Evolution Of Herbicide Resistance In Weeds Research Articles

Weed management using crop competition in Pakistan: A review

Agriculture occupies an important place in improving the living standards of farmers in Pakistan. About 90% of farm earnings rely on the cultivation of sugar, fibre, cereals and legumes. Due to lack of essential resources and technical expertise, every year thousands of farmers fail to reach maximum yield potential. Over 70% of farmers own less than 5 ha in Pakistan; therefore, it is uneconomic to employ costly mechanical and chemical strategies for the control of pests in their crops. Among these pests, we eds are considered to be the major obstacle to crop production, and can ultimately result in crop failure. Traditionally, manipulation of cropping techniques was employed for the control of weeds; later on, development of synthetic chemical herbicides made it easier to control weeds in a very short time period. However, over time the increased use of herbicides has led to the development of herbicide resistant weeds. Furthermore, increasing environmental concerns, weed population shifts, and increased managerial costs have made it difficult for farmers to control these weed species within their limited economic resources. Nowadays, scientists and research organizations are being urged to provide innovative weed management solutions, with minimal ecological impacts. Studies have revealed the importance of cultural strategies for the management of weeds in different cropping systems. Research has proved that alternation of cultural practices, and selection of competitive crop cultivars, could be a possible strategy to minimize the competitiveness of weeds. Increased crop densities, narrower row spacing, intercropping and alternation in row directions are among the weed control strategies gaining rapid attention in many countries. Unfortunately, limited information is available about weed management using crop competition in Pakistan. This review article focusses on the importance of these agronomic practices in reducing the competitive potential of weeds, for their effective and appropriate management in major crops of Pakistan. It is intended to assist researchers in the design of economically viable and eco-friendly weed management strategies, which will aid in eliminating the burden of herbicides and mechanical cultivation from farmer’s production costs.

English

Weeds cause great problems to humankind by interfering in food production, health, economic stability, and welfare. The overuse of synthetic herbicides for weed control eventually leads to the evolution of herbicide resistant weeds, which also resulted in growing public concern over their impacts upon human health. The intensification of arable farming has also had a simultaneous impact on the environment. This intensification of the landscape may disrupt natural processes, such as resistance to plant invasion. Therefore, the aim of this review is to address these production constraints by suggesting applications of allelopathy via smother cropping, because this may ease the incidence of herbicide resistance and in the process promote cultivated plant diversity and thereby maintaining healthy agroecosystems. Weed resistance to herbicides presents one of the greatest current economic challenges to agriculture. System-oriented approaches to weed management that make better use of alternative weed management tactics need to be developed. Plant roots exude a wide variety of metabolites, some of which may act as allelochemicals and mediate interactions between plants and other organisms. These metabolites are in essence chemicals from nature which may be exploited for weed management as an alternative weed control option. Smother crops, as well as its mulches, have been shown to release allelochemicals, which were inhibitory to weeds. The principal goal of smother crops is to control weeds by replacing an unmanageable weed population with a manageable smother crop. More data from the grey area where agriculture and ecology overlap will enable the greater use of ecosystem services for crop protection in agricultural production and consequently reduce the incidence of resistance to agricultural chemicals. Key words: Agroecosystems, allelopathy, cultivated ecosystems, herbicide resistance, smother cropping, mulching.

Conservation Tillage Under Threat in the United States

Conservation tillage, after being widely adopted in the past few decades, is now threatened by the development of herbicide resistant weeds. Of the 157.7 million ha of cropland across the United States (U.S.) in 2012, approximately 127.5 million ha were harvested. Roughly 44% of total cropland area used conservation tillage practices, including no-tillage. In 1989, there were approximately 0.34 ha of conservation tillage for every hectare of conventional tillage, including reduced tillage. By 2012, the ratio had grown to 1.64 ha of conservation tillage for every hectare of conventional tillage. New technology, such as herbicide-resistant crops, greatly helped to increase the adoption rate of conservation tillage practices, yet current herbicide-resistant weed challenges are a real and expanding threat to conservation tillage. Produc- ers are faced with difficult management decisions regarding herbicide-resistant weeds. Despite many of these ha currently being under United States Department of Agriculture (USDA)– Natural Resources Conservation Service (NRCS) conservation program contracts, the difficulty of controlling herbicide-resistant weeds places many ha at risk of being converted to higher-intensity tillage systems. The shift to higher-intensity tillage facilitates burial of small weed seed and/or the increase of pre-emergence and pre-plant incorporated herbicide activ- ity to control problematic weeds, especially in dry-land crop production. However, in the past decade in the U.S., there has been a renewed interest in cover crops and their management for both carbon sequestration and agronomic purposes. Start- ing in the late-1990s, scientists at the USDA – Agriculture Research Service (ARS) National Soil Dynamics Laboratory in Auburn, Alabama initiated research using a Brazilian style cover crop management technique utilizing a roller-crimper, in conjunction with herbicides, to terminate mature cover crops. High residue cover crops are increasingly being incorporated into integrated pest management recommendations to help alleviate herbicide resistance selection pressure through their weed suppressive characteristics.

Transfer of Dicamba Tolerance from Sinapis arvensis to Brassica napus via Embryo Rescue and Recurrent Backcross Breeding.

Auxinic herbicides (e.g. dicamba) are extensively used in agriculture to selectively control broadleaf weeds. Although cultivated species of Brassicaceae (e.g. Canola) are susceptible to auxinic herbicides, some biotypes of Sinapis arvensis (wild mustard) were found dicamba resistant in Canada. In this research, dicamba tolerance from wild mustard was introgressed into canola through embryo rescue followed by conventional breeding. Intergeneric hybrids between S. arvensis (2n = 18) and B. napus (2n = 38) were produced through embryo rescue. Embryo formation and hybrid plant regeneration was achieved. Transfer of dicamba tolerance from S. arvensis into the hybrid plants was determined by molecular analysis and at the whole plant level. Dicamba tolerance was introgressed into B. napus by backcrossing for seven generations. Homozygous dicamba-tolerant B. napus lines were identified. The ploidy of the hybrid progeny was assessed by flow cytometry. Finally, introgression of the piece of DNA possibly containing the dicamba tolerance gene into B. napus was confirmed using florescence in situ hybridization (FISH). This research demonstrates for the first time stable introgression of dicamba tolerance from S. arvensis into B. napus via in vitro embryo rescue followed by repeated backcross breeding. Creation of dicamba-tolerant B. napus varieties by this approach may have potential to provide options to growers to choose a desirable herbicide-tolerant technology. Furthermore, adoption of such technology facilitates effective weed control, less tillage, and possibly minimize evolution of herbicide resistant weeds.

Managing against the Development of Herbicide-Resistant Weeds: Sugarcane

Profitable sugarcane production in Florida requires effective weed management. Herbicides provide an efficient and cost-effective means of weed control, but excessive use of a single herbicide or group of herbicides with the same mechanism of action has resulted in the development of herbicide-resistant weeds. In crops such as sugarcane where a limited number of herbicides are registered, the loss of a single effective herbicide can be very costly. Thus, it is critical to manage herbicides in order to prevent or delay the development of herbicide-resistant weed populations. This revised 4-page fact sheet lists herbicides by group number, mechanism of action, chemical family, common name, and trade name. Written by D.C. Odero, B.A. Sellers, J.A. Ferrell, and G.E. MacDonald, and published by the UF Department of Agronomy, October 2014. SS-AGR-244/SC077: Managing Against the Development of Herbicide-Resistant Weeds: Sugarcane (ufl.edu)

The Kochia and Dicamba Story

Herbicides are standard weed management tools for producers. This lesson examines the development of herbicide resistant-weeds and research conducted to help predict and prevent this resistance. Discussion on populations of organisms and evolution of these populations is included.

A Novel Rice Cytochrome P450 Gene, CYP72A31, Confers Tolerance to Acetolactate Synthase-Inhibiting Herbicides in Rice and Arabidopsis

Target-site and non-target-site herbicide tolerance are caused by the prevention of herbicide binding to the target enzyme and the reduction to a nonlethal dose of herbicide reaching the target enzyme, respectively. There is little information on the molecular mechanisms involved in non-target-site herbicide tolerance, although it poses the greater threat in the evolution of herbicide-resistant weeds and could potentially be useful for the production of herbicide-tolerant crops because it is often involved in tolerance to multiherbicides. Bispyribac sodium (BS) is an herbicide that inhibits the activity of acetolactate synthase. Rice (Oryza sativa) of the indica variety show BS tolerance, while japonica rice varieties are BS sensitive. Map-based cloning and complementation tests revealed that a novel cytochrome P450 monooxygenase, CYP72A31, is involved in BS tolerance. Interestingly, BS tolerance was correlated with CYP72A31 messenger RNA levels in transgenic plants of rice and Arabidopsis (Arabidopsis thaliana). Moreover, Arabidopsis overexpressing CYP72A31 showed tolerance to bensulfuron-methyl (BSM), which belongs to a different class of acetolactate synthase-inhibiting herbicides, suggesting that CYP72A31 can metabolize BS and BSM to a compound with reduced phytotoxicity. On the other hand, we showed that the cytochrome P450 monooxygenase CYP81A6, which has been reported to confer BSM tolerance, is barely involved, if at all, in BS tolerance, suggesting that the CYP72A31 enzyme has different herbicide specificities compared with CYP81A6. Thus, the CYP72A31 gene is a potentially useful genetic resource in the fields of weed control, herbicide development, and molecular breeding in a broad range of crop species.

Herbicide Resistance: Toward an Understanding of Resistance Development and the Impact of Herbicide-Resistant Crops

Development of herbicide-resistant crops has resulted in significant changes to agronomic practices, one of which is the adoption of effective, simple, low-risk, crop-production systems with less dependency on tillage and lower energy requirements. Overall, the changes have had a positive environmental effect by reducing soil erosion, the fuel use for tillage, and the number of herbicides with groundwater advisories as well as a slight reduction in the overall environmental impact quotient of herbicide use. However, herbicides exert a high selection pressure on weed populations, and density and diversity of weed communities change over time in response to herbicides and other control practices imposed on them. Repeated and intensive use of herbicides with the same mechanisms of action (MOA; the mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide; e.g., inhibition of an enzyme that is vital to plant growth or the inability of a plant to metabolize the herbicide before it has done damage) can rapidly select for shifts to tolerant, difficult-to-control weeds and the evolution of herbicide-resistant weeds, especially in the absence of the concurrent use of herbicides with different mechanisms of action or the use of mechanical or cultural practices or both.

Why have no new herbicide modes of action appeared in recent years?

Herbicides with new modes of action are badly needed to manage the evolution of resistance of weeds to existing herbicides. Yet no major new mode of action has been introduced to the market place for about 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. However, target sites that are not utilized, for which there are inhibitors that are highly effective at killing plants, suggests that this is not true. Results of modern methods of target site discovery (e.g. gene knockout methods) are mostly not public, but there is no evidence of good herbicides with new target sites coming from these approaches. In summary, there are several reasons for a long dry period for new herbicide target sites; however, the relative magnitude of each is unclear. The economic stimulus to the herbicide industry caused by the evolution of herbicide-resistant weeds, especially GR weeds, may result in one or more new modes of action becoming available in the not too distant future.

The Effect of Rotation and In-Crop Weed Management on the Germinable Weed Seedbank after 10 Years

Agricultural production systems that reduce the use of in-crop herbicides could greatly reduce risks of environmental damage and the development of herbicide-resistant weeds. Few studies have investigated the long-term effects of in-crop herbicide omissions on weed seedbank community size and structure. A crop-rotation study was sampled 10 yr after a strictly annual rotation and an annual/perennial rotation were exposed to different in-crop herbicide omission treatments. In-crop herbicides were applied either in all annual crops (control), omitted from oats only, or omitted from both flax and oats. Seedbank densities were greatest when in-crop herbicides were omitted from flax and oats, and this treatment also reduced crop yield. Shannon-Wiener diversity differed among crops in the annual crop rotation and among herbicide omission treatments in the perennial rotation. Herbicide omissions changed the weed-community structure in flax and in wheat and canola crops in the annual rotation enough to warrant alternate control methods in some treatments. The magnitude of the effects on the seedbank parameters depended largely on the competitive ability of the crop in which herbicides were omitted. No yield response to omitting herbicides in oats indicated that standard weed management practices have reduced weed populations below yield-loss thresholds.

Managing Against the Development of Herbicide-Resistant Weeds: Sugarcane

Although it is likely that small populations of herbicide-resistant weeds are already present in the Everglades Agricultural Area (EAA), herbicide resistance is currently not a significant problem. The continued use of integrated and properly managed weed control programs should ensure that resistance does not become a major issue in the future. In order to successfully manage herbicides against the development of herbicide-resistant weeds, you must have a basic understanding of which herbicides have the same site of action. This revised 4-page fact sheet lists herbicides by group number, site of action, chemical family, common name, and trade name. Written by D.C. Odero, B.A. Sellers, J.A. Ferrell, and G.E. MacDonald and published by the UF Department of Agronomy, March 2011. SS-AGR-244/SC077: Managing Against the Development of Herbicide-Resistant Weeds: Sugarcane (ufl.edu)

Management Strategies for Herbicide-resistant Weed Populations in Australian Dryland Crop Production Systems

In most world crop-production areas, the evolution of herbicide-resistant weeds is becoming a major issue. This problem has become most severe across the Australian dryland crop-production region, where herbicide-resistant weed populations are threatening crop-production profitability and sustainability across 20 million ha. Widespread herbicide resistance has forced changes in agronomic and herbicide practices. This problem is particularly evident in Western Australia, where the frequency and distribution of herbicide-resistant weed populations appear to be greater than anywhere else in the world. Judicious use of herbicide mixtures and rotations can reduce the selection pressure for evolved resistance to any one specific herbicide. Additionally, agronomic practices, such as the double knockdown (preseeding sequential application of nonselective herbicides), increased seeding rates, and targeting of weed seed production to prevent seedbank inputs, are needed to reduce the selection pressure on all herbicides by reducing in-crop weed populations. However, these techniques are not without problems or limitations, and their weed control efficacy is inferior to that of most in-crop selective herbicides. The adoption by Australian farmers of the current limited technology is clear evidence of the value placed on the use of these alternate crop weed-control practices. The continued evolution of herbicide resistance more than justifies continuing research and development efforts to produce integrated strategies and smarter herbicide use so as to achieve sustainable weed management.

Managing Against the Development of Herbicide Resistant Weeds: Sugarcane

Producing a profitable sugarcane crop in Florida depends, in part, on effectively controlling weeds that can reduce yields. Because they provide an efficient and cost effective means of weed control, herbicides are a critical component of sugarcane weed management programs. However, in many cropping systems excessive use of a single herbicide, or group of herbicides with the same site of action, has resulted in the development of herbicide resistant weeds. This document is SS-AGR-244, one of a series of the Agronomy Department, UF/IFAS Extension. Original publication date February 2006. SS-AGR-244/SC077: Managing Against the Development of Herbicide-Resistant Weeds: Sugarcane (ufl.edu)

Continuous cropping systems and weed selection

Changes in the weed flora of agro-ecosystems can occur as long-term changes or temporary fluctuations in species composition. The rate at which weed population shifts occur depends on the selection pressure imposed, genetic variability among weed populations, plant characteristics and environmental factors. Agronomic practices associated with cropping systems including crop rotation, tillage, herbicide use, soil amendments, and mechanization of harvesting that impart a range of selection pressures on weed populations are discussed in this review. Widespread use of herbicides has had the greatest impact on weed selection in recent years. Evolution of herbicide resistant weeds presents an enormous challenge to farmers. Development of herbicide tolerant crops has provided another tool for farmers however the selection pressure on weeds and potential impact on weed population shifts will require judicious use of this technology. Simulation models provide an excellent opportunity to predict future weed population shifts in response to management practices. Further insight into future management changes on weed selection must proceed towards an investigation of the processes rather than the outcomes. In particular, this must involve an understanding of the ecological factors and processes that are likely to determine the weed responses to particular management regimes.

Transgenic crops : new weed problems for Canada?

Over 25 000 transgenic field trials were conducted globally from 1986-1997, and many transgenic crops, including soybean (Glycine max), maize (Zea mays), tobacco (Nicotiana tabaccum), cotton (Gossypium hirsutum), canola (Brassica napus, B. rapa), tomato (Lycopersicon esculentum) and potato (Solarium tuberosum) have been commercially released. There has been a high adoption rate, with at least 28 million ha reported for 1998, with herbicide- and insect-resistant plants occupying 71 and 28% of the releases, respectively. The current status of commercial production of transgenic crops in Canada is summarized. Transgenic crops have the potential to change weed communities/populations in three principal ways, via: 1 ) escape and proliferation of the transgenic plants as 'weedy' volunteers with subsequent displacement of the crop, weed and/or natural vegetation; 2) hybridization with and transgene infiltration into related weedy and/or wild species, resulting in invigorated weeds and/or alteration of natural gene frequencies in these species; and 3) genetic changes in populations of unrelated species, as a result of changes to the environment, in particular herbicide-resistant (HR) transgenic crops and the development of HR weeds. Potential risk can be estimated a priori using knowledge of the systematics of crop/wild/weed complexes. Risk must be assessed on a case-by-case basis for each crop, each country/ecological region, and for each trait. Potential weed risks will be greater if crop volunteers are predisposed to becoming weedy, are well adapted to the Canadian climate and if sexually compatible wild species are present.

Log-Logistic Analysis of Herbicide Dose-Response Relationships

Dose-response studies are an important tool in weed science. The use of such studies has become especially prevalent following the widespread development of herbicide resistant weeds. In the past, analyses of dose-response studies have utilized various types of transformations and equations which can be validated with several statistical techniques. Most dose-response analysis methods 1) do not accurately describe data at the extremes of doses and 2) do not provide a proper statistical test for the difference(s) between two or more dose-response curves. Consequently, results of dose-response studies are analyzed and reported in a great variety of ways, and comparison of results among various researchers is not possible. The objective of this paper is to review the principles involved in dose-response research and explain the log-logistic analysis of herbicide dose-response relationships. In this paper the log-logistic model is illustrated using a nonlinear computer analysis of experimental data. The log-logistic model is an appropriate method for analyzing most dose-response studies. This model has been used widely and successfully in weed science for many years in Europe. The log-logistic model possesses several clear advantages over other analysis methods and the authors suggest that it should be widely adopted as a standard herbicide dose-response analysis method.