274 Green Chemistry, 2nd ed
including microbial pests (viruses) as well. The most common transgenic crop grown in
the U. S. is the soybean, of which about 53 million acres, or 47% of the total, consisted
of transgenic varieties in 1999. This increased to 54% of the soybean acreage in 2000
and 68% in 2001. About 25 million acres of transgenic corn were grown in 1999, but
somewhat less transgenic corn was planted in 2000 because of reduced infestations of
the European corn borer. The percentage of corn that was transgenic in 2001 has been
estimated at 26%. In 1999 about 9 million acres each of transgenic cotton and canola
(grown as a source of canola oil) were grown. In 2001, it was estimated that 69% of
the cotton grown in the U. S. was transgenic. Only about 2–3% of the potato crop was
transgenic during the 1998–2000 period, and small fractions of the squash and papaya
crops were transgenic as well. In 2001, 72 million acres of U. S. farmland were planted
to transgenic crops.
The overwhelming majority of characteristics spliced into transgenic crops consist
of herbicide tolerance and resistance to insects. Insect resistance has been imparted by
addition of a gene from Bacillus thuringiensis (Bt) that causes the plant to produce a
natural insecticide in the form of a protein that damages the digestive systems of insects,
killing them. Of the acreages of transgenic crops planted in 1999, 70.2% were herbicide
tolerant, 22.2% were Bt insect resistant, 7.3% were both herbicide tolerant and insect
resistant, and 0.3% were virus resistant.
The disruption of natural ecosystems by cultivation of land and planting agricultural
crops provides an excellent opportunity for opportunistic plants — weeds — to grow in
competition with the desired crops. To combat weeds, farmers use large quantities of a
variety of herbicides. The heavy use of herbicides poses a set of challenging problems.
In many cases, to be effective without causing undue environmental damage, herbicides
must be applied in specified ways and at particular times. Collateral damage to crop
plants, environmental harm, and poor biodegradation leading to accumulation of
herbicide residues and contamination of water supplies are all problems with herbicides.
A number of these problems can be diminished by planting transgenic crops that are
resistant to particular herbicides. The most common such plants are those resistant to
Monsanto’s Roundup herbicide (glyphosate, structural formula below):
OH
OH
O
P
H
H
C
H
N
H
H
C
O
HO C
Glyphosate, Roundup herbicide
This widely used compound is a broad-spectrum herbicide, meaning that it kills most
plants that it contacts. One of its advantages from an environmental standpoint is that it
rapidly breaks down to harmless products in soil, minimizing its environmental impact
and problems with residue carry-over. By using “Roundup Ready” crops, of which by
far the most common are transgenic soybeans, the herbicide can be applied directly to
the crop, killing competing weeds. Application when the crop plants are relatively small,
but after weeds have had a chance to start growing, kills weeds and enables the crop to
get a head start. After the crop has developed significant size, it deters the growth of
competing weeds by shade that deprives the weeds of sunlight.