Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 11

If herbicide-resistant crops are accepted and used widely in coming years,
herbicide resistance in weeds will remain a concern, since herbicides used
with these crops will exert the same types of selection pressures that they do in
herbicide-tolerant, non-genetically engineered crops. Shifts in weed commu-
nity composition toward species pre-adapted to tolerate herbicides applied to
herbicide-resistant crops are also possible (Owen, 1997). In addition, transfer
of herbicide resistance from crops to related weed species through pollen
movement may create new herbicide-resistant weed populations (Snow &
Morán-Palma, 1997; Seefeldt et al ., 1998), which would have to be controlled
by different herbicides or other means.
The combination of herbicide resistance in an increasing number of weed
species,slower introduction of new herbicides,and withdrawal of older herbi-
cides means that farmers are likely to have fewer chemical control options
within the next several decades. For this reason, alternative weed manage-
ment strategies that make full use of nonchemical tactics need to be
developed.
Herbicides and water quality
Since the 1980s there has been increasing recognition that herbicides,
applied in the course of normal farming practices, have contaminated surface
and ground water in many agricultural regions (Barbash et al ., 1999; Larson,
Gilliom & Capel, 1999; United States Geological Survey, 1999). Among the
herbicides detected most frequently in drinking-water sources, there are a
number of compounds classified as probable (e.g., acetochlor), likely (e.g.,
alachlor), and possible (e.g., atrazine, cyanazine, metolachlor, and simazine)
carcinogens (United States Environmental Protection Agency, 1999). Several
herbicides contaminating drinking-water sources are also under scrutiny as
possible disrupters of human immune, endocrine, and reproductive systems
(see section “Acute and chronic effects of herbicides on human health”below).
The effects of low-level exposure to herbicides are poorly understood, but
there is considerable popular and regulatory concern over contamination of
drinking-water sources.
Herbicide contamination of the Mississippi River drainage basin has been
particularly well documented (United States Geological Survey, 1999).The 12
states that drain to the Mississippi River contain about 65% of the harvested
cropland in the USA, and fields of maize, soybean, sorghum, rice, wheat, and
cotton are dominant features of the region's landscape (United States
Department of Agriculture, 1999 b ). The Mississippi River basin receives the
majority of herbicides applied in the USA; during the late 1980s, more than
125000 metric tons of herbicide active ingredients were applied annually to