Is it wise to genetically tamper with the crops we grow, asks Prof. William Reville
It is quite likely that Roundup is the herbicide that is best known to readers of this column. Roundup is widely used worldwide in agriculture and gardening to control weeds.
But weeds are far from helpless in the face of this chemical onslaught and use the evolutionary mechanism of natural selection to develop resistance to herbicides.
This is what is happening in the case of Roundup. Several ”monster” weed species in the United States have outsmarted the chemical and biological technologies developed to control them.
This is potentially quite a problem since much commodity-crop production worldwide (including soya bean, cotton, corn and canola) has been genetically engineered by the giant biotech corporation Monsanto to become dependent on Roundup control.
The story is told in detail by Jerry Adler in Scientific American, May 2011, and illustrates the pitfalls that can await us when we make sudden significant artificial genetic alterations to plants or animals in the interests of improving efficiency, convenience and yield and, of course, of making money.
Weeds grow naturally and profusely among cultivated crops of useful plants (such as corn and cotton), compete with them for nutrients and greatly reduce crop yield if not controlled.
For example, a single giant ragweed plant growing in the midst of 30 soya plants will reduce their yield by half.
Traditionally, weeds were controlled by ploughing and hoeing but, organic herbicides were developed after WW2 and, since the 1950s, agriculture has become dependent on chemical herbicides.
Roundup, discovered in 1970 and patented and manufactured by Monsanto, is probably the best-known chemical herbicide.
The active agent in Roundup is glyphosate. It acts by inhibiting an enzyme (EPSPS) that controls the synthesis of three amino acids (chemical units used by the cell to make proteins) in plants, but not in animals.
It will kill any green plant and although very effective it was initially difficult to use in agriculture because it had to be very carefully applied to the weeds only, avoiding the crop plants. Despite the effectiveness of glyphosate, it has a relatively low toxicity and persistence in soil and, among herbicides, it is relatively benign.
Threat
Monsanto decided in the 1990s to control weeds and boost commodity-crop production by genetically engineering the major commodity crops to become resistant to Roundup.
This development came to fruition only after prodigious effort and scientific ingenuity and the investment of huge monies.
Monsanto inserted a Roundup-resistant variety of the gene EPSPS that controls the synthesis of the three amino acids into these crops.
The first ”Roundup Ready” seeds produced were soya beans in 1996; Roundup Ready corn, cotton and canola soon followed.
Weed control now became simple — douse a crop with Roundup every time weeds emerge. Roundup revolutionised the farming of commodity crops in the US and elsewhere, particularly in Argentina and Brazil.
In the US, 93 per cent of soya bean and a large proportion of corn and cotton is Roundup Ready. In addition, there is less need for tillage of Roundup Ready crops, which reduces soil erosion and fuel consumption, and nutrient runoff is also reduced. But, increasing weed resistance to glyphosate is now threatening Roundup’s reign. Biological evolution through natural selection is a powerful and successful mechanism, which is now successfully challenging Roundup.
It is only to be expected that natural selection would give a good account of itself in any contest. After all this is the mechanism that powered biological evolution from the first simple life form that arose on earth almost four billion years ago to the myriad species of life that today colonise every environmental niche on Earth.
Weeds
Natural selection works as follows: variety exists in every biological population, including weeds. Variety also continually arises naturally though mutations — random genetic changes.
While the great majority of individual weeds succumb to Roundup, some varieties are more resistant and a few individuals survive.
Constantly challenging the weeds with Roundup means that the Roundup-resistant varieties gradually come to prominence and eventually dominate the variety spectrum present in the weeds. Roundup then becomes ineffective.
About 10 species of weed in the US and 10 in the rest of the world have developed resistance to Roundup and, on average, one resistant species is added every year. Monsanto point out that these 10 resistant species are only 10 out of about 300 weeds that can infest the crops.
However, as Adler points out, the weeds that have developed the resistance are amongst the most prolific and intractable pests. They include common ragweed, giant ragweed, horseweed, Johnsongrass, water hemp and pigweed.
Some of these are awesome — giant ragweed can grow more than 3m (10ft) tall. Pigweed can grow a stalk as thick as a baseball bat, and tough enough to disable a combine harvester.
Resistance
Roundup resistance isn’t a problem in Ireland because we do not grow genetically modified plants here and Roundup is not used to control weeds in Irish agriculture.
Neither, by and large, are GM crops grown in Europe, but there is more pressure in some European countries to introduce genetically modified plants than there is in Ireland.
However, herbicide-resistant weeds can arise anywhere and do not require the availability of genetically modified plants. One way to encourage herbicide resistance is to use the same herbicide, year in year out.
This consistently forces natural selection to choose the herbicide-resistant strains of weeds and these resistant strains will come to dominate the weed population in time. This is what happened in the Roundup story.
Many farmers used Roundup as the exclusive herbicide on Roundup Ready crops. Monsanto should have more strictly advised them to use other types of herbicide, in addition to Roundup, to control weeds, inconvenient and all as this additional step would have been.
The weeds would then have to develop resistance to several chemicals in order to defeat the herbicides — a much more difficult task to achieve — and resistance to Roundup would have taken much longer to develop.
Mindful of this, Monsanto is now genetically engineering plants with inbuilt resistance to two or more herbicides. But, while this tactic is understandable it may be unwise in the long run. Natural selection will slowly but surely develop weeds that are resistant even to cocktails of herbicides.
That problem will then be exacerbated by the fact that only a limited number of chemical herbicides have been developed because advances in this area were stalled by the biotech industry in favour of the more profitable GM area.
What will we do when weeds become resistant to all available herbicides? Will we develop weeds that nothing can control?
The Roundup story is a salutary reminder of how careful we must be when we introduce artificial genetic changes into nature.
I am not against GM in principle but we must proceed very carefully and slowly. Such caution, of course, clashes with understandable human desires for convenience and profit.
The farmers wanted the convenience of controlling weeds with a simple douse of Roundup and Monsanto wanted a quick return on their massive investment in developing the GM technology.
We cannot hope to feed our teeming world population without the assistance of scientific agriculture but we must also heed the advice of ecologists, and indeed the Church, to tend very carefully to our wonderful natural inheritance of plant and animal life.
Professor William Reville is a member of staff of the Biochemistry Department and Public Awareness of Science Officer at University College Cork; http://understanding science.ucc.ie