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[SANET-MG] Dicamba ready crops
4 June 2007
Professor Joe Cummins
Dicamba ready GM crops
It is growing ever more evident that genetically modified (GM)
individual herbicide resistant crops are not sustainable. Herbicide
resistant weeds soon appear rendering the GM crop obsolete. Herbicide
resistant weeds are increasing rapidly (1,2).Herbicide tolerant crops
may themselves become “volunteer” weeds and Monsanto Company recognized
the problem they had created and patented herbicide mixtures to combat
tolerant weeds and volunteer crops (3). Herbicide tolerant weeds arise
either by mutation or by gene flow from the transgenic crops(4).The
herbicide resistance gene transferred from GM crop to a weedy relative
has been observed to transfer that gene to other weed species (5). It
has grown clear that the useful life of the original GM herbicide
resistant crops are rapidly nearing their end.
Crops resistant to the phenoxy herbicide Dicamba, have been developed as
a replacement for the outmoded resistance to the herbicides glyphosate,
glufosinate or bromoxynil. Dicamba is produced by BASF chemical
corporation and marketed as Banvel , the dimethylamine salt or as
Vanquish the diglycolamine salt (6). “Inert” ingredients in the
formulations include 3,6-dichlorosalysilic acid, dimethylnitosamine,
ethylene glycol and 2,7-dichlorodibenzo-p-dioxin. Dicamba is an old
herbicide which served alongside “agent orange” in Vietnam (7). That old
polluting phenoxy herbicide was mysteriously born again as an
environmentally friendly chemical through the magic of public relations.
Dicamba degradation was characterized in the bacterium Psuedomonas
maltophilia which can utilize the herbicide as a sole carbon source.
Three separate enzymes, an oxygenase, a ferredoxin and a reductase act
sequentially to convert Dicamba to its first degradation product
3,6-dichlorsalyicylic acid (8). The primary patent for Dicamba degrading
organisms including transgenic crops was assigned to the University of
Nebraska. The genes for the Dicamba oxygenase, ferridoxin and reductase
were used to transform crop plants. No selectable markers were required
because the transformants could be identified by the fluorescence of
3,6-dichlorsalyicylic acid (9). Success has been reported in
transforming the soybean with only the dicamba monooxygenase gene driven
by a promoter from peanut chlorotic streak virus and enhancer from the
tobacco etch virus , a chloroplast transit peptide gene from pea for
localization in the chloroplast and a transcription terminator from pea.
In this transformation, localizing the Dicamba monooxygenase to the
chloroplast, the ferridixin and reductase genes were not required to
maintain active Dicamba degradation (10)
Between 2004 and 2007 over forty field test releases of Dicamba
resistant corn, cotton or soybean were permitted in the United Sates.
These field tests were mainly undertaken by Monsanto Corporation
accompanied by one test each from the University of Kansas and BASF
corporation. Several field test releases included crops stacked with
transgenes for glyphosate résistance or for yield or oil composition.
With the exception of the University of Kansas release the donors of the
Dicamba resistance transgenes were designated confidential business
information (CBI) but CBI constructs are unlikely to be much different
from the Psuedomonas maltophilia genes used in the Kansas release(11).
However, the use of CBI designations in field releases where the public
may be exposed to toxic products is both unwise and unwarranted.
Environmental impact studies do not appear to be available for the
numerous field tests releases of either Dicamba field tests nor of the
stacked Dicamba tests and the locations of the tests are withheld from
those who may be exposed.
Since the use of Dicamba may greatly increase ,the toxicity of the
herbicide is worth reviewing. Both pure Dicamba and Banvel proved to
increase sister chromatid exchange in human lymphocytes. The study
concluded that Dicamba is a DNA damage agent and potentially hazardous
to humans (12). Dicamba proved genotoxic in mutation tests using
transgenic Arabidopsis thaliana plants (13). Dicamba proved to be a
peroxisome proliferator in rats , such activity is associated with liver
cancer in rodents (14). Dicamba is not benevolent and its increased
deposition in the environment is unwise.
Dicamba is relatively persistent in the environment and it is slowly
degraded in turf grass (15). In Saskatchewan Dicamba was found to leach
into the South Saskatchewan river , a source of drinking water for urban
populations(16) High levels of Dicamba were observed in rivers draining
tow prairie watersheds , Manitoba streams draining agricultural lands
were elevated during application period in May and June (17).
Predictably the overuse of herbicides in GM crops has resulted in the GM
crops becoming obsolescent as weeds grow herbicide tolerant. Introducing
GM crops resistant to an old polluting phenoxy herbicide seems like a
bad idea turning an environmental disaster into a real catastrophe.
Returning to sustainable agriculture seems to be the only way to deal
with the resistant weeds and the “volunteer” GM crops that show the
herbicide resistant GM crops cannot cope with natural processes leading
to resistant weeds.
1. Owen,M and Zelaya,I. Herbicide-resistant crops and weed resistance to
herbicides Pest Manag. Sci. 2005,61,301-11
2. Ho,M-W and Cummins, J. Roundup ready sudden death, superweeds and
allergens Science in Society 2004,21, 24
3. Flint,J,Probst,N and Gubbiga,N. Tank mixtires and premixtures for
weed control programs that include postmergence applications of
glyphosate plus germinicides in glyphosate tolerant soybeans 2001 United
States patent 6,239,072
4. Ellerstrand,N. Current knowledge of gene flow in plants: implications
for transgene flow Phil. Trans.R.Soc. Lond. 2003.358.1163-70
5. Zekaya,I,Owen,M and VanGessel,M. Transfer of glyphosate resistance:
evidence of hybridization in Conyza American Journal of Botany
6. United States. Department of. Agriculture. Forest Service. Pacific
Northwest Region. Dicamba Herbicide Information Profile February 1999
7. Sutton,P. The History of Agent Orange Use in Vietnam an Historical
Overview from the Veteran's Perspective 2002 Hanoi, Vietnam
8. Wang X, Li B, Herman PL and Weeks DP. A Three-Component Enzyme System
Catalyzes the O Demethylation of the Herbicide Dicamba in Pseudomonas
maltophilia DI-6. Appl Environ Microbiol. 1997 Apr;63(4):1623-26.
9. Weeks,D,Wang,X and Herman.P. Methods and materials for making and
using transgenic dicamba degrading organisms 2006 United States Patent
10. Behrens MR, Mutlu N, Chakraborty S, Dumitru R, Jiang WZ, Lavallee
BJ, Herman PL, Clemente TE and Weeks DP. Dicamba resistance: enlarging
and preserving biotechnology-based weed management strategies. Science.
2007 May 25;316(5828):1185-8.
11. Field Test Release Applications in the U.S.June 2007
12. Gonzalez NV, Soloneski S and Larramendy ML. Genotoxicity analysis of
the phenoxy herbicide dicamba in mammalian cells in vitro. Toxicol In
Vitro. 2006 Dec;20(8):1481-7
13. Filkowski J, Besplug J, Burke P, Kovalchuk I and Kovalchuk O.
Genotoxicity of 2,4-D and dicamba revealed by transgenic Arabidopsis
thaliana plants harboring recombination and point mutation markers.
Mutat Res. 2003 Dec 9;542(1-2):23-32.
14. Espandiari P, Thomas VA, Glauert HP, O'Brien M, Noonan D and
Robertson LW. The herbicide dicamba (2-methoxy-3,6-dichlorobenzoic acid)
is a peroxisome proliferator in rats. Fundam Appl Toxicol. 1995
15. Roy JW, Hall JC, Parkin GW, Wagner-Riddle C and Clegg BS. Seasonal
leaching and biodegradation of dicamba in turfgrass. J Environ Qual.
16. Cessna AJ, Elliott JA, Tollefson L and Nicholaichuk W. Herbicide and
nutrient transport from an irrigation district into the South
Saskatchewan River. J Environ Qual. 2001 Sep-Oct;30(5):1796-807
17. Muir DC and Grift NP. Herbicide levels in rivers draining two
prairie agricultural watersheds (1984) J Environ Sci Health B. 1987
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