Cultivation of herbicide-resistant crops

USA: ‘Superweeds’ encouraged by GM plants?

Farmers in the US are increasingly facing problems with weeds that have developed resistance to certain herbicides. A recent study blames the problem on the large-scale cultivation of genetically modified crops.

Weed control in soybean farming: Repeated use of the same herbicide active ingredient has encouraged the development of resistant weeds.
Photograph: ARS-USDA

Glyphosate-resistant populations of Palmer amaranth or pigweed (Amaranthus palmeri) have been spreading across the south-eastern US since 2005. The weed is very hard to control.

Horseweed (Conyza canadensis) is another glyphosate-resistant weed affecting large areas of the US. The northern limit of the main area affected by horseweed borders on the area affected by A.palmeri and in some places they overlap.

The fact that, sooner or later, the use of herbicides leads to the emergence of resistant weeds has been known since the 1970s. The emergence of resistant weeds is documented in a publicly accessible database by the Weed Science Society of America. The database currently documents 194 species with 341 biotypes - local populations of a species – around the world that are resistant to at least one herbicide. Most herbicide-resistant weeds occur in the US.

In a study published in November 2009 in the US, the author Charles Benbrook presents the view that the cultivation of genetically modified herbicide-resistant crops exacerbates the problem of resistant weeds. The US grows around half of the world’s genetically modified crops. The majority are crops that are resistant to the broad-spectrum herbicide glyphosate (known under the brand name Roundup).

Resistant weeds due to repeated use of the same herbicide

Chemical synthetic herbicides have been available since the 1940s. A herbicide has to control the weeds that occur on the fields in which a crop is grown, but must not harm the crop itself or reduce the harvest yield. Different weeds are associated with each crop and in conventional agriculture they are controlled using different herbicides. The time window in which the herbicide can be applied is usually very small and depends on the growth phase of the crops and weeds.

In conventional farming, crop rotation requires a change of herbicide. The same herbicide is only used again and again in a short space of time with monocultures. It is this that provokes the spread of resistant weeds. The first resistant weeds developed in the 1970s. They were resistant to photosynthesis inhibitors, a commonly used group of active ingredients. Most of them were resistant to atrazine, which was used in maize monocultures for years. Today there are 68 known weeds around the world that are resistant to a photosynthesis inhibitor.

In the early 1980s new active ingredients appeared on the market: ALS (acetolactate synthase) and ACC (AcetylCoA-Carboxylase) inhibitors. These herbicides were also applied continually for years. Within ten years a large number of weeds around the world were resistant to these two groups of active ingredients; today there are 107 weeds, and 38 species. Resistances to other groups of active ingredients are also known to exist.

In the 1980s people began breeding herbicide-resistant (HR) crops, initially using conventional breeding methods. These represent an innovation in weed control because the complementary herbicide (the herbicide to which the crop is resistant) can be applied at any time during the growth phase without harming the crop. The conventionally bred HR crops were resistant to photosynthesis inhibitors and ALS inhibitors. Genetic engineering methods were then used to develop crops that are resistant to glyphosate and glufosinate. Since these are broad-spectrum herbicides, it was possible to insert the herbicide resistance into different types of crop. This means that crop rotation no longer automatically entails a switch to a different herbicide. However, repeated use of the same herbicide for years at a time encourages the development of resistance in weeds – in both genetically modified and conventional crops.

Herbicide rotation delays resistance development

The first genetically modified HR soybean varieties arrived on the US market in 1996, joined a few years later by cotton and maize varieties. In the case of HR soybeans, until recently only glyphosate-resistant seed was available. HR varieties now account for 90 per cent of soybean cultivation in the US, and for 80-90 per cent of cotton grown in the south-eastern US. Although glyphosate has been used since the 1970s, in the mid-1990s there was no record of any glyphosate-resistant weeds. By 2009 there were 16 species worldwide, nine of them in the US.

In the south-eastern US, where these weeds are particularly widespread, farmers frequently grew HR soybeans or HR cotton, or both in alternation for years at a time, which means that they continually used glyphosate for weed control. Further to the north and west, in the northern corn belt region, glyphosate-resistant weeds are currently found only on a small scale or in isolated instances. In this area, soybeans are grown in rotation with maize. Glyphosate-resistant maize has been on the market only since 1999 and was slow to catch on. However, rising cultivation of HR maize is now increasing the selection pressure on weeds in the northern corn belt as well.

Countermeasures are now being introduced to prevent the further spread of glyphosate-resistant weeds.

The seed companies have set up sustainable resistance management plans that favour farmers who use glyphosate in alternation with other herbicides – including those sold by their competitors. Some glufosinate-resistant soybeans that were developed a long time ago have recently become available. At the same time, applications have been submitted in the US for the authorisation of various new genetically modified varieties that are resistant to several different groups of active ingredients. Cultivation of these varieties could make it easier for farmers to switch regularly between herbicides with different active ingredients. Charles Benbrook fears, however, that if these plants are grown, farmers will start spraying several different herbicides at once, which will lead to the development of ‘superweeds’ with resistance to several different groups of active ingredients. Here too, the development of resistance will depend on whether these plants are grown continually over several years. At the moment, there are only isolated instances of weeds that are resistant to glyphosate and other herbicides - with one exception: in Missouri there are significant populations of common waterhemp (Amaranthus rudis) that are resistant to glyphosate, ALS inhibitors and another group of active ingredients.