Science in Society Archive

Ready for Dicamba Ready GM crops?

Turning an environmental disaster into a real catastrophe. Prof. Joe Cummins

Herbicide tolerant GM crops near obsolete

It has become all too clear that genetically modified (GM) herbicide-tolerant crops are not sustainable, as herbicide resistant weeds soon appear, rendering the GM crops obsolete [1, 2] ( Roundup Ready Sudden Death, Superweeds, Allergens... , SiS 28) . In addition, herbicide-tolerant crops may themselves become ‘volunteer' weeds. Monsanto Company had recognized the problem, and created patented herbicide mixtures as early as 2001 to combat weeds resistant to its formulation of glyphosate (Roundup) and volunteer ‘Roundup Ready' GM crops [3]. Herbicide-resistant weeds arise either by mutation or by gene flow from the transgenic crops [4]. The herbicide-tolerance gene from GM crops has been found to transfer to weedy relatives [5]. In short, the useful life of the original GM herbicide-tolerant crops is rapidly nearing its end.

So, crops resistant to the phenoxy herbicide Dicamba have been developed as a replacement for the near-obsolete tolerance to glyphosate, as well as 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, and ethylene glycol and 2,7-dichlorodibenzo-p-dioxin.

Dicamba an old companion of “agent orange”

Dicamba is actually an old herbicide that served alongside “agent orange” in Vietnam [7], and has been resurrected 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 to the chloroplast and a transcription terminator from pea. In this transformation, which localized 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 40 field-test releases of Dicamba resistant corn, cotton or soybean were approved in the United States; all undertaken by Monsanto Corporation, except for one test each from the University of Kansas and BASF Corporation. Several field test releases included crops stacked with transgenes for glyphosate tolerance, 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), though 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 Dicamba resistant GM crops or GM crops with stacked traits, and the locations of the tests are withheld from those who may be exposed.

Dicamba not safe nor environmentally friendly

The toxicity of the Dicamba 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 human [12]. Dicamba proved genotoxic in mutation tests using transgenic Arabidopsis thaliana plants [13]. Dicamba was found to be a peroxisome proliferator in rats, an activity associated with liver cancer in rodents [14]. Dicamba is certainly not safe or environmentally friendly, and its increased deployment must be resisted.

Dicamba is relatively persistent in the environment and it is only 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 two prairie watersheds, Manitoba streams draining agricultural lands were elevated during the herbicide application period in May and June [17].

Predictably, the overuse of herbicides in GM crops has resulted in the GM crops becoming obsolete as weeds become herbicide resistant. Introducing GM crops resistant to an old polluting phenoxy herbicide seems like a measure of sheer desperation, turning an environmental disaster into a real catastrophe.

The only way to deal with the problem of herbicide resistant weeds and volunteers is to return to sustainable, organic agriculture, free of polluting herbicides.

Article first published 20/06/07


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