Assessing soybean stands is an important exercise to determine plant populations and uniformity that may affect soybean yields.
Assessments of soybean plant stands help identify issues with planter performance, evaluate the impact of planting conditions and determine the need for replanting. Poor plant populations can significantly reduce soybean yield, making it essential for farmers to annually evaluate soybean emergence.
Why it’s important
Poorly emerged soybean plant populations can be caused by several factors. They can be due to issues with planter performance. Uneven plant uniformity in the row or emergence dates can signal seed roll in the furrow before closing wheels pass over, or variable seed depth.
If this occurs, planter maintenance and adjustments in the offseason can help make corrections for next year’s planting season.
Unfortunately, poor plant emergence and spacing is not always a result of planter performance. Soil conditions at and following planting can impact soybean emergence. When combined with poor seed quality or nontreated seeds, this can have devastating effects on plant populations.
Cool soil temperatures also can put soybean seed in a vulnerable position for increased risk of disease. This is largely because early planting into cool soils results in more days between planting and emergence. Learning from planting conditions in one growing season can help inform farmers what makes for ideal conditions in future growing seasons.
Regardless of why lower-than-expected plant populations occur, timely soybean plant stand assessments allow an opportunity to decide if replanting is needed. Uniform but lower plant populations may not warrant replanting, whereas nonuniform and low plant populations may. Remember, low plant populations can lead to reduced competition among plants, increased weed pressure and, ultimately, lower yields. Ensuring an optimal plant population is crucial for achieving the best possible outcomes.
How to assess plant populations
There are several methods for assessing soybean plant populations, each with its own advantages and disadvantages.
The most common method involves physically counting plants in a given area (typically 1/1000th of an acre or 17 feet, 5 inches in 30-inch rows), and taking note of plant-to-plant spacing and large gaps. While accurate, it can be time consuming and labor intensive.
Often, short row lengths are counted and converted to soybeans per foot. In 30-inch rows, 6 soybeans per foot equals 105,000 plants per acre. In 15-inch rows, you can expect 3 soybeans per foot for the same population.
Modern technology allows for aerial assessments using drones equipped with cameras and sensors. This method provides a comprehensive view of the field and can quickly identify areas with poor plant populations. Presently, drone assessments cannot accurately determine plant populations. However, they can easily identify field areas with thin canopy structures, an indicator of low plant populations or lower seedling vigor.
Ideal soybean plant populations
The relationship between plant population and yield is well-documented. Higher plant populations generally lead to higher yields, but only up to a certain point — 100,000 plants per acre. Beyond this, overcrowding can reduce yield due to increased competition for resources. In fact, the higher the seeding rate and plant population, the greater the amount of in-season plant mortality that can be expected.
The ideal soybean plant population varies depending on field productivity and growing season weather conditions. Fields or field areas with higher soybean productivity can withstand lower plant populations without loss of soybean yield.
In high-productivity zones, 80,000 plants per acre at the time of harvest may be suitable. Generally, it is recommended to have 100,000 plants per acre. However, while plant populations of 80,000 or higher would not warrant replanting, it may not be feasible to replant soybeans, even with lower plant populations.
Replant or ‘thickening up’ considerations
When considering replanting, farmers must weigh several factors:
- Yield potential of the existing soybean stand
- Yield potential of replanted soybean
- Costs of replanting (e.g., seed, labor, fuel and equipment)
While the existing plant stand may be lower than 100,000 plants per acre, it may still have higher yield potential than an ideal plant population that is planted late. It is hardly ever advised to do a complete replant for soybeans.
Even with plant populations of less than 60,000 plants per acre, it is better to “thicken up” the existing plant stand by interplanting additional soybean to achieve a final plant population of greater than 100,000 plants per acre. Replanting can be a costly and time consuming, so it's essential to carefully evaluate the potential benefits and drawbacks.
Quickly assess after emergence
Assess plant populations and uniformity quickly after emergence to allow for actionable post-planting decisions. Additionally, making plant assessments allows for the early detection of soybean health that may have season-long effects. To learn more about soybean stand assessment, visit crops.extension.iastate.edu/encyclopedia/stand-assessments-soybean.