As soybeans emerges, take time to evaluate soybean stands. This involves taking stand counts and visually assessing seedlings to see growth and development appears normal. This can help alert us to potential issues that may due to planting, insects, diseases, and/or environmental.
Stand counts in soybeans are very similar to stand counts in corn. Soybean stand is not as critically linked to yield as with corn; however, early season stand counts are important to evaluate the germination and emergence of the planted soybeans. Several helpful methods are available to estimate soybean stand. The traditional method is identical to the method most commonly used for corn, however, because soybean populations are much higher than corn, soybean stand counts are often taken using an alternative approach with estimates based on soybean per foot or using a hula-hoop method to reduce the number of soybean being counted.
Whichever method is used to obtain an estimate of plant stands, repeat the process several times throughout the field to provide a more complete picture of the average plant population as well as identifying good and bad areas of the field. Do not intentionally avoid areas with missing plants, but if one stand count seems unusually low or different than other counts, keep that one separate and make note of it. Emerged plant populations are often 10-20% lower than the original seeding rate. With early-season soybean planting, a final stand of 100,000 evenly spaced plants is a desired, but soybeans have a significant ability to branch and yield well even in thinner stands. Additionally, while taking stand counts be looking at emergence uniformity (differences in crop stages), plant spacing, and seedling vigor.
Traditional Stand Count Method
- Measure out 1/1000th of an acre based on your row width. For example, with a 30-inch row spacing, measure out a length of 17 feet, 5 inches.
- Count the number of live plants in the measured area.
- Repeat this process multiple times across the field to provide a more accurate estimate of the number of plants per acre.
- Average the counts and multiply the average number of plants by 1,000 to obtain the plant population per acre.
- When taking stand counts, randomly select these locations and do not intentionally avoid areas that have missing plants or gaps. Try to avoid selecting rows representing the same planter row unit. If one count does not seem to fit the other counts, keep that count separate and make note of where that area is at in the field. Additionally, be looking at emergence uniformity (differences in crop stages), plant spacing, and seedling vigor.
Alternative Stand Count Methods
It is not uncommon to see 20-inch, 15-inch, 10-inch, or sometimes even 7.5-inch row spacing soybean in Iowa. Taking stand counts using the traditional method above can be arduous if soybeans are planted in narrow rows, so an alternative method may be more conducive to quickly getting stand count estimates.
Counting plants per foot of row: One alternative is to estimate the total plant stand by counting of soybeans in an individual foot of row using Table 2. Like other methods, multiple transects are necessary to get a representative estimate of the stand across a field. These counts can be done more quickly than measuring 1/1,000 of an acre, but it may require more counts if stands are particularly uneven. It is recommended to count the number of soybeans in three to five feet of the row to get an estimate of beans per foot. This approach minimizes bias to avoid areas with poor plant spacing, large gaps or thin plant populations.
Hula Hoop Method: Another alternative method to take stand counts uses a hula hoop. To use this method, toss a hula hoop to arbitrarily select an area for a stand count. Count the number of plants inside the hula hoop and multiply the number of plants by a multiplication factor to estimate the number of plants per acre (Table 3).
If a hula hoop does not have the exact diameter of those listed in the table below, determine the estimated stand by using the equation [Population = Plants Counted / (3.14 * hoop radius * hoop radius / 43,560)]. There are also tables available with plant populations already calculated based on the number of plants and hula hoop diameters (see Table 3 in this link).
Investigating poor stand assessments
If an area has a lower plant stand, uneven emergence, variable plant spacing, or poor seedling vigor, investigate what may be causing that to occur and how widespread it appears to be in the field. While some issues may not be remedied this growing season, determining the cause of poor stands assessments can help prevent or minimize that issues from happening again in the future.
Dig up seedlings and check the plants for symptoms of the following:
- Seed rots and seedling blights
- Herbicide injury
- Insect issues like seedcorn maggot, black cutworm, true armyworm, or even a non-insect pest like slugs, millipedes, and isopods
- Planter issues (seed depth, sidewall compaction, seed drop and seed spacing)
Soybean Replant Considerations
If the emerged soybean population is significantly lower than expected, replanting or, more likely, “thickening up” soybean may be considered. Unlike in corn, rarely do soybean fields require replanting; however, planting additional soybean on an offset pattern (typically angled or perpendicular) from the original rows is usually sufficient to improve stands. Due to the soybean plant’s growth habit and reproductive capabilities, soybean can coexist at different crop development stages without an issue.
In order to determine whether replanting is likely to be of benefit, consider the following factors:
- Yield potential of the current stand for the original planting date using tables in Soybean Replant Decisions (PM 1851).
- Yield potential of a replanted stand at the replanted date of planting and estimated plant population using tables in Soybean Replant Decisions (PM 1851).
- Compare the original yield potential with the replanted yield potential. Be sure to factor in replant costs like seed, fuel, time/labor, and additional weed control.
- In general, replanting should be considered if uniform stands are less than 75,000 plants per acre when planted prior to mid-May or less than 50,000-60,000 plants per acre when planted late May into June.
- Soybeans can compensate for low stands through increased branching and pod set.
- Soybean seed treatments are often not necessary for replanted soybeans due to warmer soils at the time of planting but consider a fungicidal seed treatment if the field is prone to seedling diseases.
- Low or uneven soybean populations may be more susceptible to weed issues due to lack of early-season suppression from the plant canopy, so additional postemergence herbicide may be necessary.
- Reference herbicide labels for any products already applied to the field to be certain residual products do not disallow replanting.
- Maturity selection, seeding rate, and row spacing may all need adjustment due to the shorter timeframe to harvest and lack of competitive ability of late planted soybeans. If already planting a well-adapted soybean variety to the area, do not change maturity selection unless replanting occurs in late June, but do consider increasing seeding rate or narrowing row spacing to allow for more competition with weeds.