Glyphosate was used as a broad-spectrum, non-selective herbicide for nearly 40 years, with few concerns about resistance until the mid- 1990s. A number of factors have combined to increase the selection pressure for glyphosate-resistant weeds in several cropping systems. These include an increase in reduced- or no-tillage systems that depend on preplant weed control with glyphosate; the development of glyphosate-tolerant crops in which glyphosate can be used during the growing season; environmental quality concerns that have reduced the use of some preemergence herbicides; and a significant decrease in glyphosateprices. Since the initial report of a glyphosate-resistant weed in 1996, a total of 24 glyphosateresistant species have been reported around the world, and 14 of these are present in the United States. Target site. Glyphosate inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase in the shikimate pathway. Inhibition of EPSPS by glyphosate causes an increase in shikimate and leads to plant death by disrupting several key metabolic processes. Resistance to glyphosate results from point mutations in the EPSPS gene, indoor vertical farming causing amino acid substitutions in the protein that affect the ability of the glyphosate molecule to bind to the enzyme; the enzyme is thus less sensitive to inhibition by glyphosate.
These mutations confer moderate levels of resistance, which are nevertheless enough to enable plants to survive and selection for resistance to occur when glyphosate is applied repeatedly. Non-target site. The ambimobile nature of glyphosate transport within plants is a major reason for its effectiveness, and alterations in glyphosate translocation within the plant can confer relatively high levels of resistance in certain weeds. In certain glyphosate-resistant bio-types of Lolium spp. and horseweed , glyphosate failed to accumulate in growing points and roots and tended to remain concentrated in the leaves rather than translocating throughout the plant. For the most part, the precise mechanisms contributing to reduced translocation are not yet well understood, and details are still being elucidated.Herbicide-resistant weeds are an issue around the world; but the greatest problems with resistance tend to be found in countries with highly industrialized agricultural cropping systems, due to their greater reliance on herbicides. Herbicide-resistant weed bio-types have been reported in at least 60 countries and include about 350 unique species-herbicide group combinations worldwide . Due to intensive, high-input cropping systems, the United States has a greater number of resistant bio-types than any other country, followed by Australia , Canada , Spain and France , and Israel, United Kingdom, Germany, and Brazil .
Herbicide-resistant weeds around the world and throughout the United States are dominated by the photosystem II inhibitors and by ALS inhibitors as a result of the widespread use of these diverse herbicide classes in broad-acreage cereal and grain crops. Some of the most troubling herbicide-resistant bio-types are multiple-resistant bio-types. In vineyard and orchard systems, several regions have reported substantial problems with resistance to glycines and paraquat, as well as problems with multiple-resistant bio-types due to repeated use of post emergence materials.A number of factors affect the degree of selection pressure for herbicide-resistant weeds . However, if preventive measures are taken to reduce selection pressure, herbicide resistance can be avoided or delayed. As outlined previously, repeated use of the same herbicide or herbicides with the same mode of action can select for weeds that are resistant to or tolerant of that mode of action. As an herbicide controls the susceptible bio-types, its repeated use causes the resistant bio-types to gradually build up in the population . The adage “why fix something that is not broken” may not be applicable to herbicide-resistance management. For example, it is tempting to repeatedly use a certain herbicide that is very effective against a certain weed species or a community of weeds. However, such a process imposes a higher selection pressure on the target weed and ultimately leads to the buildup of a resistant population.
Therefore, a major goal of herbicide resistance management is to reduce selection pressure. In this context, rotation and tank mixes become important resistance-management tools and often are used as the first line of defense against the selection of herbicide-resistant weeds. Rotation can mean the use of different crops in a sequence, as in the case of annual cropping systems. In this type of cropping system, herbicides with different modes of action can be used in different phases of the rotation. Resistance to one weed control tactic does not easily evolve for several reasons. The weed populations are disrupted by the use of herbicides with different modes of action and also by the cultural operations that differ for each particular crop. For example, time of planting, type of tillage, and use of interrow operations may vary for these different crops, thus creating an unfriendly environment for weeds in which to adapt because of these continuous changes in selection pressure. However, in perennial cropping systems, like orchards and vineyards, it is impractical to firequently rotate crops. Similarly, crop rotation opportunities can be limited by economic or environmental constraints. Non-crop areas such as roadsides, canal banks, and industrial sites also have few rotational alternatives. Therefore, in these systems, rotation or tank mixes of herbicides with different modes of action should be a part of the management plan to prevent the buildup of weeds resistant to those particular modes of action. When herbicides with different modes of action are used in rotation or mixtures, the selection pressure for any one herbicide is reduced. Thus, the weeds will have difficulty adapting to this continuous alteration in selection pressure. Selection pressure on susceptible weeds from herbicides with longer residual activities is higher than that from herbicides with shorter or no residual activities, because one treatment can result in the exposure of multiple weed cohorts to the herbicide. However, when herbicides with no residual activity are used multiple times in a season, selection pressure is equally high and can lead to selection for herbicide-resistant weeds, as has been observed with glyphosate-only weed control programs. In fact, short-term residual herbicides in combination with postemergence herbicides are being recommended for management of glyphosate-resistant weeds in many cropping systems.Resistance mitigation seeks to diversify weed control methods in order to delay the evolution process; it does this by reducing the selection pressure exerted through the use of herbicides. Target site resistance is conferred by an alteration that causes loss of plant sensitivity to herbicides with specific mechanisms of action. It isclear, therefore, that one way of dealing with the problem is by switching to another herbicide that is effective on the same weed species but that has a different mechanism of action . The use of herbicide mixtures or sequences involving herbicides with different mechanisms of action can protect the herbicides and delay the evolution of resistance to both, since mutants with resistance to one herbicide would be controlled by the other herbicide and vice versa. However, the recurrent use of the same herbicide mixture could theoretically select for bio-types with resistance to both herbicides . Non-target-site resistance may involve different herbicides and the enhanced expression of mechanisms that are common in plants and thus easily selected for. If several herbicides share a common degradation route, such as the ubiquitous P450 monoxidation, their use will select for the same mechanism of resistance in bio-types that will be resistant to all the herbicides, rolling benches even if these herbicides are used in mixtures or sequences with each other. Thus, combining or changing herbicides to control non-target-site-resistant bio-types becomes very difficult. Nontarget-site resistance may involve the accumulation of genes contributing partial resistance levels.
From this discussion of resistance mechanisms in herbicide-resistant weeds, it should be clear that resistance cannot be mitigated only by switching or combining herbicides in production systems that rely solely on the intensive use of selective herbicides for weed control. Instead, herbicide-resistance management requires the integrated diversification of chemical and non-chemical weed control methods to reduce selection pressure for resistant weed bio-types. Herbicides are one of the most effective tools for weed management; however, they must be used judiciously. They should be one of the many tools in a weed-management toolbox rather than the only tool, or else we are at risk of losing effective herbicides due to the evolution of herbicide-resistant weeds.The adoption of glyphosate-resistant crops has increased dramatically in the last decade. Most of the increase in crop acreage is attributable to glyphosate-resistant soybean, corn, canola, and cotton.Glyphosate-resistant sweet corn, alfalfa, and sugarbeet are approved and expected to be planted in many parts of the western United States in the coming years. The outcomes of this unprecedented adoption of glyphosate-resistant crops have been many, but perhaps most dramatic is the simplification of weed control tactics; growers can now apply glyphosate at high rates and at multiple times during the growing season without concern for crop injury. When using herbicides with short or no residual activity, such as glyphosate, optimal weed control often requires sequential herbicide applications, and the timing relative to weed emergence is important. When glyphosate is applied two or three times annually at high rates, it imposes a high selection pressure on weeds. Repeated use over several years may cause a shift in weed composition toward species that naturally tolerate glyphosate. Several weedy species that are common in the western United States—such as yellow nutsedge, common lambsquarters, black nightshade, panicle willowweed, creeping buttercup, cheeseweed, burning nettle, filaree, purslane, and morningglory—are relatively tolerant of glyphosate. To avoid weed species shifts, it is obvious that weed management practices besides the use of glyphosate need to be integrated into glyphosate-resistant cropping systems. With the rapid adoption of glyphosate-resistant crops and the corresponding increase in glyphosate use, the evolution of glyphosate-resistant weed populations has rapidly escalated. Currently, there are 13 confirmed glyphosate-resistant weed species in the United States , including five species in California and Oregon . Glyphosate resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of glyphosate normally lethal to the wild type. Weeds contain a tremendous amount of genetic variation, and the repeated use of a single herbicide can select individuals with genetic mutations that confer resistance. Although repeated use of glyphosate can select resistant populations, no evidence has been presented to demonstrate that glyphosate induces mutations directly. Resistant individuals can exist in a field at low firequency prior to glyphosate application. These individuals survive applications of glyphosate and reproduce to increase their numbers in a population. Several important mechanisms can confer glyphosate resistance, including an altered glyphosate target site, changes in translocation, and gene overexpression. In contrast to many other cases of herbicide resistance, glyphosate-resistant weeds usually have relatively low levels of resistance compared with susceptible plants. In addition, young glyphosate-resistant weeds that are 1 to 2 inches tall are often less resistant to glyphosate than are more developed plants measuring 6 to 9 inches tall. Typically, weeds resistant to herbicides with other modes of action have similar levels of resistance at all growth stages. In the western United States, glyphosate-resistant weeds are not widespread . However, because of the rapid adoption of glyphosate-resistant crops, such as corn, canola, alfalfa, cotton, and sugarbeet, as well as the declining glyphosate prices inrecent years, growers are more firequently using glyphosate to replace in-crop cultivation and other herbicides. The reliance on herbicides with the same mode of action for extended periods can certainly contribute to weed shifts and the selection of bio-types with resistance to glyphosate.A single weed control measure is not likely to provide effective and sustainable weed management over time, as weed populations may have different responses to a single weed management practice. In addition, weeds have different life cycles and may escape treatment if they emerge after herbicide treatment, especially if a nonresidual herbicide is applied. Relying on one weed management practice can lead to weed shifts or selection for resistant bio-types; thus, it is critical to integrate as many weed management strategies as possible into a system. Glyphosate is a great weed management tool, but it must be used in conjunction with other herbicide types and practices to ensure effective weed control in the short and long term. In most western states, in-crop cultivation has traditionally been a significant component of weed management programs. In addition to controlling weeds, cultivation may improve early season crop growth in compacted soils. On most soils, however, cultivation is of no value beyond weed control and can sometimes have negative effects.