The losses caused by weeds to our major crops are estimated to be about Rs 65 billion. These losses can be reduced by means of weed prevention, eradication and chemical control, said Dr Muhammad Sarwar Khan, Director, Center of Agricultural Biochemistry and Biotechnology (CABB), University of Agriculture, Faisalabad.
Chairing a departmental meeting on engineered weed management and perceptions, he said that engineered herbicide tolerance in plants provides a superior mechanism of weed control in crop husbandry using broad spectrum herbicides. Nevertheless, resistance development in plants particularly of weed, and the carcinogenic effects of glyphosate on human beings are reported as implications of herbicide applications.
He said chemically controlling weed is most effective method in the developed world. It constitutes about 55 % of the total pesticides used in these countries. In Pakistan, the herbicides are specifically imported from USA, Europe, Japan, or China and their usage is increasing rapidly. These herbicides are selective in nature and kill weeds with limited damage to crop plants. This also releases thousands of people from the drudgery of hand weeding. Being selective in nature, the use of these herbicides is species-dependent where dose of the chemical is determined, keeping in view the genetic makeup, age, growth rate, morphology, physiology and biochemistry of the plant. Hence, these are used as pre- or post-emergence and specific to broad or narrow leaf pants. Specificity of the herbicides necessitates the development of non-selective broad spectrum herbicides that do not discriminate plants based on their age, growth rate, morphology, physiology and biochemistry. However, use of non-selective herbicides demands a novel technology that allows desired plants to survive while leaving others susceptible.
He said glyphosate is the world leading herbicide as it is affordable and GM crops have been developed with a trait which can tolerate the herbicide dosage. About 80 % of the GM crops have resistance to glyphosate. However due to the over-reliance on glyphosate and glyphosate herbicide resistant crops, weeds are rapidly getting resistant to this class of chemicals, contrary to the pre-roundup ready arena when nearly 400 herbicides with 17 different modes of action were used. Further, global consensus is building due to its health related issues.
In this regard, glufosinate has been introduced in the market as an alternative to glyphosate. Glufosinate kills weeds as a contact herbicide and it must be applied to smaller plants. Development of glufosinate-resistant traits has been reported worldwide in corn, soybean and cotton until recently. Although glyphosate beats it in cost and more restrictive application timing relative to weed size, but it is found to be more advantageous as it kills all weeds and plants that have developed resistance against glyphosate. To add more in this account, no weed has been formally reported as glufosinate-resistant yet. The trait has also been developed in sugarcane in Pakistan, and is being tested for field dose applications.
He said the journey that was started from mechanical methods to chemical control; and then to genetically modified crops with post-emergence application of non-selective herbicides has paid off in the form of controlled losses. However, even today there are losses due to weeds, part of them being due to the evolution of herbicide-resistant weeds and reliance on a single commercial herbicide and limited number of genetically modified crops with diverse herbicide-resistant traits. Further, apart from resistance development in weeds against glyphosate there is an evolving story that glyphosate causes health risks in human beings. So, the scenario can become worse if new herbicides are not developed readily.
He said looking into details both agencies used different approaches to risk assessment. IARC has assessed the cancer hazard, considering the likelihood that a chemical might, at least in some circumstances, cause cancer in humans. Whereas, EFSA is principally concerned with whether there is sufficient confidence that a pesticide will not pose an unacceptable risk to human health or the environment when used following the conditions of its approval. EFSA assessed some studies conducted by industry groups that were excluded from the IARC analysis. The IARC team looked only at evidence that is in the public domain and available to independent scientists to review (For details; an article by Daniel Cressey in Nature issue November 13, 2015).
He said herbicide resistant crops have a great potential in the simplification of weed management. Handled judiciously, these crops may be beneficial to the environment but these should be carefully evaluated prior to releasing into a cropping system, especially when GM crop possess weedy characters or may outcross to related weeds. Further, extensive and continuous use of a single herbicide should be avoided to exclude the possibilities of resistance development in plants and precautionary measures should be taken to safeguard human health.
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