Soybean inoculation: will the extra cost pay off?

A three-year on-farm study funded by the Michigan Soybean Committee examined the effects of soybean inoculant applied as a seed treatment and post-emergence application.

High-yielding soybeans have a high nitrogen (N) requirement (over 300 pounds per acre) throughout the growing season, most of which (60-70%) is typically supplied by synbiotic nitrogen.2– fixing bacteria (rhizobium) living in the nodules of soybean roots. Recent research by Moretti LG et al. (2018), “Can additional inoculations increase soybean nodulation and grain yield? was conducted to test the hypothesis that additional soybean inoculations during the growing season will increase nodulation and available nitrogen to meet the higher nitrogen requirements of high-yielding varieties.

This research, conducted under Brazilian climatic conditions on clay soil (54% clay, 33% sand, 13% silt), found that soybean nodulation and yield increased with post-emergent inoculant applications. . To determine if the results of this study are applicable to farmers in Michigan, particularly those with coarse-textured soils, major components of the research were replicated on farms in southwestern Michigan in 2019-2021. See Table 1 for the locations and soil types of fields used each year.

Table 1. Soil type for each field used in the study.
Year of construction The type of soil
Sturgis Dryland 2019 Loamy Sand (Spinks)
Sturgis irrigated 2019 Sandy loam (Hillsdale) and loamy sand (Spinks)
Hickory Corners Arid Land 2019 Limon (Kalamazoo)
Sturgis 2020 Sandy loam (Oshtemo)
Constantine 2020 Limon (Kalamazoo)
Sturgis 2021 Sandy loam (Hillsdale) and loamy sand (Spinks)
Nottawa 2021 Sandy loam (Oshtemo)

In 2019, soybeans were planted at 15-inch or 30-inch row spacing in May on three fields, one of which was under center pivot irrigation while the other two were dryland. The plots were 10 or 15 feet wide by 100 or 200 feet long. The treatments were repeated four times in each field.

Treatments with Bradyrhizobium japonicum inoculant included: no seed inoculant (NI); seed inoculant alone (SI); seed inoculant plus additional inoculant at V3 (SI plus V3); and seed inoculant plus additional inoculant at R2 (SI plus R2). Inoculant (TerraMax Liquid ST/IF available from TerraMax, Inc., Bloomington, Minnesota) was applied to all SI treatments using a cement mixer at the rate indicated. Post-emergent inoculant applications were made using a knapsack sprayer with drop nozzles directed at the base of each row at the manufacturer’s recommended rate.

Nine plants were dug up in each plot at V3 and R5 and the nodules were counted. Leaves were also collected and analyzed for N concentration. Final stand counts were made and a 5 foot wide strip was harvested in the middle of each plot with a small plot combine.

No significant effect of inoculation on nodulation (number or mass of nodules), leaf N concentration, or final grain yield could be detected statistically at either sampling time. . Counting nodules proved to be difficult and time-consuming, and was determined not to be an effective or efficient method to measure the impact of inoculation treatments due to the inherent variability in formation of nodules and difficulty in performing accurate counts.

Another challenge of this study was to ensure that the B. japonicum inoculant has reached the root zone after post-emergence inoculations. Watering is essential to carry the inoculum down to the root zone, especially during the hot weather typical of the V3 and R1 growth stages. For this reason, it was decided to use only irrigated sites for subsequent replications of the study.

In 2020 and 2021, the nodulation impact measurement protocol was modified to measure ureide concentration in soybean stalks as explained by Herridge and Peoples in “Ureide assay for measure nitrate fixation by nodulated soybean calibrated by 15N methods. Ureides are chemicals produced from nitrogen stored in nodules and then transported to aerial structures in soybeans. The higher the ureide concentration in the stems, the greater the N production in the nodules. Collection of stem samples at R2 and R5 and analysis of ureide concentration replaced root digging and nodule counting performed in 2019. All other aspects of the procedure remained the same for the second and third years.

A summary of soybean yield for each field is shown in Figure 1. There was no statistically significant impact of inoculant treatment on yield at any site-year. The ureide analyzes are still in progress so no data is yet available on the impact on nodulation.

Figure 1. Effect of B. japonicum inoculation on soybean yield over seven site-years from 2019 to 2021. NI=uninoculated, SI=inoculated seed, SI+V3=inoculated seed plus post-emergent inoculation at V3, SI+R2= seed inoculated plus postemergence inoculation at R2.

The absence of significant yield differences is probably due to the presence of B. japonicum in soils prior to application of treatments at levels sufficient to support soybean growth. It is generally understood that rhizobium persists in soils when legumes have been grown in a field in the recent past, thereby masking any effect of the application of additional inoculum. Some believe that rhizobium may not persist from one soybean rotation to the next in lighter textured soils that are subject to high summer temperatures and dry conditions. However, the results of this three-year study in southwestern Michigan did not support this supposition. Previous research conducted at MSU by Schulz and Thelen, “Effects of Soybean Seed Inoculant and Fungicide Seed Treatment on Soybeans,” showed that soybean yield increased by an average of 1.3 bushels per acre. in six of the 14 site-years in fields that had been rotated to soybeans. inoculation a cost effective practice.

Thanks to the Michigan Soybean Committee for funding this project, to TerraMax, Inc. for the inoculant donation, and to the collaborating farmers for their cooperation.

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