Part 1, Section 6: Soybean

Section 6 Table of Contents

Soybean

ESTABLISHMENT

Replant Decisions

Deciding to replant a soybean crop is difficult and involves many interacting factors: crop growth and development characteristics, plant response to reduced plant populations, weather conditions, soil conditions, calendar date, distribution of existing plant stand, cost/price relationships, and the potential yield of the existing stand vs. the potential yield of the replant stand.

When making a replant decision, follow a set procedure that includes these factors:

  1. Estimate the yield of a full stand at the original planting date.
  2. Determine the population and distribution of the existing stand.
  3. Estimate the yield potential of the reduced stand.
  4. Estimate the yield potential of a replanted full stand.
  5. Estimate the cost of replanting.
  6. Compare the value of reduced stand to replanted stand.

The estimated yield of a full stand at the original planting date is the producer’s anticipated yield for the field under normal management practices. Faculty at the University of Delaware (B. L. Vasilas, et al.) have described a technique for evaluating a soybean stand. It is useful in estimating both the percentage of stand loss to gaps and the population in undamaged areas of the field. The procedure is based on pacing off sections of row and then pacing off gaps in those sections. A description of the method follows.

  1. Flag off sections of row at least 20 paces in length. Record the number paces. Do this in several locations in the field.
  2. Pace off each gap in the flagged area. Record the number of gaps one pace in length, two paces in length, etc. Not all gaps will be a whole number of paces. Round up or down to the nearest whole number. Do not count gaps less then one-half pace. Determine the total length (in paces) and the percent of row lost to gaps for each field location (see example in Table 1.6-3).
  3. Record the number of plants per foot in sections of row not reduced by gaps. Do this for several locations in the field. Use a tape measure to obtain an accurate count.
  4. Average, over all field locations, the percentage of row lost to gaps and the plants per foot of row in undamaged sections of row.
Table 1.6-3. Sample calculation for a section of row 20 paces long
Length of gap Number of gaps Length of row lost
Source: University of Delaware
1 pace 1 1 pace
2 paces 1 2 paces
3 paces 1 3 paces
Location total 6 paces
Length of row section = 20 paces
Length of row lost to gaps = 6 paces
Percentage of row lost to gaps = (6/20) × 100 = 30%

Using this information and the values in Table 1.6-4, you can estimate the yield of the reduced stand. The percentage of the stand lost to gaps is the average value obtained from the above procedure, and the remaining plants per foot of row is the average number of plants counted in the undamaged sections of row. For example, assume the following:

From Table 1.6-4, the estimated yield from the reduced stand would be 45 (50 × 0.9) bushels/acre.

Table 1.6-4. Yield response of full-season soybean (in 30- and 7-inch row spacings) to deficient stands.
  Remaining plants per foot of row
30-inch rows: 8 6 4
7-inch rows: 2 1.5 1
(Plants/acre (×1000) 140 105 70
Stand lost to gaps (percent) Full yield potential (percent)
Source: University of Illinois
0 100 97 95
10 98 96 93
20 96 93 91
30 93 90 88
40 89 86 83
50 84 81 78
60 78 75 73

The estimated yield from planting can be obtained from Table 1.6-5. Assume the original planting was on May 12, and the stand was evaluated on June 4 and could be replanted on June 5. The estimated yield would be approximately 45 bushels/acre, or the same as not replanting. Remember that the cost of replanting must be considered, and there is no guarantee that replanting will give a full stand.

Table 1.6-5. Yield response of soybean to changes in planting date.
Date Full yield potential (percent)
Developed form Ohio, Indiana, Minnesota, and Pennsylvania data.
May 10 100
May 20 98
May 30 95
June 10 88
June 20 76