Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 10

resistance in weeds include using individual herbicides with different modes
of action sequentially and using mixtures of herbicides with different modes
of action concurrently (Gressel & Segel, 1990; Wrubel & Gressel, 1994). The
underlying assumption in these strategies is that weeds are less likely to
evolve resistance to several unrelated compounds than to a single compound.
The evolution of weed biotypes with resistance to multiple classes of herbi-
cides is a real possibility, however. This phenomenon is common in insects
(Georghiou, 1986) and has been observed in Lolium rigidum in Australia
(Burnet et al ., 1994; Gill, 1995) and Alopecurus myosuroides in the UK (Holt,
1992). Of particular interest is the ability of weeds to evolve resistance to dis-
tinct classes of herbicides as a consequence of exposure to, and selection by,
chemically unrelated herbicides. Burnet et al . (1994) reported, for example,
that a L. rigidum population in Victoria had become resistant to nine different
chemical classes of herbicides after 21 years of exposure to five herbicides in
only five classes. Lolium rigidum is a major cropland weed in southern Australia
and, as a species, has demonstrated resistance to most of the major herbicide
chemistries used there (Powles et al ., 1997).
Increasing costs of research, development, and registration are reducing
the rate at which new herbicides are introduced into the marketplace.The cost
to a company of developing and registering a pesticide product increased
from $1.2 million in 1956 to an estimated $70 million in 1991 (Holt &
LeBaron,1990; Leng,1991).Concomitantly,the chances of a newly discovered
chemical becoming a legally registered product have decreased greatly; Holt &
LeBaron (1990) cited the odds as 1 in 1000 in 1956,compared with 1 in 18 000
in 1984.Increased costs of toxicological testing and legal work associated with
the regulatory process are also leading many agrichemical firms to not seek re-
registration for the use of herbicides in crops that occupy only small areas,
e.g., vegetables and fruits (Anonymous, 1989).
Partly as a consequence of rising costs for discovering,developing,and reg-
istering new herbicides, agrichemical firms have merged with seed and bio-
technology companies to produce new crop varieties with resistance to
existing herbicides, especially glyphosate, glufosinate, bromoxynil, and sul-
fonylurea, cyclohexanedione, and imidazolinone compounds (Duke, 1999).
Many of these varieties have been produced using recombinant DNA technol-
ogies.Worldwide in 1999,herbicide-resistant,transgenic varieties of soybean,
maize, cotton, rapeseed, and other crops were planted on 28 million ha
(Ferber,1999).The broadscale deployment of these and other genetically engi-
neered crops has been met with controversy in Europe, Japan, the USA, and
elsewhere because of environmental and consumer concerns.Thus,the extent
to which herbicide-resistant crops will be used in the future is uncertain.