Research Update About Soybean Inoculants
In a White Paper on this website, there is a section on “Inoculants” that provides a summary about the use of Rhizobium-containing inoculants (N-fixing bacteria) for soybean production. Pertinent points from that summary follow.
• An appropriate inoculant is cheap, generally less than $3/acre. Thus, cost is not a factor in deciding whether or not to inoculate.
• Do not apply inoculants to gain a yield increase. Rather, apply them to ensure that nitrogen fixation will be sufficient for the crop to realize the yield potential from the planted site.
• There is overwhelming evidence that applying inoculants to soils that have recently been cropped to soybeans provides no yield benefit.
• The cheapness of inoculants warrants their application when soybeans have not been grown recently on a site and the risk of insufficient native soil inoculum is high. The importance of this fact is because there is no option after planting but to apply expensive nitrogen fertilizer to overcome the effects of poor nodulation.
• With the change in cropping systems that is occurring in Mississippi, it is a good idea to inoculate when soybeans are planted on a site that has had continuous cotton or corn or if the site has not been cropped to soybeans in the last 4 to 5 years.
A Mar. 2018 article titled “Bradyrhizobium japonicum Inoculants Have Little Effect in Kentucky’s Soybean-Corn Rotation” by Rod et al. (Crop, Forage, & Turfgrass Management, Mar. 22, 2018) provides further support for the above statements about inoculant use. Pertinent details about the research and findings reported in that article follow.
• Field trials were conducted at two locations in Kentucky in 2016.
• Full season (late May planting date) and doublecrop (late June planting date) plantings were evaluated on silt loam soils at both locations.
• All trials evaluated plantings made with treatments of two inoculant products applied to seed at 1X and 2X rates, plus a non-inoculated treatment.
• The full-season trials were preceded by a corn crop, and the doublecrop trials were preceded by winter wheat that followed corn.
• Seed yield and number of root nodules were similar among all inoculated treatments and the non-inoculated treatment in both the full season and doublecrop experiments. Yields from all treatments in the full season experiments were within 1.2 bu/acre of each other (48.4 to 49.6 bu/acre), while yields from all treatments in the doublecrop experiments were within 2.4 bu/acre of each other (46.3 to 48.7 bu/acre).
• The additional inoculant (2X rate) did not result in a yield increase or a difference in number of nodules compared to the 1X inoculant treatment.
• These results support current Univ. of Kentucky recommendations regarding use of soybean seed inoculants, and indicate that the annual application of these products is an unnecessary expense.
The above results from this Kentucky research further affirm the information contained in the above-cited White Paper.
Composed by Larry G. Heatherly, Sept. 2018, larryheatherly@bellsouth.net