Soybean Growth and Development & Management Information for Replant Decisions

Determining the Yield Potential of Remaining Plants

Research in Minnesota has shown that the plant population can vary widely with little effect on yield. Soybean plants can produce good yields at low populations by setting more pods per plant and filling more of the seeds in each pod. Branch number also increases at low populations to provide more pod sites. At plant populations below 50,000 plants per acre, yield from the branches is significant. Some of these branches can break off the main stem prior to harvest and increase harvest loss, so extra care is necessary during combining.

Plants may not be uniformly distributed in lower plant populations. Uneven spacings or gaps between plants can result in lower yield (the amount depends on the size and number of gaps). A low population of uniformly spaced plants can yield equal to an evenly spaced normal stand, but yields can be lower if large gaps are present.

Stand reduction measurements are used to determine potential yield loss and assist in a replant decision. Extensive research in various areas of the United States has shown that yields are not affected by population reductions until they drop well below 125,000 plants per acre. Table 3 shows that stand reductions and gaps created at various times during the growing season have little effect on final soybean yield.

The first step in determining the number of remaining live plants is to carefully examine plants from various parts of the field. Determine the stage of growth now and at the time the damage occurred (use the previous material on growth and development to assist you in this task). Then carefully examine several plants to determine their potential to recover from the damage.

Plants which are cut off below the cotyledons, as shown in Figure 4 , will never recover because there are no axillary buds to provide regrowth. In your plant population evaluation these should be considered as dead.

Figure 4. Soybean stem cut off below cotyledons.

While some of the plants damaged as shown in Figure 5 eventually die, most can regrow from one or both of the axillary buds located at the cotyledon attachment point to the main stem. The rate and intensity of regrowth is influenced by the amount of remaining cotyledon tissue which supplies energy for this process. Three situations are shown. In order of probability of regrowth A > B > C because of the relative amounts of cotyledon tissues present to supply the energy required for regrowth.

Figure 5. Soybean stems cut off immediately above the cotyledons, with various amounts of cotyledon tissue remaining.

Plants cut off above the unifoliolate leaf node can recover by growth from any of the four axillary buds present on the main stem. Functional green leaf tissue and cotyledons are important in generating energy to drive regrowth. Even though the leaves are shredded and torn they can still supply energy to the developing growing points. Axillary buds usually develop rapidly after the main stem is cut off. Usually one new stem becomes dominant and can be mistaken later for an original stem unless you examine the base of the plant carefully. This initial regrowth can be visible within 3 or 4 days if conditions are favorable. Figure 6b shows a typical regrowth pattern 10 to 14 days after cutoff damage similar to that shown in Figure 6a .

Figure 6. Soybean plant with a main stem cut off immediately above the unifoliolate node and a typical regrowth pattern.

Regrowth of damaged plants is sometimes very slow and often is slower than replanted soybeans, especially if large amounts of leaf tissue have been removed and hot, dry conditions prevail. This can result in a slight delay in maturity of a hail damaged field because of the time spent in the recovery period.

In some previous publications, leaf loss and amount of damaged leaf tissue was a part of the yield loss determination process, but extensive research has indicated that leaf damage during the early vegetative (V) stages has an insignificant effect on final yield because of the soybean plants ability to compensate for early season damage.

Using all this information, carefully reexamine plants in several areas of the field and count the number of LIVE PLANTS per foot of row (make several of these counts in various areas of the field) then use Table 4 to determine the plant populations per acre for your row width. Then determine the expected yield for this population using Table 3 .

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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