Weed management and herbicide resistant weeds: a case study from wheat growing areas of Pakistan

Evolution of herbicide resistance in weeds is among the serious challenges that agriculture face today. In Pakistan herbicide use over last 6 decades has enabled earlier planting of short-season crops and improved weed control in wheat as well as other cropping systems. This has greatly favored the use of more and more herbicides. Conversely, this exercise increased the evolution of resistant biotypes. Wheat being Pakistan's number one staple crop faces daunting weeds problem. In order to understand the status of herbicide resistant weeds in wheat fields, survey was conducted for two consecutive years (2015-2017) in different regions of Pakistan. A questionnaire was designed to collect data from three hundred farmers randomly selected in all four provinces of Pakistan on herbicides used in wheat, history of crop cultivation regimes and weed species that have seized to respond to field rates of herbicides. Seed of suspected biotypes were collected from the survey sites and preserved in our gene pool for the confirmation of the resistance in the laboratory by bioassay and molecular level in future. Farmers (96%) perceived that Phalaris minor Retz. was the most important annual weed of wheat crop followed by Avena fatua L. (94%). Customarily, three aryloxyphenoxy propionate herbicides, clodinafop-propargyl, fenoxaprop-P-ethyl and diclofop-methyl were used to control different grass weeds such as P. minor, A. sativa L. and A. fatua. In farmers' opinion, high yield of wheat was associated with the continuous use of these herbicides. Resistance-suspicious weeds were reported from all studied areas (Punjab, KPK, Sindh, and Baluchistan provinces) in fields with the herbicide use history of five to seven years or more. This study suggests that for long-term avoidance of herbicide resistance, in our wheat growing systems we will require embracing practices that decrease selection pressures favoring resistant weeds. Achieving these goals requires an understanding of the evolution and dynamics of resistant populations. It is recommended that for the effective management of resistant populations of P. minor and A. fatua, farmers' education on ecologically sustainable weed management is critically important. This study will provide a basis for examining the questions that are relevant to understanding herbicide resistance evolution in Pakistan and that may help determine appropriate weed management strategies