Glyphosate Resistance in Weeds: The Transgenic Treadmill
Institute of Science and Technology, March 3, 2010
Straight to the Source
Before 1996, weeds were not observed to have evolved resistance to glyphosate in the field, but since then, the introduction of transgenic glyphosate tolerant crops has led to evolution of a number of resistant weeds as the result of the greatly increased use of the herbicide particularly during the post-emergent growth of the crops. Glyphosate reisistant Asiatic dayflower (Commelina cumminus L) common lambsquarters (Chenopodium album L) and wild buckwheat (Polygonum convolvulus L) are reported to be increasing in prominence in some agro ecosystems as are populations of horseweed (Conyza canadensis (L) Cronq) .
In regions of the USA where transgenic glyphosate resistant crops dominate, there are now evolved glyphosate-resistant populations of the economically damaging weed species Ambrosia artemissifolia (rag weed), Ambrosia trifida L.(great ragweed), palmer pigweed (Amaranthus palmeri), common water hemp (Amaranthus rudis) , rough fruit amaranth (Amaranthus tuberculatus) and various Conyza (horse weed ) and Lolium (rye grass) species.
Likewise, in areas of transgenic glyphosate resistant crops in Argentina and Brazil, there are now evolved glyphosate resistant populations of Johnson grass (Sorghum halepense) and Mexican fireplant (Euphorbia heterophylla) . These herbicide resistant weeds pose a clear threat to the transgenic crops dominating North and South America .
There is no simple remedy for the evolution of resistance to glyphosate. Interestingly, the inventor of both glyphosate and the herbicide tolerant crops, Monsanto Corporation, does not appear to be engaged in finding remedies for the invasion of resistant weeds. There has been an effort to remedy the invasion of resistant weeds by the academic community. Simulation modeling has been developed. Glyphosate use for weed control prior to crop emergence is associated with low risks of resistance. These models are based on assumptions that low risks can be further reduced by applying glyphosate in sequence with other broad-spectrum herbicides prior to crop seeding . Post-emergence glyphosate use, however, associated with glyphosate-tolerant crops, very significantly increases the risks of resistance evolution. Annual rotation with conventional crops reduces these risks, but the proportion of resistant populations can only be reduced to close to zero by mixing two of three post-emergence glyphosate applications with herbicides that have alternative modes of action. Weed species that are prolific seed producers with high seed bank turnover rates are most at risk of glyphosate resistance evolution. The model is especially sensitive to the initial frequency of resistance alleles, and other genetic and reproductive parameters, including weed breeding system, dominance of the resistance trait and relative fitness, influence rates of resistance. (Although these assumptions may be quite irrelevant in view of numerous physiological mechanisms of the 'fluid genome' that can produce resistant mutations in plants exposed to non-lethal levels of glyphosate, as discussed in  (GM Crops Facing Meltdown in the USA, SiS 46).
Over the past decade, the most problematic weeds in agronomic cropping systems have shifted away from perennial grass and perennial broad leaf weeds to primarily annual broadleaf weeds, although the glyphosate resistance mechanisms in weeds are currently poorly understood . It appears that post-emergent use of glyphosate may be a main contributor to evolved glyphosate resistance, but eliminating post-emergent herbicide treatment practically eliminates all the advantages of and hence the need for herbicide tolerant crops.