In 2006, a group of farmers gathered in York for a discussion about on-farm research projects for the coming year. They were looking at ways to reduce input costs without affecting yield.

One question asked was: "What is the effect of seeding rates on soybean yields?" To look at the question, several farmers chose tried four seeding rates (90,000, 120,000, 150,000 and 180,000 seeds per acre) in 30-inch row spacings.

Ten years later, the research continues with the same results: reducing soybean seeding rates from 180,000 or 150,000 seeds per acre to 120,000 seeds per acre doesn't statistically reduce yields in 30-inch rows in silty clay loam and silt loam soils in south-central and southeastern Nebraska.

Results of 16 studies showed for seeding rates of 180,000, 150,000 and 120,000 seeds per acre, average yields were 66.9, 66.5, and 66.2 bushels per acre, respectively (see Figure 1).

FIGURE 1: Yield results of on-farm seeding rate studies from 2006 to 2010.
Average final stands: 90,000 = 83,010 plants per acre; 120,000 = 106,269 plants per acre;
150,000 = 132,059 plants per acre; and 180,000 = 156,626 plants per acre

The data set for this study includes:
• latest soybean varieties, as the research was conducted from 2006-16
• erect and bushy type varieties in growth architecture
• higher- and lower-yielding situations
• 14 irrigated fields and two non-irrigated
• hail events occurring from V2 to R2 in fields
• seed treated in some fields and others without (determined by farmer by planting date)
• pod and seed count data collected in some years (data showed similar numbers of seeds per acre and ultimately yield per acre)
• observations of increased plant branching at lower seeding rates and difficulty in telling the seeding rate treatments apart as the season progressed

The early studies within this data set all had seed germ of at least 90% listed on the seed bag, and in all but two situations, the farmers were able to achieve 90% or greater of their planted stand. The two exceptions were seeded at 180,000 seeds per acre where they achieved 88%.

Cost savings and recommendation
Surveys conducted via CropWatch and at pesticide trainings found most Nebraska farmers planting an average 150,000 seeds per acre. Our recommendation based on our research is to consider reducing your soybean seeding rate to 120,000 seeds per acre and aiming for a final plant stand of 100,000 plants per acre.

Economically, if you dropped your seeding rate from 150,000 to 120,000 seeds per acre, you would save $10.69 per acre, assuming a $60 per bag seed cost at 140,000 seeds.

What about narrow rows?
The same question regarding the effect of reduced populations on soybean yields exists today for farmers switching to narrow-row soybeans. In 2016, two on-farm research studies were conducted in Richardson and Washington counties, with 15-inch row soybeans. Both fields contained silt loam or silty clay loam soils.

Seeding rates of 90,000, 120,000, 150,000 and 180,000 seeds per acre were planted in the Washington County field with nonsignificant yield differences of 76, 77, 77 and 76 bushels per acre, respectively, with the 90,000 rate resulting in the highest marginal net return. The farmer was able to achieve 91% or greater of original planted stand.

Seeding rates of 116,000, 130,000, 160,000 and 185,000 were planted in the Richardson County field. There were no statistical yield differences between 185,000, 160,000 and 130,000 seeding rates in this study with yields of 68, 68, and 67 bushels per acre, respectively. The 116,000 seeding rate resulted in a yield of 66 bushels per acre. Heavy crusting affected final plant populations in the field, resulting in final stands of 87,667, 99,417, 113,667 and 126,333, along with seeding rates of 116,000, 130,000, 160,000 and 185,000, respectively. The 116,000 seeding rate resulted in the highest marginal net return for this study.

Test it for yourself
Growers looking for ways to reduce soybean inputs this year can try this on their own farms. Protocols can be found at the Nebraska On-Farm Research Network website. You can also download the Nebraska On-farm Research app available in Apple and Android to help you set up your plot design to obtain scientific results. Or contact any of the authors or anyone involved in our Nebraska On-Farm Research Network for additional questions or for help setting up your research project.

This report comes from UNL CropWatch. Learn more at cropwatch.unl.edu.