1. Human Health and Ecological Risk Assessment for 2,4-D - USDA Forest Service
2. Dow's solution to glyphosate-resistant weeds: 2,4-D resistant crops

GMWatch comment: Earlier today we put out a bulletin (item 2) about a Dow-sponsored study that touts GM crops engineered to tolerate the herbicide 2,4-D as a 'solution' to the problem of glyphosate-resistant superweeds a problem caused by widespread use of GM glyphosate-tolerant crops. Glyphosate-resistant weeds are fast making GM Roundup Ready technology redundant.

Dow's employees paint a reassuring picture of 2,4-D in their study, claiming it has "environmentally friendly properties such as short environmental persistence and low toxicity to humans and wildlife." The authors promote GM 2,4-D tolerance as an important advance that "can help preserve the productivity and environmental benefits of herbicide-resistant crops".

One of our subscribers has told us about a summary of scientific findings compiled by the USDA Forest Service of the risks of the herbicide 2,4-D to health and the environment (item 1). While it takes the conservative approach typical of government regulatory assessments of pesticides, it is rather more honest than the Dow study in its assessment of 2,4-D's toxicity - and comes up with decidedly less reassuring conclusions.

The biggest irony here is that Dow's 'solution' to glyphosate-reistance has already failed. Weed species resistant to 2,4-D already exist, according to the pesticide industry-financed website weedscience.org (Herbicide Resistant Weeds Summary Table. July 26, 2010, www.weedscience.org, http://www.weedscience.org).   
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1. Human Health and Ecological Risk Assessment for 2,4-D
USDA Forest Service
Sept 30, 2006
http://www.fs.fed.us/foresthealth/pesticide/pdfs/093006_24d.pdf

EXCERPTS:
For many pesticides, including 2,4-D, accidental exposure scenarios, some of which are extremely conservative and perhaps implausible, lead to risk quotients that exceed the level of concern. 2,4-D is, however, somewhat atypical because many non-accidental exposure scenarios i.e., exposures that are plausible under normal conditions of use also exceed the level of concern and often by a very substantial margin.

Unless steps are taken to mitigate risks, workers involved in the application of 2,4-D and members of the general pubic who consume vegetation contaminated with 2,4-D could be exposed to 2,4-D levels greater than those which are generally regarded as acceptable. In some cases, the exceedances are substantial. Similarly, adverse effects in the normal use of 2,4-D salts or esters could occur in groups of nontarget organisms including terrestrial and aquatic plants, mammals, and possibly birds. Adverse effects on aquatic animals are not likely with formulations of 2,4-D salts except for accidental and extreme exposures at the upper ranges of application rates. The ester formulations of 2,4-D are much more toxic to aquatic animals and adverse effects are plausible in sensitive species and sometimes in relatively tolerant species. The results of this risk assessment suggest that consideration should be given to alternate herbicides and that the use of 2,4-D should be limited to situations where other herbicides are ineffective or to situations in which the risks posed by 2,4-D can be mitigated. ...

Based on recent studies published in the open literature, 2,4-D is toxic to the immune system and developing immune system, especially when used in combination with other herbicides. The mechanism of action of 2,4-D toxicity is cell membrane disruption and cellular metabolic processes. The molecular basis for 2,4-D toxicity to human lymphocytes and nerve tissue is likely the induction of programmed cellular death known as apoptosis.
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2. Dow's solution to glyphosate-resistant weeds: 2,4-D resistant crops

Wright, T. R., G. Shan, et al. (2010). "Robust crop resistance to broadleaf and grass herbicides provided by aryloxyalkanoate dioxygenase transgenes." Proc Natl Acad Sci U S A 107(47): 20240-20245.

Free in full:  http://www.pnas.org/content/107/47/20240.full.pdf+html

Abstract: Engineered glyphosate resistance is the most widely adopted genetically modified trait in agriculture, gaining widespread acceptance by providing a simple robust weed control system. However, extensive and sustained use of glyphosate as a sole weed control mechanism has led to field selection for glyphosate-resistant weeds and has induced significant population shifts to weeds with inherent tolerance to glyphosate. Additional weed control mechanisms that can complement glyphosate-resistant crops are, therefore, urgently needed. 2,4-dichlorophenoxyacetic acid (2,4-D) is an effective low-cost, broad-spectrum herbicide that controls many of the weeds developing resistance to glyphosate. We investigated the substrate preferences of bacterial aryloxyalkanoate dioxygenase enzymes (AADs) that can effectively degrade 2,4-D and have found that some members of this class can act on other widely used herbicides in addition to their activity on 2,4-D. AAD-1 cleaves the aryloxyphenoxypropionate family of grass-active herbicides, and AAD-12 acts on pyridyloxyacetate auxin herbicides such as triclopyr and fluroxypyr. Maize plants transformed with an AAD-1 gene showed robust crop resistance to aryloxyphenoxypropionate herbicides over four generations and were also not injured by 2,4-D applications at any growth stage. Arabidopsis plants expressing AAD-12 were resistant to 2,4-D as well as triclopyr and fluroxypyr, and transgenic soybean plants expressing AAD-12 maintained field resistance to 2,4-D over five generations. These results show that single AAD transgenes can provide simultaneous resistance to a broad repertoire of agronomically important classes of herbicides, including 2,4-D, with utility in both monocot and dicot crops. These transgenes can help preserve the productivity and environmental benefits of herbicide-resistant crops.