Genetic Gain X Management Interactions In Soybean: I. Nitrogen Utilization.

Soybean yields in the United States have increased 21.3 kg^-1 ha^-1 yr^-1 since 1911 with speculation to the source(s) of yield gains. Our objective was to determine yield gains associated with greater nitrogen (N) utilization in cultivars released from 1923 to 2008 in maturity group II (60 cultivars) and maturity group III (57 cultivars). A field study was initiated in 2010 at 4 locations: Indiana, Illinois, Wisconsin, and Minnesota. Cultivars were exposed to two N sources: soil N + N fixation vs. fertilizer N. Fertilizer N totaled 560 kg N ha^-1  in a split application at planting and V5. Typical management practices were followed and hand weeding was used for weed control. Soil samples were taken at planting and after harvest to assess soil N content. Pictures were taken weekly during the growing season for analysis of canopy closure and total light interception.  Whole plant above ground biomass samples were collected at R6.5. Biomass samples were partitioned into pods and stover at Indiana. Nitrogen concentrations of grain, pods, and stover were collected using a combustion analyzer. In 2010, preliminary results varied at all locations. Analysis of canopy closure at the Indiana location indicated a decrease in number of days to canopy closure with additional fertilizer N. Many of the maturity group II cultivars grown in Minnesota and Wisconsin had little response to fertilizer N; whereas, maturity group III cultivars in Illinois and Indiana demonstrated some positive yield response to fertilizer N. The first year of field study was mixed and field research will continue through the 2011 growing season.