Soybeans: Replant Decisions

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Replant Decisions
   • Compensation and Plant Spacing (Gaps)
   • Evaluating Stand Reductions
   • Evaluating Returns From Replanting
   • Patching or Thickening Thin Stands
2. Updates on Soybeans: Replant Decisions
3. Related links...

Replant Decisions

Assessment of a poor soybean stand should not occur too quickly. Fields with a plant reduction of 50% do not need to be replanted if plant loss is uniform and the stand is healthy. Every year, soybean stands are replanted needlessly. Numerous studies and field experience have demonstrated that keeping an existing stand was often far more profitable than replanting. Replanting gives no guarantee of a perfect stand.

There is no formula on which to base a replant decision. Every decision is based on factors surrounding individual situations. Information needed to make a replant decision includes:

• The population and health of the existing stand. Normal seeding rates include a margin of safety to ensure emergence of an adequate stand.
• The cause of the low plant population. A number of factors can cause reduced soybean stands. These include soil crusting, herbicide injury, frost, hail, insects and diseases. 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, the use of a Phytophthora-resistant variety plus a seed treatment should be considered.
• The uniformity of the remaining plant stand.
• The yield potential of the existing stand compared to the yield potential of a replanted stand. Yield potential begins to decline after the optimum planting date and declines throughout June (see Table 4-7, Effect of Planting Date on Yield, Plant Height and Maturity).
• The costs of replanting and possibly additional weed control costs in thin stands.

Figure 4-2.
Reduced Stand in the Field

On June 15, a reduced stand of solid seeded (18-cm rows) soybeans had an average 10 healthy plants per hoop (measured using a 76 cm (30 in.) diameter hula hoop). This equates to a population of 222,220 (10 x 22,222 plants per hectare) (see Table 1-3, Determining Plant and Pest Populations Using a "Hula" Hoop). The yield potential of this stand is 90% of a full stand (see Table 4-11, Expected Yield of Soybeans in Optimum and Reduced Stands). Replanting is not justified (due to lower yield potential and higher cost).

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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. Ontario research has found that a 33% reduction in the stand (distributed uniformly over the field) will not significantly affect yield.

Plants in thin stands branch profusely, making them heavy and more prone to lodging. Branched plants tend to bear more of their pods near the ground. Consequently, harvest losses can be slightly higher in these stands. In trials with thin stands, lodging did not become a problem until populations dropped below 60% of a full stand. Stand reduction is rarely uniform.

Evaluating Stand Reductions

It is important not to make a quick decision about replanting. Accurately assess the stand for the population, spacing and health of the remaining plants. To determine plant population, refer to the hula-hoop method mentioned in the section Determining Plant Population and Pest Infestation Levels.

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Evaluating Returns From Replanting

The numbers in Table 4-11, Expected Yield of Soybeans in Optimum and Reduced Stands are used to determine the percentage yield for a field that was planted on time but suffers some stand reduction. This yield potential will only be realized if the plants remaining in the field remain healthy and free of weeds. Compare the potential return of leaving this stand to the returns from a delayed replanting.

Patching or Thickening Thin Stands

In cases of poor stand establishment, replanting alongside the established seedlings to patch up or thicken the existing stand seldom improves yields. Repair planting often leads to timing difficulties with weed control and harvest date. When dealing with unacceptably thin stands, significant yield improvement was only achieved when the original soybean stand was so poor that it had to be destroyed and a new stand re-established.

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Updates on Soybeans: Replant Decisions

No updates available at this time.

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Related links...

• Soybeans

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