What to Expect with August Soybean Plantings

Canoeing a soybean field. Photo: Adam Barnes,
Livingston County ANR Agent

Carrie Knott, Extension Agronomist-Princeton, University of Kentucky

Chad Lee, Extension Professor and Agronomist, University of Kentucky, Lexington

Many river and creek bottoms in Kentucky have flooded existing crops due to significant rainfall the past 2 months.  As a result many producers are asking: 

How late can I re-plant soybeans?  

What maturity group should I plant?

Both questions will come with the "It depends....." qualifier.

How late you can plant will depend on the yield loss you are willing to sacrifice.  University of Kentucky research indicates that yield losses can be significant beginning with June plantings (Tables 1 & 2).  Regardless of maturity group yield losses of about 15% can be expected with early June plantings, about 25% yield loss in late June, and almost 40% with early July plantings.  Additional research has also shown that yield losses of about 1.5% per day are expected beginning in June (Figure 1).  These estimates can easily be used to determine expected yield loss for each specific production system.

Let's assume an expected full-season yield of 96 bu/A.  If an August 1 re-plant occurs the expected yield is: 

Calculate Yield Loss: 

July 6 planting = 37% yield loss

July 6 to Aug 1=26 days x 1.5%/day=39%

37% + 39% = 76% yield loss

Calculate Expected Yield:

96 bu/A x (1-0.76) = 23 bu/A

If we assume an expected double-crop yield of 65 bu/A planted on June 20, re-planting August 1 would result in follow yield expectation:

Calculate Yield Loss: 

June 20 to Aug 1=42 days x 1.5%/day=63% yield loss

Calculate Expected Yield:

65 bu/A x (1-0.63) = 24 bu/A

The expected yields calculated are assuming fairly high average yields.  Be sure and use your own farm yield averages to determine expected yield for your system.

In terms of maturity group selection, at this point in the season you will be limited to what your local seed supplier has on hand.  If you do have a choice, you will have to 'pick your poison' so to speak. Early maturities, late-group II or early-group III, are at risk of podding on the soil surface while later maturity soybeans, mid- to late- IVs, are at risk of frost or freeze damage in the fall.  A very late frost would allow the Group IV's to escape damage but there is no way to accurately predict when a killing frost will occur. The odds are against Group IV's at this point.

The best way to determine if you should re-plant is to get out your pencils (or the calculator on your smart phone).
1. Calculate expected yields for your farm. (Be realistic. We're not trying for a yield contest at this point.)
2. Calculate all input costs:
           Don't forget the cost of
                   Your time
                    Fuel
                    Equipment Usage
                    Seed
                    Chemicals
                    What else??
3. Calculate the expected profit/loss.
4. Make your decision.

It is very difficult to see a field that is not planted and do nothing, but in this case it may be more profitable to simply keep weeds under control by mowing and/or spraying than to try and re-plant soybeans this late in the season.

Table 1. Soybean Yield and Yield Loss of Mid-Group IV Soybean Variety Planted on Different Dates, 2008-2011.

Table 2. Soybean Yield and Yield Loss of Late-Group II Soybean Variety Planted on Different Dates, 2008-2011.

Figure 1. Expected Daily Yield Loss for Late-Planted Soybeans.