by Mike Staton

Michigan soybean producers have consistently identified planting rates as the highest priority topic to evaluate in on-farm replicated trials. Producers wanted to evaluate the effect of low planting rates on soybean yield and income.

The two factors driving the increased interest in reducing soybean planting rates are seed cost and white mold. To help Michigan soybean producers make planting rate decisions, the Soybean On-farm Research Program conducted a total of 67 on-farm replicated trials from 2015 to 2021.

Nearly half of the planting rate trials were conducted in Tuscola and Sanilac Counties, so the Thumb area has been well represented.

Four target planting rates (80,000, 100,000, 130,000 and 160,000 seeds per acre) were compared at all but one location, where the lowest rate was not included. Stand counts were taken to determine the actual final plant stands at each location in all years.

None of the varieties planted in the trials were straight-line or thin-line plant types, and a complete seed treatment was used at 52 of the locations.

Because we conducted the trials over seven years, we learned how the planting rates performed over a range of growing conditions. Planting and emergence conditions were nearly ideal in 2015, but they were much more challenging in the following years as evidenced by the average stand loss shown in Table 1.

Statewide record yields were achieved in 2015 and 2016, and nearly again in 2021. However, lower yields occurred in 2017 because of excessive early rains and a lack of rain in August and September. Yields rebounded in 2018 but fell again in 2019 because of planting delays and dry weather in August. 

The figure below shows the average yield and income for all 67 locations. When all 67 sites were combined, the yields from the highest two planting rates were nearly identical, and they beat the 100,000-seeds-per-acre planting rate by 1 bushel per acre and the 80,000 rate by only 2.7 bushels per acre.

The 100,000-seeds-per-acre planting rate generated the most income, while the 160,000-rate produced the least income.

Two of the trials — Sanilac in 2015 and Saginaw in 2018 — were infested with white mold showing that reducing soybean planting rates can also be an effective management practice for reducing yield and income losses from white mold (Table 2).

At both sites, the lowest planting rate produced nearly $90 per acre more income than the highest planting rate. The figure below shows how planting rates affected white mold in the 2018 Saginaw location. This site was planted in 30-inch rows.

Staton is a Michigan State University Extension field crop educator and soybean specialist.