Field Studies to Evaluate Potential Differences in the Decomposability of Bt and Non-Bt Corn Residue.

Some reports suggest that the genetically-modified Bt corn residue may have higher lignin content and that the residue may be more resistant to decomposition.  If true, then there are implications for both farming practices, e.g., tillage and planting, as well as global carbon budgets. We conducted four separate field studies to evaluate the relative composition and decomposition of Bt and non-Bt corn residues. In the first study, we found no differences in 22-month residue decomposition rates between Bt (cry1Ab and cry3Bb1, singly and stacked) and non-Bt corn hybrids from a single seed manufacturer.  In the second study, we found no differences in decomposition between Bt (stacked cry1Ab and cry3Bb1) and non-Bt residues (from two different manufacturers) over one year, nor any evidence that root exudates containing modified Bt endotoxins may inhibit decomposition activities.  A third study conducted to determine if corn rootworm pressure influences the relative decomposition of Bt (cry3Bb1) and non-Bt-protected corn found no differences between these two residues.  In a fourth study, the potential for European Corn Borer (ECB) damage of non-protected corn to influence residue decomposition rates was examined.  Using hybrids from three manufacturer-maturity series (CL344, DKC46, DKC50/51), above-ground biomass from hybrids for each manufacturer-maturity series with no Bt gene (base genetics), Bt (cry1Ab) active against ECB, and Bt (cry3Bb1) active against corn rootworms were harvested from replicated plots where there was a strong natural ECB infestation.  The assessments of ECB damage to the stalks showed that ECB protected hybrids had far less damage than non-protected hybrids. However, stalk sections from the ECB-protected hybrids did not decompose more slowly than their corresponding non-protected isolines.  We conclude that there is little evidence for significant variations in residue composition and decomposability that may be related to the presence of Bt genes in corn hybrids.