Do Crop Rotation and Tillage Influence Seed-applied Inoculant Decisions?

David Marburger, Spyridon Mourtzinis, John Gaska, & Shawn Conley, Department of Agronomy

Soybean has the unique ability to form a symbiotic relationship with a soil bacterium, Bradyrhizobium japonicum. This relationship results in biological nitrogen fixation, a process in which atmospheric nitrogen (N) is converted to plant-available N in exchange for photosynthetically derived carbon. Because of this symbiotic relationship, soybean growers typically do not apply N fertil- izer, but will apply inoculants containing B. japonicum on or near the seed to ensure that adequate bacterial infection and subsequent biological nitrogen fixation can occur (Schulz and Thelen, 2008). Current university recommenda- tions suggest using inoculants when planting in fields with no previous history of soybean, where soybean has not been planted in the last 3 to 5 years, for soils with pH <6.0, and for sandy texture soils (i.e., low organic matter soils) (Pedersen, 2004; Abendroth et al., 2006). Although much work has examined soybean yield response to inoculant use in fields with or without a previous history of soybean, there is a general lack of information examining inoculant use under different crop rotations and tillage systems. Our objective was to measure soybean yield response to seed-applied inoculants as influenced by crop rotation and tillage system.

Field trials were conducted from 2009 through 2011 within a long-term corn- soybean rotation study established in 1983 near Arlington, WI. This study contains two tillage systems: conventional and no-till. Conventional tillage was accomplished with one pass of a chisel plow in the fall and two passes with a field cultivator in the spring before planting. Within each tillage system, there are seven crop rotations containing soybean: continuous soybean (SS); soybean rotated annually with corn (SC); first-year soybean after 5 consecutive years of corn (1S); and two (2S), three (3S), four (4S), and 5 years (5S) of con- tinuous soybean after 5 years of corn. Finally, within each crop rotation, there were three seed-applied rhizobia inoculant treatments: a non-treated control; Optimize Soybean (contains B. japonicum); and Excalibre (contains B. japoni- cum and B. elkanii). To view the results of the study and read the full article, please follow the link below: