Weeds threaten the productivity of agricultural systems by affecting the quantity and quality of crop production. Archaeological studies have revealed archaism of weed control efforts by humans is equal to the antiquity of agriculture itself. In other words, ever since mankind began farming, the confrontation between humans and weeds began. Farmers have exploited all their strength for eradication or containment of the weeds, once with bare hands and doing hand weeding and developed and used new more effective and time-labor consuming tools over time. The results of these efforts have led to the development and completion of the weed control method. Of all these methods and approaches, herbicides have been a turning point in the history of weed management programs.
Herbicide use is increasingly being adopted around the world. Many developing countries are facing lack of enough workers to hand weed fields since millions of people migrate from agricultural to urban areas. That’s why, herbicides
are cheaper and more readily available than labor for hand weeding in these countries. Shortages of hand weeders and need to increase crop yields are the most important reason for increasing use of herbicide. In many parts of the world, herbicides are being increasingly used to replace tillage in order to improve environmental conditions. Herbicide approach reduces erosion, fuel use, greenhouse gas emissions and nutrient run-off and conserves water in comparison with tillage and conventional tools.
Discovery of the weed killing properties of the phenoxyacetic herbicides in Britain and the United States during 1942 to 1944 marked the real beginning of the herbicide phase of the ‘‘Chemical Era of Agriculture.’’ These new synthetic herbicides promoted broad-leaved weeds control especially in cereals significantly. The discoveries of phenoxyethyl sulfate, and the phenoxypropionic and the phenoxybutyric acids followed during the early 1950’s. The development of improved organic soil sterilizer herbicides progressed rapidly during the 1950’s and 1960’s with the substituted ureas and uracils, the chlorobenzoic acid and phenylacetic acid derivatives, the s-triazines, triazoles, and other heterocyclic derivatives coming along almost simultaneously. There has been increased emphasis on selective herbicides, especially among the carbamates and carbanilates, the amine, acetamide, and anilide group, the toludines and nitriles, as well as among the s-triazines and substituted urea compounds.
The sulfonylurea herbicides were discovered in 1975 and immediately set a new standard for chemical weed control. The subsequent worldwide development effort has led to the commercialization of many different active ingredients in over a dozen major crops, and additional development candidates have been announced. Some of the reasons for the rapid commercial acceptance of sulfonylurea herbicides include their exceptionally low application rates (often a factor of 10-100 times lower than conventional herbicides), favorable environmental and toxicological properties, and compatibility with the trend
towards postemergence weed control. Discovery of new herbicide mode of actions continued which led to introduction of Protoporphyrinogen Oxidase (PPO) and 4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors.
Nowadays there is serious limitations in the way of development and introduction of new herbicide family with novel mode of actions. In fact, no new major herbicide mode of action has been introduced in a commercial herbicide active ingredient in the last 20 years. There are probably several reasons for this. New potential products may have remained dormant owing to concerns that glyphosate-resistant (GR) crops have reduced the market for a new herbicide. The capture of a large fraction of the herbicide market by glyphosate with GR crops led to significantly diminished herbicide discovery efforts. Some of the reduced herbicide discovery research was also due to company consolidations and the availability of more generic herbicides. Another problem might be that the best herbicide molecular target sites may have already been discovered.
Weed problems and chemical weed control
Weeds are a serious threat to crop production as they reduce the yield of wheat, barley, rice, maize, and chickpea on average by 23%, 21%, 35%, 20%, and 50%, respectively. Orobanche spp., Avena ludoviciana (Durieu.), Convolvulus arvensis (L.), Sorghum halpence (L.) Pers, and Cuscuta compestris (Y.) are the most important weed species that compete with major crops. Recently, some newly introduced and invasive weeds, including Hordeum spontaneum (K. Koch.), Cynanchum acutum (L.), Physalis divaricata (L.), and Azolla filiculoides (Lam.), have become a very serious problem in a wide range of crops.
Chemical control is the predominant weed management practice. In addition, mechanical weed control methods including soil tillage and hand weeding are applied to a lesser extent. 2,4-D + MCPA (in cereal crops), clodinafop-propargyl (in wheat crop), haloxyfop-r-methyl ester (in broadleaved crops), tribenuron-methyl (in wheat), nicosulfuron (in maize), trifluralin (in oil crops), metribuzin (in potato), glyphosate (in orchards and non-cultivated areas), and paraquat (in waste lands and between crop rows) are the most commonly used herbicides. There are currently 14 unique cases (species x site of action) of herbicide-resistant.
The most important and newly emerged challenges are to manage the present noxious and invasive weed species. Increasingly, the evolution of herbicide-resistant biotypes in wheat and other important crops would be another challenge in the future. In addition, the adoption and extension of integrated weed management strategy, addition of suitable adjuvants to herbicide tank mixture, and use of proper sprayers would remain critical challenges in weed management practices. The integration of weed control methods such as crop rotation,
tillage, planting date and pattern, herbicides, and allelopathy would lead to the effective and sustainable management of weeds.