Tuesday, November 14, 2006

Variation in soybean yield and composition: association with soil properties and SCN.

Jose Rotundo, Mark Westgate, and Greg Tylka. Iowa State University, 1301 Agronomy Hall, Ames, IA 50011-1010

  There is increasing interest in quantifying the spatial variation in yield and composition in order to define homogeneous management areas. The presence of Soybean Cyst Nematode (SCN) and the heterogeneity in soil across a field may determine differences in yield and composition at relatively small spatial scales. A 3-year field study was conducted to assess the influence of SCN and soil variation in soybean yield and composition. One SCN-susceptible and one SCN-resistant cultivar were planted in alternating four rows strips. Eight sampling points were defined within each strip and yield, seed composition, soil physical parameters and SCN eggs were measured. Firstly, we focused on average differences between SCN cultivars across different soils. Secondly, we quantified the magnitude of within-field variation in soybean yield and composition. Finally, we attempted to identify explanatory variables that accounted for the observed within-field variation. Results showed that the SCN-resistant variety performed better than the SCN-susceptible variety both for yield and seed composition. Yield ranged from 500 to 3500 kg/ha. Yield was reduced in both varieties in soils associated with depressed topographic positions.  Most of the variation was explained by soil elevation; the higher the elevation the higher the yield.  The effect of elevation was normalized to the SCN-resistant values to reveal the residual variation in yield at each field location, 20% of which was explained by SCN eggs counts.  In addition, yield of SCN-susceptible cultivar was negatively correlated with soil pH, indicating a possible influence of SCN on soybean yield.  Although within-field variation in seed protein and oil was detected, it was not associated with measured soil properties or SCN egg counts. Current research is focused on identifying the environmental factor(s) responsible of variation in seed composition.