Compared to corn and wheat, soybeans are more prone to poor stand establishment because the seedling must pull the cotyledon seed leaves through the ground to emerge. Deciding whether it is worth replanting a poor stand can be difficult because plant stand reductions are rarely uniform. Often, it is best to treat parts of a field separately. Do not assess a poor soybean stand too quickly, since more seedlings may still emerge. Fields with a plant reduction of 50%, do not need replanting if plant loss is uniform and the stand is healthy, see Table 1, Expected Yield of Soybeans in Optimum and Reduced Stands. Numerous studies and field experience have demonstrated that keeping an existing stand is often more profitable than replanting. Replanting also gives no guarantee of a perfect stand.
Table 1. Expected Yield of Soybeans in Optimum and Reduced Stands
| % Of Full Stand | Row Spacing | Expected Final Yield as % of Optimum | |||
| 18 cm (7 in.) | 36 cm (14 in.) | 53 cm (21 in.) | 76 cm (30 in.) | ||
| 100% | 553,300 plants/ha (223,900 plants/acre) | 402,600 plants/ha (162,900 plants/acre) | 392,700 plants/ha (158,900 plants/acre) | 405,100 plants/ha (163,900 plants/acre) | 100% |
| 80% | 442,100 plants/ha (178,900 plants/acre) | 323,600 plants/ha (131,000 plants/acre) | 131,700 plants/ha (127,00 plants/acre) | 323,600 plants/ha (131,000 plants/acre) | 100% |
| 60% | 331,000 plants/ha (134,000 plants/acre) | 242,100 plants/ha (98,000 plants/acre) | 237,100 plants/ha (96,000 plants/acre) | 244,500 plants/ha (98,900 plants/acre) | 100% |
| 40% | 222,300 plants/ha (90,000 plants/acre) | 160,600 plants/ha (65,000 plants/acre) | 158,100 plants/ha (64,000 plants/acre) | 163,000 plants/ha (66,000 plants/acre) | 87% |
| 20% | 111,200 plants/ha (45,000 plants/acre) | 81,500 plants/ha (33,000 plants/acre) | 79,000 plants/ha (32,000 plants/acre) | 81,500 plants/ha (33,000 plants/acre) | 62% |
Compensation and Plant Spacing (Gaps)
Soybean plants have an amazing ability to compensate for thin stands. Soybean plants can fill interplant spaces up to about 30 cm (12 in.) within or between rows without any yield loss, provided weeds do not compete for this space. According to studies in Ontario, a 33% stand reduction spread uniformly over the field will not have an impact on yield.
Plants in very thin stands branch profusely, making them heavy and more prone to lodging. Branched plants tend to bear more of their pods near the ground. As a result, these stands may experience greater harvest losses.
Every decision to replant is based on specific field’s circumstances. When deciding whether to replant, the following information is required:
- The population and health of existing stand. Normal seeding rates include a margin of safety to ensure emergence of an adequate stand.
- The cause of the low plant population. Several factors such as soil crusting, herbicide injury, frost, hail, insects, and diseases can reduce soybean stands. For instance, in a wet year, damping-off is likely to be caused by two fungal classes – Pythium and Phytophthora. In this situation, if the stand is to be replanted, consider the use of a variety resistant to Phytophthora plus a seed treatment. If seed corn maggot have destroyed the stand, an insecticide seed treatment is necessary when replanting.
- The uniformity of the remaining plant stand.
- The yield potential of the existing stand to the yield potential of the replanted stand. Yield potential begins to decline after the optimum planting date and declines throughout June.
- The cost of replanting and additional weed control in thin stands.
Evaluating Stand Reductions
Accurately assess the population, spacing, and overall health of the remaining plants. Refer to Factsheet, Methods for Determining Plant and Pest Populations, for information on calculating plant populations. In narrow rows the hoop method is usually quicker, but in wide rows counting the number of plants per foot of row is more accurate. Figure 1, demonstrates a hoop being used to count the number of live plants.
Figure 1. A hoop can be used to quickly determine the number of plants per acre.
Table 1, Expecting Yield of Soybeans in Optimum and Reduced Stands, provides an estimate of the yield potential compared to a full stand. It is important to note that Table 1 is based on the number of healthy plants remaining in a thin stand, spaced uniformly, and kept free of weed competition.
Calculating Returns from Replanting
To calculate returns from replanting, the following is required:
- Estimate the yield from a full stand using the initial planting date.
- Determine the population of the existing stand. See Factsheet, Methods for Determining Plant and Pest Populations.
- Estimate the yield potential of the reduced stand. See Table 1, Expected Yield of Soybeans in Optimum and Reduced Stands.
- Estimate the yield potential of the replanted full stand. A later replanting date will have a reduced yield potential. See Factsheet, Planting Date and Soil Temperature.
- Calculate the cost of replanting.
- Evaluate the value of reduced stand to replanted stand.
Example:
If there was a full plant stand in a field planted on May 12th, the estimated yield potential is 3 t/ha (45 bu/acre). When assessing a thin field on June 5th, the average population of a reduced stand of solid-seeded soybeans in 18-cm row spacing (7-in. row) is 222,220 plants/ha (90,000 plants/acre). The yield potential of this stand is 87% (2.6 t/ha or 39 bu/acre) of a full stand based on Table 1, Expected Yield of Soybeans in Optimum and Reduced Stands). Replanting on June 6th, the yield expectation would be about 2.8 t/ha (41 bu/acre) due to the later planting date (3 t/ha or 45 bu/ac x 92% = 41 bu/ac), from Factsheet, Planting Date and Soil Temperature. Replanting would not be justified in this situation given the cost of seed and planting, as well as the risk associated with replanting.
Soil Crusting
Following a driving rain or ponding water, crusting can occur which can inhibit soybean emergence. The crust can break the hypocotyl arch (the portion of the plant that lifts the cotyledons above the soil surface). If soil is prone to crusting and there is heavy rain, plan to break the crust before the seedlings are attempting to break through. Often, “Crust-busting” is carried out too late to improve plant stands. Soybean emergence and soil crusting can be aided by light tillage using a rotary hoe, harrows, coulter cart, planter, or seed drill. Some growers have reported that running a 76 cm (30 in.) row unit planter over a crusted field is sufficient to bust the crust. As a result of these operations, there will be at least a 10% loss of emerged beans. A higher stand loss will occur when the hypocotyl arch is breaking the surface. In uniformly thin stands (e.g., 60%) where full yield potential already exists, “crust-busting” is not be required,
Patching or Thickening Thin Stands
Thickening a stand is preferable to removing a thin stand and starting over. However, replanting alongside the established seedlings to patch up or thicken the existing stand only improves yields when the stand is very thin. The planted seedlings are so far behind in development that they are unable to compete with even a thin original stand. However, to optimize yields, thickening a thin stand is a better option than removing the original stand of thin soybeans. If patching or thickening is contemplated, use the same variety, and do not destroy the original stand.
Normally, a plant stand of 220,000 plants/ha (90,000 plants/acre), in 19 cm (7.5 in.) rows or more does not need to be patched or thickened. Very heavy clay soils need to have less than 250,000 plants/ha (110,000 plants/acre) before thickening is worthwhile.