Submitted to: Soil/Water Research, Progress Report
Publication Type: Experiment Station
Publication Acceptance Date: 4/15/2009
Publication Date: 7/2/2009
Citation: Lehman RM, Osborne SL, and Rosentrater KA. 2009. Field Studies to Evaluate Potential Differences between Bt and non-Bt Corn Residue. 2008 Progress Report #SOIL PR 08-21. Ag. Exp. Stn., Plt. Sci., SDSU. Brookings SD 57007. Available on line at: http://plantsci.sdstate.edu/soiltest/datafiles/pr%2008-21.pdf Interpretive Summary: 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 three residue decomposition studies using chopped litter in buried litter bags to simulate the effect of chopping and tillage operations on the residue and found that Bt-containing hybrids decomposed at rates similar to their non-Bt near-isoline under these conditions, regardless of corn rootworm pressure. When ECB-damaged residues were tested using whole stalk sections (Study #4), Bt-protected stalk sections did not decompose more slowly than the non-protected stalk sections. No differences in residue composition or stalk mechanical strength were found among hybrids that could be related to the presence of Bt genes.
Technical Abstract: 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 evaluated the decomposition of three Bt-containing corn residue compared to non-Bt corn residue (all hybrids from a single seed manufacturer) under field conditions using the litter bag approach. At seven sampling points spanning 22 months year, no difference in decomposition rates were observed among the four hybrids. A second study was conducted to determine if the production of root exudates containing modified Bt endotoxins may influence decomposition rates. Four corn hybrids were used for this study: Bt and non-Bt containing near isolines from two different manufacturers. These residues were buried within rows of Bt and non-Bt producing corn hybrids in June, 2006. No differences between the Bt and non-Bt residues were observed. Decomposition of residues buried in the root zone of the Bt-producing hybrids was not inhibited. A third study to determine if corn rootworm pressure influences the relative decomposition of Bt- and non-Bt-protected corn found no significant differences in the rate of decomposition of chopped residues between the rootworm-damaged CL344 and the rootworm-protected CL344CRW hybrids at intervals of either 236 or 371 days of burial in the soil. 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 four manufacturer-maturity series (CL44, DKC46, DKC50/51, DKC60), above-ground biomass from hybrids for each manufacturer-maturity series with no Bt gene (base genetics), Bt active against European Corn Borer, and Bt active against corn rootworms were harvested from replicated plots at the Eastern South Dakota Soil and Water Research Farm in 2006 when there was a strong natural infestation of ECB at this site. For each of the hybrids, whole stalks were collected and ECB damage assessed using the number of tunnels and total length of tunnels in the stalks. For the decomposition portion of the study, whole stalk segments were used to allow proper consideration of the stalk damage to the residue decomposition. Because whole stalk sections were used, a larger 13-mm mesh was used for the litterbags that permitted access to most soil invertebrates. 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.