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|Soybean Inoculant Strategies Final Report (April 18 2018)
Determine if there is any additional yield benefit to:
- Using in-furrow granular inoculant (instead of seed-applied liquid)
- Double inoculating (seed-applied liquid + granular in-furrow) (compared to seed-applied liquid only)
- Increasing the rate of inoculant (from 1x to 2x)
- Using "enhanced" inoculant products
Selection of the appropriate Bradyrhizobium japonicum inoculant formulation, rate and combination of products is dependent on field history, equipment available, cost of inoculant and environmental conditions. The objective of this study was to compare fourteen inoculant products, formulations, rates and combinations across a range of locations and field histories in Manitoba. Field experiments were conducted in 2014, 2015 and 2016 at Melita, Carberry, Carman, Roblin and Beausejour. Four of the site-years tested had a history of soybeans and five of the site-years had no history of soybeans. Site-years were combined and analyzed based on this cropping history.
Inoculation had important economic implications on fields with no history of soybean. Averaged across all site-years without a history of soybean, inoculant treatments increased number of nodules per plant by 20, yield by 15 bu ac-1 and protein by 4.8% compared to the uninoculated control. On fields with a history of soybean, there was no difference in yield, number of nodules or seed protein between inoculant treatments and the uninoculated control. Regardless of field history, under the optimal seeding and plant establishment conditions encountered in this study, inoculant product, rate or combination did not have an effect on nodule number per plant or seed yield. There are several possible explanations for the lack of response to double inoculation in this trial which cannot always be guaranteed under field conditions. Therefore, MPSG recommends using a double inoculation strategy on fields with a limited history of soybean and a single inoculation strategy after at least two successfully nodulated soybean crops have been established on a particular piece of land. See MPSG’s Soybean Fertility Factsheet for more details regarding inoculation recommendations.
The minimum number of nodules required to reach 90% of maximum yield was approximately ten nodules per plant at the R4 stage. The R1 stage, however, permits assessment of nodulation failure prior to the ideal window to apply rescue nitrogen fertilizer (R2-R3).