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Factors that can contribute to less than ideal soybean stands include: planting into a poor seedbed, planter adjustment problems, poor quality seed, soil crusting, inadequate or excessive soil moisture, seedling diseases, and numerous environmental issues. Understanding the cause of an inadequate soybean stand is important to help prevent a reoccurrence of the problem if the decision is made to replant. Spotty stand reductions throughout a field can be caused by poorly drained areas, sandy soil patches with inadequate soil moisture, or even soil compaction in certain areas. Before deciding to replant, estimate the stand for population and uniformity, and examine yield potential of the existing stand. When evaluating soybean stands, only count plants that have a good chance of survival. Soybean plants cut off below the cotyledon by hail, or other means, have no potential for regrowth. However, soybeans are able to grow out of some leaf tissue damage with minimal effect on yield potential.
To evaluate the plant population for 30-inch rows, count the number of plants in 17 feet 5 inches of row and multiply the number of plants by 1,000 to determine plants per acre. For 15-inch rows, count the number of plants in 34 feet 10 inches of row and multiply by 1,000. Repeat these counts in several locations in the field.
Another method for evaluating soybean stands in any row spacing, especially drilled, is to use the hoop method. Measure the diameter of the hoop, toss it in the field and count the number of plants inside the hoop. Do this in at least 5 to 10 locations in the field. Multiply the average number of plants by the appropriate factor listed in Table 1 to determine the number of plants per acre. Notice that having a diameter of 28¼ inches allows you to simply multiply by 10,000 to obtain the number of plants per acre. This hoop size can be made by cutting anhydrous tubing to 88¾ inches and joining it to form a circle.
Numerous studies have examined the yield potential of various soybean stands. What appears to be a significant soybean stand reduction does not automatically translate into a significant loss of yield potential. Soybean plants are well adapted to compensate for gaps in the field. Gaps of less than 2 feet in diameter can be filled in by branches of adjacent soybean plants. A summary of yield potential from reduced stands is presented in Table 2. Established plant stands of 8, 6, and 4 plants per foot of row in 30-inch rows (equals approximately 140,000, 105,000 and 70,000 plants per acre, respectively) differed in full yield potential by only 5 percent. Final soybean plant stands of 73,000 plants per acre or more consistently yielded 90 percent or more of maximum yield potential. With an initial planting population of 140,000 plants/acre, a 50 percent stand loss resulted in a 16 percent yield loss in areas of the field with no skips or gaps. With an initial planting population of 70,000 plants per acre, a 50 percent stand reduction resulted in a 22 percent loss of yield. Assuming an original yield potential of 60 bu/acre, a field planted at 70,000 plants per acre with a 50 percent loss of stand could still yield 47 bu/acre.
Reduced soybean stands allow more light to penetrate to the soil surface. This increases the potential for more weed seed germination and weed competition, as well as soil water evaporation and increased soil temperature. All of these factors can affect nodulation, biological nitrogen fixation, and nutrient and water availability. Make weed control a high priority, especially in fields with reduced stands, in order to help maximize the yield potential of the existing crop.
Whigham, K., Farnham, D., Lundvall, J., and Tranel, D. 2000. Soybean replanting decisions. PM 1851. Iowa State University. https://store.extension.iastate.edu/Product/Soybean-Replant-Decisions. Davis, V. M. 2010. Soybean seeding rates for 2010. The Bulletin, no. 4. University of Illinois. http://bulletin.ipm.illinois.edu Davis, V. M. 2010. More on soybean stands and the decision to replant. The Bulletin, no.8. University of Illinois. http://bulletin.ipm.illinois.edu Web sources verified 4/21/2016. 130516023043