Eliminate Soybean Inoculant in Fields with a History of Beans (Save $1.50/ac)
December 1, 2008
Jennifer M. Rees
Inoculating soybeans with products containing the bacterium Bradyrhizobia japonicum is a common practice and considered an inexpensive insurance against soybean yield loss. This bacterium forms a symbiotic or beneficial relationship with soybean roots in which nitrogen-producing nodules are formed, allowing for nitrogen fixation to occur.
As producers strive to find a "silver bullet" to significantly enhance soybean yields, they may look to one of the soybean inoculant or combined inoculant and growth promoter products new to the market. These products are said to contain more aggressive strains of B. japonicum, fix more nitrogen, and increase yields over previous inoculant products.
Added Inoculant Often Proves Unnecessary
UNL research conducted from 2001 to 2004 at the South Central Agricultural Laboratory near Clay Center, and by the Greater Quad County On-farm Research group and Nebraska Soybean and Feed Grains Profitability Project all showed that adding soybean inoculant did not significantly increase yields on fields with a history of soybeans. The B. japonicum remained in the soil after previous inoculations and was able to inoculate the new soybean planting. These fields had all produced soybeans in the previous four to five years.
In 2005-2006, Greater Quad County On-farm Research producers tested a few new products to determine if treated fields would have a significant yield increase over untreated checks. These studies were conducted in irrigated soybeans with a field history of soybeans (corn and soybean rotation).
The products tested in 2005 included Cell Tech 2000, a liquid inoculant; Vault, a liquid seed treatment which includes a biofungicide and plant activator; and SoySuperb, a peat-based (now liquid) plant growth-promoting rhizobacteria. Cell Tech 2000 yielded 75.5 bu/ac compared to the untreated check at 75.4 bu/ac in four replications with no statistical significant difference. Vault yielded 77.1 bu/ac compared to the untreated check at 76.1 bu/ac in 11 replications with no significant difference. The SoySuperb yielded 76.1 bu/ac compared to 74.2 bu/ac in 19 replications. This was statistically significant at the 99% level.
In 2006, SoySuperb and Vault were compared to untreated checks with 12 replications in producers' fields. There was no significant difference in yields with SoySuperb, Vault, and the untreated check yielding 72.7 bu/ac, 73.5 bu/ac, and 72.9 bu/ac respectively. Thus, UNL's recommendation remains that in most situations, soybean inoculants are not needed in fields with a history of soybeans in the past four to five years.
When to Inoculate
If the field has not produced soybeans in the past four to five years or has never produced soybeans, an inoculant is needed for nitrogen fixation to occur. Positive yield results from inoculation also can occur where the soil environment does not support long-term bacteria survival. These environmental conditions could be due to
- pH below 5.0 or above 8.0,
- sandy soils,
- fields experiencing flooding of more than four days, or
- conditions where crops often experience severe water stress.
In these cases, a yield response may be possible or likely due to re-inoculation.
To determine if your field needs re-inoculation, use the Soybean Inoculation Decision Worksheet for "Old" Fields in the NebGuide, Soybean Inoculation: Applying the Facts to Your Fields (G1622), available online at http://www.ianrpubs.unl.edu/sendIt/g1622.pdf