Over the past couple of weeks, many soybean fields have experienced challenging conditions from soil crusting shortly after planting to more recent hail events. The result has been uneven emergence, and, in some cases, plant populations are below what was originally targeted. As conversations turn to replanting decisions, it is important to remember that soybeans have a remarkable ability to compensate for reduced plant populations.
Results from recent work conducted by Iowa State University (Figure 1) highlight a consistent takeaway: soybean yield plateaus sooner than many expect. Seeding rates around 100,000 plants per acre are already near maximum yield, and reductions below that threshold do not automatically translate to large yield losses. In fact, populations in the 80,000 to 100,000 plants per acre range typically maintain near-optimal yield and even plant populations between 60,000 and 80,000 plants per acre can still produce strong yields when plants are healthy and evenly distributed.
Soybean’s ability to branch and compensate allows them to maintain yield even when plant populations are reduced. Research has shown that as few as 50–75K uniformly distributed plants per acre can still achieve around 95% of maximum yield under favorable conditions.
Soybean plants from this Iowa State University experiment showed compensation happens through increasing branching and producing higher numbers of pods and seeds per plant. Some plants from the 30,000 seeding rate treatment had more than 300 pods and 1000 seeds per plant (Figure 2). Obviously, we would not recommend going this low, but this is an example of how soybean plasticity can help when plant populations are lower than planned.
How does this apply to fields impacted by crusting or hail?
Repair planting may be better than a complete replant. The goal of repair planting is to thicken up the existing plant population to achieve greater than 100,000 plants per acre at harvest. A general rule of thumb is to the initial emerged plant population plus the repair planting seeding rate to equal 140,000. For example, if a field averages 60,000 plants per acre, the target would be to repair plant another 80,000 seeds per acre to achieve an optimal plant population while preserving the earlier-emerged plants. The initial emerged plants have the highest yield potential.
The first step is to get out and assess the existing plant population. Count plants and evaluate their uniformity and health. If plants are evenly spaced and still able to recover, the field may have more yield potential than it appears at first glance. Replant decisions can sometimes reduce profitability, especially when factoring in delayed planting and added costs. Plant population losses that leave fewer than 60,000 plants per acre or stand loss patterns that leave areas of more than 6 to 10 square feet in size without any plants, should be considered for full replant.
Fields with reduced plant populations due to crusting or hail may still be capable of high yields, especially when populations remain above roughly 80,000 plants per acre and are evenly distributed. Before replanting, take time to assess what you have. You may find that the best decision is not to start over, but simply to repair and move forward.
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- Research Scientist II
- Associate Professor
- Extension Cropping Systems Specialist