ASKED TO name a chemical pesticide, I’m sure most readers would cite Roundup. Roundup is widely used in agriculture and gardening to control weeds. But weeds are far from helpless in the face of this chemical onslaught. Weeds use the evolutionary mechanism of natural selection to develop resistance to pesticides.
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 become dependent on Roundup control. The situation is detailed by Jerry Adler in Scientific American, May 2011.
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. Traditionally, weeds were controlled by ploughing and hoeing but, since the 1950s, agriculture has become dependent on chemical pesticides.
Roundup, manufactured by Monsanto, is the best-known chemical pesticide. The active agent in Roundup is glyphosate. It acts by inhibiting an enzyme 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. Despite the effectiveness of glyphosate, it has a relatively low toxicity and persistence in soil and, among pesticides, it is relatively benign.
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 by inserting a Roundup-resistant variety of the gene 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 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, which reduces soil erosion and fuel consumption, and nutrient runoff is reduced. But, increasing weed resistance to glyphosate is threatening Roundup’s reign.
Biological evolution through natural selection is a powerful and successful mechanism, which is now successfully challenging Roundup. It 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. Constantly challenging the weeds with Roundup means that the Roundup-resistant varieties come to prominence and soon dominate the variety spectrum present in the weeds. Eventually Roundup becomes ineffective.
About 10 species of weed in the US have developed resistance to Roundup. 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.
Roundup resistance isn’t a problem in Ireland because we do not grow genetically modified plants here. There is more pressure in some European countries to introduce genetically modified plants than in Ireland.
Herbicide-resistant weeds can arise anywhere and do not require availability of genetically modified plants. One way to encourage herbicide resistance is to use the same herbicide, year in year out, consistently forcing natural selection to choose the herbicide-resistant strains of weeds. These resistant strains will come to dominate the weed population in time. It is best to use several types of herbicide because in order to defeat herbicides the weed must now develop resistance to several chemicals – a much more difficult task to achieve.
Part of the reason some weeds have developed resistance to Roundup so quickly is that many farmers used Roundup as the exclusive herbicide on Roundup Ready crops. Resistance would have taken much longer to develop if other herbicides had been used in addition to Roundup. Monsanto is now genetically engineering plants with inbuilt resistance to two or more herbicides. This does not seem wise. What will we do when weeds become resistant to these? Will we develop weeds that nothing can control?
Prof William Reville is a member of staff of the biochemistry department and public awareness of science officer at University College Cork; understandingscience.ucc.ie