Soybean Yield Expectations when Planting in July (and possibly August) in Kentucky

 

Carrie Knott, Extension Agronomist-Princeton, University of Kentucky

The 2016 soybean season has quickly become a challenge for much of the state. For soybean fields planted in June, stands were reduced because of the extremely dry conditions across much of the state. Some regions received less than 2” for the entire month of June. These struggling soybean stands were then inundated with significant rain starting over the Fourth of July weekend and not ending until last week. The official precipitation recorded by University of Kentucky Agricultural Weather Center was as much as 10” for parts of the state. However, localized reports of more than 16” occurred. This led to considerable flooding in the state or at a minimum extended periods of saturated soil conditions. As such, many producers are considering replanting soybean fields.

The first question on most minds is “How much yield should I expect from late-planted soybeans?”.
Based upon a 6-year (2006 to 2011) study that examined two relative maturity groups (late group II and mid group IV) the most recent estimate of Kentucky’s soybean yield loss per day is:

0.50% yield loss per day for mid group IV soybean beginning May 9 (Figure 1)

Figure 1. Expected soybean yield loss per day for mid-group IV relative maturity after optimal planting date of May 9.
Based upon 6 years of data with 7 planting dates. 

and 0.42% yield loss per day for late group II soybean beginning April 20 (Figure 2)

Figure 2. Expected soybean yield loss per day for late-group II relative maturity after optimal planting date of April 20.
Based upon 6 years of data with 3 planting dates. 

To calculate expected soybean yield, based upon planting date:

1.       Calculate the number of days after the optimal planting date

a.       Optimal Planting Date

                                                                           i.      May 9 for mid group IV

                                                                         ii.      April 20 for late group II

2.       Calculate expected yield loss

a.       Multiply the number of days past the optimal planting date by yield loss per day

b.       Yield Loss per Day

                                                                           i.      0.50% for mid group IV

                                                                         ii.      0.42% for late group II

3.       Multiply average soybean yield by expected yield loss

EXAMPLE 1: Plant a mid group IV maturity group on July 19

                Days past May 9, the optimal planting date, is 71 days

                                22 days in May + 30 days in June + 19 days in July = 71 days

                Expected yield loss, 0.50% per day x 71 days = 35.5% loss for a July 19 soybean planting

                Soybean yields average 80 bu/A for your farm

                              80 bu/A x 35.5% = 28.4 bu/A expected for a Mid Group IV soybean planted July 19

 

EXAMPLE 2: Plant a late group II maturity group on July 19

                Days past April 20, the optimal planting date, is 90 days

                                10 days in April + 31 days in May + 30 days in June + 19 days in July = 90 days

                Expected yield loss, 0.42% per day x 90 days = 37.8% loss for a July 19 soybean planting

                Expected soybean yield,

Your typical soybean average for late group II soybeans is 65 bu/A

                               65 bu/A x 37.8% = 24.6 bu/A expected for a Late Group II soybean planted July 19

 

Additional Considerations

When choosing a cultivar for July or August plantings consider one that is 0.5 to 1.0 relative maturity less than a full season cultivar.  This may help reduce the risk of frost damage in the fall, due to an early frost event. Full-season soybean cultivars range from 3.5 to 5.0 relative maturities in Kentucky. Relative maturities between 4.0 to 5.0 are typically considered a full-season soybean in far Western Kentucky (Purchase area) and southern Kentucky (south of the Western Kentucky Parkway). Relative maturities of 3.5 to 4.5 are more typical full-season soybean cultivars for the rest of Kentucky. 

Be aware that reducing the relative maturity of the soybean cultivar by more than about 1.0 relative maturity may increase the risk that the pods will be near or at the soil level. This is because the internode distances are typically reduced as planting date is delayed, which can have a much more dramatic effect on shorter relative maturities.                                 

The calculations above for expected soybean yield assume “normal” weather conditions occur after planting. That means adequate precipitation occurs, maximum temperatures are not excessive during flowering and seed pod development, and frost does not occur before soybean maturation. If any of these, or other stress events occur, soybean yield may be markedly lower than estimated above.