What glyphosate does to plants

December 2010

Weed-killers kill weeds. Just how they kill the weeds doesn't matter too much, so long as they do it efficiently.

During the many years that glyphosate (the active ingredient in Roundup brand weed-killer) could only be used to clear fields before planting began, or for paths and roadsides, little research was carried out to find out what it actually did to the weeds, besides kill them.

When GM Roundup Ready plants resistant to glyphosate appeared on the scene, we were presented with a clever technological trick which seemed to be based on the science of how the weed-killer worked. Glyphosate, we were told, kills by reacting with and inactivating an enzyme vital for the manufacture of many essential proteins in plants. The GM plants get round this by generating their own novel version of this enzyme which can't be knocked out by glyphosate. So we have weeding made easy: the GM seedlings survive spraying with Roundup while the weeds around them die.

But, are the life-processes really so simple?

Some varieties of Roundup Ready soya have a disturbing habit of turning yellow after being sprayed with Roundup. They recover, but it can take up to two weeks. This wasn't meant to happen.

When university crop scientists began belatedly to investigate the effects and side-effects of glyphosate on GM plants, what they found amounted to significant disruptions throughout the plants' physiology.

Roundup Ready plants can't get rid of the Roundup they absorb: glyphosate remains there until the plant is fully mature. Very little of the chemical stays bound to the enzyme it de-activates. Most glyphosate ends up being translocated to active growing areas, while a small amount is degraded to AMPA (aminomethylphosphonic acid) which is toxic to the plant. There's lots of scope here for the weed-killer to perpetrate wider metabolic mischief.

One major mischief comes from the fact that glyphosate reacts, directly and indirectly, with plenty of other vital substances in the plant besides the one enzyme replaced by genetic engineers. Foremost amongst these are the essential mineral micro-nutrients (such as magnesium and calcium) which glyphosate binds to and makes unavailable to the plant. After spraying, glyphosate can interfere with these micro-nutrients at all stages: in the soil, during root uptake and transportation to the leaves and growing points, and in all the plant's tissues.

The metabolic pathway glyphosate disrupts is a very major one: one-fifth of the carbon fixed by green plants (from carbon dioxide in the air using photosynthesis) is channeled into this pathway. The function of the pathway includes the production of a huge range of proteins (such as enzymes and structural materials), besides lignin, plant hormones and the plants' immune mechanisms. Glyphosate can potentially create mischief in all of these.

Recently published studies have revealed that glyphosate-treated GM plants have:

  • low leaf chlorophyll (i.e. the green colour which carries out photosynthesis)
  • low respiration
  • low water-use efficiency (i.e. an attribute that enables the plant to cope with drought)
  • reduced biomass of the shoot and roots
  • reduced leaf-area
  • short plant height
  • reduced amino acid content (i.e. the building blocks for all proteins)
  • reduced micro-nutrient content, with the exception of nitrogen
  • reduced lignin content (lignin is an important structural component providing mechanical support and compression strength to the plant, especially its vascular system, and contributing to protection of the seed coat)
These parameters are all part of the intricate physiological processes essential to life. How much any one of these disruptions causes, is caused by, or exacerbates the others can't be unravelled. For example, reduced photosynthesis must result in less energy being available to drive everything else, while low micro-nutrient availability will impair the enzyme activity needed for many life processes. Other interacting factors are now coming to light in glyphosate-treated areas, such as a higher incidence of plant disease and disturbances to beneficial soil microbes.

In summary, the metabolic pathway which is broken by glyphosate and allegedly mended by a GM elastoplast enzyme is clearly not made entirely whole. The picture emerging is that Roundup Ready plants are chronically sick.

First generation Roundup Ready soya was hyped as having “provided about 5% more crop” than conventional soya. In the event, it proved to yield more than 5% less. This is hardly surprising considering the fundamental physiological impairments wrought by the glyphosate it absorbs. Now, we have a second GM generation, Roundup Ready 2 Yield, again hyped as having improved yield. This new Roundup Ready soya has been manufactured using a less disruptive gene insertion technique, an improved DNA construct, and an elite soya cultivar. The studies found that it does, indeed, have less impairment of photosynthesis and respiration compared with its prototype, but these are cancelled out by an even greater reduction in leaf area and biomass (resulting in a reduced level of functional chlorophyll). Also, Roundup Ready 2 Yield doesn't seem to be yielding terribly well.

In West Virginia, officials are investigating Monsanto for unfair or deceptive practices in the marketing of its latest GM soya. Farmers there have relied on advertising claims by the Company that its Roundup Ready 2 Yield soyabeans would yield 7-11 percent more than the original GM ones. It seems the yields do not live up to these claims and do not justify the increased prices charged for them.

Monsanto described this allegation as a 'misunderstanding’. The Company's response was that it could provide data demonstrating the performance of Roundup Ready 2 Yield, based on 40,000 soybean yield records collected between 2007 and 2009. These data apparently give a rolling average yield benefit of more than 7 percent compared to competitors' seeds also engineered to tolerate Monsanto's Roundup herbicide.


This last statement has been emphasised because, typically, the Company has generated a huge amount of data based on comparisons with different (possibly irrelevant) seed varieties. It hasn't compared like-with-like. This trick can be used to statistically 'prove' just about anything you want. It may be good sales-talk, but it's bad statistics and it's certainly not science.

The science is telling us that all glyphosate-tolerant plants are sick, and sick plants are vulnerable to stress and disease. Expecting a good yield from sick plants is scientifically implausible.

Most GM crops in the pipeline seem to consist of elite varieties bred by traditional methods, with a herbicide-tolerance gene added in to allow them to be patented and sold at a premium, along with the herbicide. If we are going to 'feed the world', science tells us that these GM glyphosate-resistant (and probably other herbicide-tolerant) crops are definitely, and intrinsically, NOT the answer. Don't let anyone try to tell you otherwise.


  • Zobiole et al., 2010, Glyphosate affects lignin content and amino acid production in glyphosate-resistant soybean, Acta Physiologiae Plantarium 32
  • Zobiole et al., 2010, Glyphosate reduces shoot concentrations of mineral nutrients in glyphosate-resistant soybeans, Plant Soil, 328
  • Zobiole et al., 2010, Glyphosate affects photosynthesis in first and second generation of glyphosate-resistant soybeans, Plant Soil article published on-line 2.07.10
  • Carey Gillam, West Virginia investigating Monsanto for consumer fraud, Reuters 25.06.10
  • Monsanto 1997 Report on Sustainable Development including Environmental, safety and Health Performance
  • Tsuioshi Yamada, 2009, Glyphosate interactions with physiology, nutrition, and diseases of plants: Threat to agricultural sustainability?, European Journal of Agronomy

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