Prior posts have noted that a wide variety of substances, including hazardous substances, can act as selection factors in fostering "evolution" amongst impacted organisms. Now, researchers have found that many weeds, including some of significant economic importance because of the crops impacted, are resistant to multiple herbicides. This should come as no surprise to readers of this blog.
Resistance in weeds has been a factor known to agronomists for decades. But researchers point to the introduction of glyphosate immune crops in 1996 as a key event because it resulted in the very heavy application of glyphosate on croplands. Today, nearly 98% of U.S. soybeans, 88% of U.S. cotton, and 70+% of U.S. corn come from cultivars resistant to glyphosate. It should be no surprise then that such heavy application selected for resistant weed strains, accelerating their widespread presence. Other weeds are now showing resistance to other widely used herbicides.
For example, Hybrid Goat Grass has shown signs of tolerance to the herbicide imazamox in fields planted with wheat breed to be immune to that herbicide. However, in this case the resistance is developing not exclusively from herbicide exposure but also from the spread of resistant genes in the pollen of wheat, the weed's distant cousin, through interbreeding. Although farmers can target the hybrid weed with other herbicides, they risk simultaneously impacting the wheat.
Not surprisingly, weeds immune to one herbicide will generally prove resistant to other herbicides that utilize the same mode of action, usually the chemical's disruption of an enzyme essential to plant (weed) function. For example, Common Water Hemp has risen to become a major weed in Midwestern soy and corn production. Studies have found that it is now immune or resistance to five different herbicide groups. The result is that soy farmers are nearly out of options for herbicides. In Australia, it has been reported that some weedy ryegrass species are resistant to seven different herbicide types.
In the 1970's, I had the honor of co-authoring with Dr. Dale Bottrell a CEQ report on Integrated Pest Management; Dr. Bottrell has gone on to write widely about IPM [for his well reasoned assessment of IPM and its failure to be truly implemented, see http://www.issues.org/16.3/ehler.htm]. We foresaw problems of this type, not because we were great prognosticators, but because the problems were already manifesting themselves with insect pests in (for example) cotton crops. The lessons clearly have yet to be learned. Whether the pest is a plant or an insect, the fundamentals of environmental science apply. Exposure to any substance can act as a selection factor, yielding resistant or "evolved" strains.
Reports and data bases on the topics noted in this blog can be found at http://www.weedscience.org/In.asp, http://pubs.acs.org/doi/abs/10.1021/jf102652y, http://pubs.acs.org/doi/abs/10.1021/jf102704x, http://pubs.acs.org/doi/abs/10.1021/jf104719k, http://pubs.acs.org/doi/abs/10.1021/jf101286h, http://onlinelibrary.wiley.com/doi/10.1002/ps.2100/abstract, http://pubs.acs.org/doi/abs/10.1021/jf103389v, and http://pubs.acs.org/doi/abs/10.1021/jf103797n.