Vol. 51—1997 - NorthEastern Weed Science Society

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151
HTC's can
scientists re
foreseeable f
pesticides is
control, wher
insects, it i
crops can be
herbicides th
some of the h
fer benefits in the environmental arena. Most growers and weed
gnize there are no feasible alternatives to herbicides in the
ure. However, it is obvious use of herbicides and other
nder increasing scrutiny by the general public. Unlike insect
crops can be genetically trangformed to be resistant to
hard to imagine a crop that is resistant to weeds. However,
enetically transformed such that they are resistant to
t have more environmentally favorable attributes than perhaps
rbicides currently in use.
Perhaps of greatest concern at the current time is groundwater
contamination Triazines and chloroacetamides are of particular concern as
they have bee detected in a number of groundwater monitoring studies in
various state. Herbicides usually are detected in only a small percentage of
the wells sam led and the detects are usually under the worst case scenarios.
Nevertheless, there have been some detections that appear to result from
labelled uses of the herbicides. The EPA, as part of its overall groundwater
protection pr ram, will soon be requiring states to develop pesticide
specific stat management plans to reduce or eliminate herbicide contamination
of groundwate. Certain widely used triazines and chloroacetamides will be in
the first gro p for which such plans must be developed. The plans likely will
place signifi ant restrictions on use of these herbicides. Cultivars with
tolerance of erbicides with good environmental attributes, such as
glyphosate, w uld alleviate this problem.
An area wh re HTC's, if used wisely, can have a major impact is in
herbicide res stance management. Weed resistance is viewed by many weed
scientists as a major potential threat to US agriculture. Although resistance
to a number 0 herbicides in various chemical families has been identified,
the greatest oncern is probably with the ALS inhibitors. As a group, ALS
inhibitors se to be more prone to resistance evolution. The real concern
arises from t e potential for widespread usage of ALS inhibitors on a number
of crops. T re is now one or more ALS inhibitors that can be used in every
major agrono ic row crop in the US except tobacco (Nicotiana tabacum L). ALS
inhibitors a ewidely used in soybean. Soybean often is rotated with corn,
and there ar a number of ALS inhibitors that can be used in corn. If use of
triazines an lor chloroacetamides is reduced or eliminated because of
groundwater r other concerns, growers will depend more heavily on ALS
inhibitors i corn. In the Southeast and to a lesser extent in the Virginia­
Carolina and Southwest production areas, ALS inhibitors are used in peanut
(Arachis ~~Qa~lL.). Pyrithiobac sodium {sodium salt of 2-chloro-6-[(4,6­
dimethoxy-2- yrimidinyl)thio)benzoic acid}, another ALS inhibitor, was
registered r cently for cotton. To emphasize the potential seriousness of
resistance t ALS inhibitors, a biotype of common cocklebur (Xanthium
strumarium L ) cross resistant to imidazolinones and pyrithiobac sodium was
identified i Missouri before pyrithiobac sodium was even registered.
HTC's suc as bromoxynil-tolerant cotton, glyphosate-tolerant cotton and
soybean, and glufosinate-tolerant corn and soybean can bring new chemistry
into the sys em to help in managing currently resistant weeds and to avoid
further resi tance evolution. On the other hand, if the new technology is
used wisely, HTC's could increase the problem with weed resistance. As is
case in so ny other areas of crop production, management will be the key to
success or f ilure. Imidazolinone-tolerant corn, for example, increases the
likelihood 0 greater use of imidaaolinone herbicides. Use of imidazolinones
on corn gro in rotation with soybean which also receives an ALS inhibitor
would clearl not be beneficial in terms of resistance management. On the
other hand, n regions where corn often is grown continuously, imidazolinonetolerant
cor could be beneficial if the grower rotated imidazolinones with
other herbicides such as triazines.
The same argument could be made for sethoxydim-tolerant corn. Biotypes of
johnsongras [Sorghum halepense (L.) Pers.) and several annual grasses are
known to be [resistant to sethoxydim. sethoxydim-tolerant corn gives one the
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