Assessing Soybean Stands

Assessing Soybean Stands: Evaluating soybean stands soon after emergence provides information to determine if adequate populations exist for maximum yield potential. It may also be necessary to evaluate stands after hail storms to see if significant damage occurred that requires replanting or adding additional plants to the field. Guidelines are listed below for assessing soybean stands with additional information on maturity selection and yield potential by planting date.

Table 1: Using a hoop to take stand counts.

Stand counts can be taken a number of ways and using a hoop is an easy way to get several counts easily. It is important to take multiple counts across all affected areas to best determine current stands.

Hoop Diameter 28″ 30″ 32″ 34″ 36″
Multiplication factor 10,192 8,878 7,808 6,912 6,165

Formula for calculating area of a circle:

Factor = 43,560/ (inch radius of hoop2 x 3.14)/ 144

Table 2: Length of row equal to 1/1000th acre for several row widths.

Row Width (inches) Length of Row Needed to Represent 1/1000th Acre
30 17’ 5″
20 26’ 2″
15 34’ 10″
10 52’ 3″
8 65’ 4″
7.5 69’ 7″

 

Stand counts can also be taken by measuring the distance equal to 1/1000th acre and counting the emerged plants in that area. This can be difficult in narrow rows as the length equal to 1/1000th acre increases as row width narrows. While it’s possible, but more work to measure off the 1/1000th acre in narrow rows using a shorter length and calculating plants per foot of row also provides a good estimate of stand establishment. For instance, stands can be estimated by counting the number of plants in 10 feet of row and then calculating the number of plants per foot. That number of plants per foot can be multiplied by length of one row that is equal to the number of square feet per acre. See table 3 below.

Table 3: Multiplying factor for plants per foot to equal plants per acre.

Row Width (inches) Multiply plants per foot by the factor for row width to estimate plants per acre
30 17,424
20 26,136
15 34,848
10 52,272
8 65,340
7.5 69,696

Using the data in Table 3, one plant per foot in 7.5″ rows equals a final stand of 69,696 plants per acre.

Once the stand data is collected, then the chart below can be used to estimate the yield potential of the existing stands. Other factors to consider are original planting date and replanting date, stand uniformity, soil conditions, especially soil moisture, soybean maturity and the costs of replanting. The table below provides an estimate of yield potential for various stand counts and row widths. Data is from the University of Nebraska.

Table 4: Stand reduction/yield loss chart.

 

 

 

 

 

Both the original planting date and the potential replant date need to be considered when making a replant decision. As with corn, delayed planting negatively impacts soybean yield potential. Generally, the percentage of decline in yield loss per day of delayed planting is not as steep with soybeans as corn. The data from Purdue University in the table below shows yield potential for several planting dates.

Table 5: Effect of planting date on soybean yield for mid and full-season soybean varieties.

Planting Date                        Yield as Percent of Normal (mid-season variety)                        Full-Season Variety
May 20                                    100                                                                                                                        100
May 30                                     96                                                                                                                          94
June 10                                     92                                                                                                                         90
June 20                                    82                                                                                                                          78
June 30                                    70                                                                                                               Not Recommended
July 10                                     60                                                                                                                Not Recommended

Factors to Consider:

1. Estimate of optimum yield for field with normal stand ____

2. Estimate of reduced stand yield ____ (Table 4)

3. Estimate of yield from delayed planting ____(Table 5)

4. Projected costs of replanting for: seed, fuel, machinery, labor, additional herbicides _____

5. Compare the estimate for the yield of the reduced stand to the estimate of yield from replanting-minus the costs for replanting. If the potential income from replanting minus the added costs is greater than the gross income estimate for the reduced stand then replanting is a good decision.

References: University of Nebraska, Purdue University

DW: 5/2015

 

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