Replanting Strategies for Soybeans

INSIGHTS

• The decision to replant an existing soybean stand can be complex where if estimated yield loss from a later replanting date exceeds the loss from a reduced stand, it may be more beneficial to keep the original stand and consider interplanting.
• Accurate assessment of the original plant stand is important, as replanting may not always be the best option.

Figure 1. Soybean plant emerging through heavily crusted soil.

Introduction

Unpredictable weather events can impact the timing of soybean emergence and influence stand establishment (Figure 1), and assessment of early season soybean stands sometimes leads to a question of whether to replant or fill in an existing stand. Previous studies suggest that a final soybean stand greater than 100,000 plants per acre usually produces the highest yield potential. Soybeans can compensate for reduced stands to a certain degree through branching, and often later-planted soybeans have a reduced capacity to maximize yield potential. Significantly reduced plant stands can increase problems with weed control and often lead to lower yields, making the decision to keep an existing stand complicated. If stands are found to be inadequate for maximum yield potential, common approaches to increase stands are filling-in existing stands or removing existing plants with herbicide or tillage and replanting. Both methods may increase plants per acre, but interplanting an existing stand can create differences in soybean maturation and increase grain moisture at harvest.

An accurate assessment of the existing stand is an important first step in making a replant decision. Visual assessments of existing soybean stand can sometimes be unreliable, as narrower row spacings may increase the space between individual plants and give the appearance of a poor stand. Existing stands can be measured using a hula hoop or by measuring linear feet of row and converting to plants per acre for a more accurate estimate.

2024 Soybean Replant Strategy Trials

Agronomy in Action research trials were established in Malta, IL, Slater, IA, and Waterloo, NE, to compare soybean yield response of interplanting into poor stands as compared to tilling up a reduced stand and replanting. Established stands were compared to stands replanted at the unifoliate growth stage. Plant stands of 140,000, 100,000, 60,000, and 40,000 plants per acre were established in 30-inch rows to understand yield penalties of a possible reduced stand if deciding not to replant. To understand the effect of gaps in final stands, devitalized seed was mixed at various ratios with viable seed prior to planting to achieve similar final stands as the reduced stand plots mentioned earlier, only with gaps. Replanted plots were established with GPS by shifting the guidance line 6 inches from the previous row. Delayed planting date plots at 140,000 and 180,000 plants per acre were also established to simulate a scenario where an original stand was removed completely with tillage and replanted.

Impact of Reduced Stands

Established seeding rates of 140,000 and 100,000 plants per acre produced statistically similar yields (Graph 1), showing that established stand levels were adequate to maximize yield. Yields of seeding rates below 100,000 were significantly lower indicating there weren’t enough plants per acre to maximize yield. Overall, gaps in plant stands reduced final yields 8-12% less than similar plant per acre populations with even plant spacing (Graph 2). Previous multiyear trials have shown a similar 8% yield loss associated with gaps.¹ Although stand gaps can have negative impacts, unlike corn, soybeans are better able to compensate for small gaps by branching and bushing out into the gaps to better utilize all available sunlight.

Graph 1. Soybean yields at established seeding rates in 2024.

The reduced stand and various replant strategy yield responses can be compared to the 140,000 seeding rate that was established early in the season to better understand which strategy recovered the most yield potential (Graph 2). Replanting by termination of the original stand or by filling in the existing stand was unwarranted when the original stand levels were 100,000 or greater. Interplanting stands that were 60,000 or lower yielded similarly to stands that were terminated and completely replanted. Attempting to improve the final stand above 140,000 did not prove beneficial in this trial, however previous trials have shown a benefit to an increase in seeding rate at later planting dates.

Graph 2. Soybean yields across all trial treatments in 2024.

Planting Date Effect on Yield

Graph 3. Soybean yield response to planting date in 2024.

Planting date is another important factor to consider in replant scenarios. The average replanting date across these three trials was 4 weeks after the original planting date. Complete replanting at a rate of 140,000 plants per acre yielded less than stands of 100,000 plants per acre that were established early. Increasing the seeding rate to 180,000 plants per acre at the complete replanting date showed no benefit.

In our trials the average delay was 4 weeks, but ultimately the actual calendar date planted on can have larger impacts. A separate 2024 planting date trial at Slater, IA, showed different rates of yield loss than observed in earlier years and locations (Graph 3). Planting in early April brought the highest yielding soybeans. Late-April and mid-May planting dates produced similar yields, and there was significant yield loss by delaying planting until June. Yield response to early planting dates is most likely due to the longer period of vegetative and reproductive growth stages compared to later planting dates.²

Summary

Replanting may not be needed unless stands fall below 100,000 plants per acre or large gaps are present. If estimated yield loss from a later replanting date exceeds the loss from a reduced stand, it may be more beneficial to keep the original stand and consider interplanting. If the decision is made to replant, this trial indicates interplanting instead of completely replanting may not be as concerning as previously anticipated. Accurate early stand assessment can help determine whether replanting would be a viable option. Replanting or interplanting existing stands might also increase costs of tillage, seed, and herbicide product per acre, and those considerations should be weighed against the potential for increased yield.

References

¹Moore, S.H. 1991. Uniformity of Plant Spacing Effect on Soybean Population Parameters. Crop Science, 31: 1049-1051, cropsci1991.0011183X003100040041x

²Iowa State Extension publication, Soybean Replant Decisions. PM1851