The possibility that some Bt-containing corn hybrids have higher lignin content than their non-genetically modified counterparts represents an unintended effect with no known mechanism, says Michael Lehman of the USDA-ARS-North Central Agricultural Research Lab in Brookings, S.D.

The research microbiologist says there is widespread anecdotal information within the agricultural community that plant residue from some genetically modified corn hybrids may be resistant to degradation, and implement manufacturers now market improved or alternative tillage machinery to deal specifically with tough Bt corn residue.

"If more aggressive tillage is required to handle a growing amount of tougher residue, then decades-long gains in soil and water quality achieved through conservation tillage may be at risk," Lehman says.

Additional studies comparing Bt and non-Bt corn residue composition and evaluating residue decomposition under laboratory conditions have produced ambiguous results he says. At present, there are no published experimental data comparing the decomposition of Bt and non-Bt corn residue under field conditions.

ARS researchers undertook a 22-month field study using the litter-bag approach to compare residue decomposition rates of four corn near-isolines with the following genetics:

* Cry1Ab transgene active against European corn borer (Ostrinia nubilalis Hübner);

* Cry3Bb1 transgene active against corn rootworms (Diabrotica spp);

* Stacked with both transgenes;

* Base genetics with no genetic modification.

Experimental procedures were used to approximate the exposure of the residue when it's chopped and buried during combining and tillage operations conducted in the fall. The researchers also reported on the compositional and physical properties of the residue used for the decomposition study.

"We found no differences in the decomposition of chopped Bt and non-Bt corn residue under the field conditions existing at our research site," Lehman says. "We also found no difference in compositional properties among Bt and non-Bt corn residues, but physical testing suggested differences in residue mechanical strength."

Lehman says there is insufficient data existing to explain the perceived resistance of some genetically modified corn residues to decay.

"It remains possible that the apparent persistence of Bt corn residue observed by agricultural producers may be related to the overall amount of residue produced, regardless of residue compositional qualities, or the relative sturdiness of the Bt+ residue compared to non-Bt residue where insect pressure is present," Lehman says.

Lehman says it's possible that where insect pressure is present, non-Bt-protected corn stalks will be weaker and Bt-protected stalks will be relatively more resistant to decay. In this case, research studies which examine the decomposition of shredded or ground residue may not find any differences correlated with the presence of the transgene.