By Bob Sampson
Genetically modified seeds that are resistant to a low-toxicity
herbicide, glyphosate, have a positive environmental impact when compared to
other technologies to combat weeds, according to a recent University of
Illinois study.
"With the emergence of glyphosate-resistant (GR) weeds, the
environmental consequences of alternatives to the use of genetically
modified seed are of increasing importance," explained Gerald Nelson, a
professor in the U of I Department of Agricultural and Consumer Sciences.

Nelson and his doctoral student Justin G. Gardner conducted a study
that simulated the environmental effect of abandoning the
glyphosate-resistant seed if weeds develop immunity to it. They utilized a
well-known mammalian toxicity measure, the LD 50 dose for rats (the volume
of pesticide needed to kill 50 percent of a test population of rats), to
assess one potential environmental impact. They simulated the consequences
for corn, soybeans, and cotton.

"With conventional tillage, we found that the use of GR seeds reduces
the number of LD 50 doses applied per hectare by 17 percent to 98 percent,
depending on the crop," said Nelson. "With no-till, the use of GR seeds
reduces LD 50 doses only in corn.

"If farmers switch to conventional seeds because of GR-resistant weeds
but maintain the same tillage practices, our simulation suggests that LD 50
doses could increase by as much as 100 LD 50 doses per hectare in soybeans,
and 500 LD 50 doses per hectare in cotton, or 11.4 and 19.8 percent,
respectively."

Reducing LD 50 doses per hectare generally depends on the crop and
whether the tillage system changes.

"Because no-till replaces mechanical weed control with chemical weed
control, we expect it to increase the LD 50 doses as in fact it does for all
three crops, even with the use of GR seeds," Nelson noted.

The simulation assumed the extreme case of all farmers switching to
non-GR seed technology. Nelson added that due to the small number of farms
that use GR seed in the sample, the corn results are suspect.


[www.aces.uiuc.edu]