By Mike Staton, Soybean Educator

Soybean growers across Michigan identified soybean planting rates as one of the highest-rated topics they wanted to see evaluated by the SMaRT (Soybean Management and Research Technology) program in on-farm replicated trials. When asked if they wanted us to evaluate the effect of higher planting rates or lower planting rates, the growers overwhelmingly voted for low planting rates.

There are two main factors driving the interest in reduced soybean planting rates: seed costs and white mold. Soybean seed is typically the second highest variable expense in producing soybeans and reduced planting rates have the potential to reduce yield losses associated with white mold.

To evaluate the effect of low soybean planting rates on yield and income, 11 planting rate trials were conducted in 2015 and 11 more were conducted in 2016. Four target planting rates of 80,000, 100,000, 130,000 and 160,000 seeds per acre were compared in both years. Stand counts were taken to determine actual final plant stands at each location in both years. The complete results from the planting rate trials are available in the “SMaRT 2016 On-farm Research Report.”

The planting rate trials produced mixed results in 2015 at the individual locations. At three sites, the 160,000 planting rate produced the highest yield. However, it also produced the lowest yield at two other locations. The lowest three planting rates each produced the highest yield in three different trials. When all the locations were combined and analyzed, the yield for 80,000 planting rate was 1.8 bushels per acre lower than the 100,000, 130,000 and 160,000 planting rates. However, there was no difference in the yields produced by the highest three planting rates.

The more challenging conditions occurring in 2016 favored the higher planting rates. The 160,000 planting rate beat the 80,000 rate at six locations, the 100,000 rate at four locations and the 130,000 rate at two locations. The 130,000 planting rate beat the 80,000 rate at five sites, the 100,000 rate at two sites and the 160,000 rate at two sites.

When all the 2016 locations were combined and analyzed, the two highest planting rates produced identical yields and they yielded 1.3 bushels per acre higher than the 100,000 rate and 2.7 bushels per acre more than the 80,000 rate. When all 22 sites (2015 and 2016) were combined and analyzed, the highest three planting rates produced similar yields and beat the lowest rate by 1.5-2.2 bushels per acre.

Projected market prices ($9.20 per bushel) and conservative seed costs ($60 per 140,000 seeds) were used to determine the income (gross income minus seed cost) produced by the four planting rates. The figure below summarizes the average yields and income produced by the four planting rates in 2015 and 2016. In 2015, the lowest two planting rates generated more income per acre than the higher two planting rates. In 2016, the lowest three planting rates were more profitable than the highest planting rate. The highest planting rate was the least profitable in 2015 and 2016.

There are several factors that contributed to the success of the reduced planting rates in these trials. The varieties planted in all the trials had the medium-bush to bush-type growth habit. This enabled the plants to produce robust branches at the lower planting rates. The second reason the lower planting rates may have performed so well was that complete seed treatments were applied in all but three of the locations.

I want to be clear that Michigan State University Extension does not recommend reducing planting rates to 80,000 or 100,000 seeds per acre. However, the data clearly shows that some very low plant stands can produce excellent yields. Growers can use this information when making replant decisions. They can also compare their planting rates to the rates evaluated in the trials to see if they have an opportunity to increase farm income by reducing planting rates in 2017.