 days at all five locations. Therefore, MG IV through VI varieties can be planted without fear of frost injury before maturity.

•    For a July 1 planting date, the estimated growing season length exceeds 118 days at all five locations. Therefore, MG IV through VI varieties can be planted without fear of frost injury before maturity.

•    For a July 20 planting date, the estimated growing season length is between 99 and 110 days. MG IV through V varieties will have reached R7 by this time. MG VI varieties are probably a safe selection at the southernmost locations in the table.

•    For an August 10 planting date, the estimated growing season length is estimated to be less than 90 days at all locations. MG IV varieties should have safely reached R7 by this time.

•    I am not aware of any ultra-late plantings where maturity dates were recorded. The information above should only be used as a guide. Deciding which MG’s/varieties should be chosen is an arbitrary decision. Use SOYMAP and SOYSTAGE to refine MG choice and length of growing season for varieties in each MG.

A publication titled “After the Flood: Row Crop Replanting–MSU-ES Pub. 2680” that was published in 2011 by the MSU Extension Service offers helpful tips for managing soybeans that are planted late following receding flood waters.

Composed by Larry G. Heatherly, Updated June 2021, larryh91746@gmail.com