Finding answers to herbicide resistance

Posted on August 13, 2015

Agriculture has been an integral part of human life since the earliest times. When our nomadic ancestors began to settle and grow their own food, human society changed forever. Throughout history, humans have had a very intimate relationship with the land and no matter how sophisticated civilizations became, they could not deny their dependence on agriculture for their continued growth and survival. Despite the fact that many of us firmly believe that a good meal is just a trip to the supermarket away, our dependence on agriculture for survival remains a reality and access to advanced biotechnology in the agricultural sector, especially in developing countries like South Africa, has become crucial for boosting agricultural productivity so as to ensure continued food security.

It is estimated that in the next three to four decades food production will have to be doubled in order to keep pace with burgeoning human populations – the global human population is projected to increase from the present six billion to more than nine billion by 2050. This has led to an important movement in agricultural research towards molecular aspects of plant growth and development, and although the use of genetically modified (GM) crops is controversial, its role in ensuring food security for the world’s growing population is a reality.

Plant genetic engineering methods have been refined almost to an art form over the last 30 years, and GM crops have been widely adopted across the globe. According to TerrAfrica (a NEPAD-led partnership with representation in 24 African countries that support innovative solutions to sustain landscapes, address land and water degradation and adapt to the changing climate), GM crops may increase land productivity in Africa where 49% of soil is heavily degraded, as these crops could be engineered to endure harsher conditions and be less susceptible to climate changes such as drought, which is a leading cause of food insecurity in Africa.

Genetically modifying a plant involves inserting one or more genes into its natural genetic coding in order to add desirable traits such as resistance to a particular disease or substance. The genes that are inserted may come from the same type of plant, from a completely different plant species, or even from a totally unrelated organism, making it possible to add attributes that would otherwise have been impossible. Effectively, GM crop biotechnology has reduced the time it would take to improve a crop to only a few years, from the thousands of years it took when wild plants were first cultivated some 10 000 years ago, and from the decades required in modern-day classical plant breeding methods.

GM crops have many improved traits that make them attractive to commercial farmers and governments alike. Herbicide-resistant cotton, soybeans and maize, for example, have been extensively farmed all over the world for years, and can be ‘weeded’ with herbicides that are more effective, less toxic and cheaper than the alternatives. In South Africa, specifically, approximately 85% of maize, 100% of cotton and 92% of soybeans currently grown are transgenic varieties that are tolerant to the herbicide glyphosate. Glyphosate is a broad-spectrum systemic herbicide that is commonly used to kill weeds, especially annual broadleaf weeds and grasses known to compete with commercial crops.

The Monsanto/BE at UP Collaborative Research Programme is a joint research initiative between the University of Pretoria (UP) and Monsanto South Africa. Monsanto is a multinational agrochemical and agricultural biotechnology company and a leading producer of genetically engineered (GE) seed, as well as of the herbicide glyphosate, which it markets under the Roundup® brand.  The collaborative programme was initiated in September 2012 for an initial three-year period and is based in the Department of Plant Production and Soil Science at UP.

Glyphosate-tolerant crop technology has been effective in controlling a broad spectrum of weeds without risk to transgenic crops and South African farmers have used it continuously since it was first introduced in 1976. Unfortunately, as is the case with most human inventions aimed at manipulating nature, problem situations related to glyphosate-resistant weeds cropped up soon after the first introduction of Roundup Ready® crops in 1996. It is important to note though that these problems were not linked to Roundup Ready® crops specifically, but rather to the high frequency of use of glyphosate in orchards and vineyards in the intervening years since the herbicide was first introduced in 1976. About five years after the release of Roundup Ready® crops in the United States (US), weed resistance to glyphosate was observed for the weeds Amaranthus tuberculatus (waterhemp) and Conyza canadensis (Canadian horseweed). Today, 32 weed species in several countries in both the southern and northern hemispheres have proven resistance to glyphosate and it is expected that the number of resistant species will increase as glyphosate use increases in current and future glyphosate-tolerant crops. It is known that herbicide resistance in weeds is a technical problem that can be managed and even neutralised through sound weed resistance management strategies that incorporate best practices.

The research component of the collaborative programme at UP aims to monitor whether glyphosate-resistance in weeds is developing and to assess the plant mechanisms involved. The emphasis is on factors affecting weed resistance, as well as ways to overcome it and avoid it from taking hold. The programme also considers opportunities for amending current research protocols that apply to weed resistance research, suggesting what kind of amendments are likely to improve research techniques and make research findings applicable to practical situations. 

An important downstream facet of the programme is the dissemination of research findings in both scientific and popular media, with the ultimate aim of providing facts and perspective on the issues pertaining to GM crops, weed resistance to glyphosate, as well as the associated best agricultural practices. By following both a research and an advocacy approach, the programme aims to inform global efforts to manage herbicide resistance effectively and to contribute knowledge that can increase farmers’ profit margins and secure rural livelihoods, especially in developing countries on the African continent. The research programme is led by Dr Carl Reinhardt (better known as Prof Charlie); the academics in charge of projects in the programme are Dr Juan Vorster and Prof Hannes Robbertse.

The weed Conyza bonariensis, commonly known as hairy fleabane, has been chosen as the first of many weeds to be included in investigative research on glyphosate-resistant weeds. In South Africa, three species, (C. bonariensis, C. canadensis (Canadian horseweed), and C. sumatrensis (tall fleabane), are well-known weeds that were first noticed about a century ago, with infestations of one or more species occurring in every province. To date, only three weeds have evolved resistance to glyphosate in South Africa, namely C. bonariensis, Plantago lanceolata and Lolium sp, and no confirmed cases of resistance to glyphosate have been found in areas of the country (i.e. the summer rainfall region) where genetically modified crops are planted. Although these three glyphosate-resistant cases are restricted to the Western Cape region, all three species are common in other parts of South Africa. Of the 32 weed species for which glyphosate resistance have been proven worldwide, 20 species also occur in South Africa, which means we have to be vigilant.

More than 20 populations from the Western Cape and 8 from South Africa’s summer rainfall region were sampled by Prof Charlie and his research team in 2012 and 2013. These populations are now used in experiments by students in the Monsanto/BE at UP Collaborative Research Programme on various topics surrounding the resistance of Conyza bonariensis to glyphosate, under the supervision of Prof Charlie, Dr Juan Vorster and Prof Hannes Robbertse. 


- Author Ansa Heyl

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