Cattle are not affected by the toxins but humans may be severely impacted. Goats, sheep, feral pigs, and deer are also considered animals of significant risk for shedding E. coli O157:H7. Others, such as elk, coyotes, and raccoons, have also been shown to harbor this pathogen. Other wild animals, including birds, can acquire the bacteria from various sources, serve as a transient reservoir, or mechanically vector E. coli O157:H7 bacteria across a landscape. Although relatively limited in scope, studies assessing the seasonal association of E. coli O157:H7 in wildlife have generally concluded that the prevalence is very low or generally not detected in most regions studied . However, when there is a local potential source of E. coli O157:H7, such as a nearby dairy operation or feedlot, the prevalence can be much higher, and transmission between plant and animal agriculture may be demonstrated by genetic matches in isolates from the source and in associated rodents and birds visiting both areas . The two most common ways that E. coli O157:H7 can be spread from cattle into the environment and agricultural landscapes are through the land application of raw, uncomposted manure and through runoff of manure or lagoon water into streams and irrigation ditches. Bioaerosols of buoyant fine particulates have been suggested as another probable source of localized spread. Implementation of good agricultural practices as defined by the commodity specific food safety guidelines for the production and harvest of lettuce and leafy greens will help minimize risks of contamination of crops with E. coli O157:H7 . For hedgerows, the GAPs for leafy greens will likely require periodic monitoring of fields adjacent to wildlife habitat, cannabis drying system both for evidence of intrusion by animals of significant risk for carrying E. coli O157:H7 , as well as smaller known or potential vectors such as rodents and birds.
Presence of these smaller animals may also indicate the attraction of predators, such as coyotes, also shown to be potential vectors. If there is evidence of intrusion by animals, the production block must undergo a detailed food safety assessment by appropriately trained food safety personnel. The Salmonella spp. food safety issue is essentially the same as that for E. coli O157:H7, but the focus tends to turn to habitat for birds, reptiles, rodents, and amphibians. Apart from reptiles, in some areas, the prevalence and frequency of transmission again tends to be low except in association with significant point sources such as dairy, poultry, cattle, and swine production operations. However, Salmonella seems to have a much more prevalent environmental phase, so there is building evidence for a baseline that one is unlikely to escape. Hence, a mitigation treatment to reduce the threat of salmonellosis is needed if tolerated by the crop . Although much research remains to be done on the epidemiology of E. coli O157:H7, hedgerows around farms may actually help reduce the risk of E. coli O157:H7 by helping to trap and filter harmful pathogens in dust and irrigation or storm water runoff .Endemic and invasive weeds are important management concerns in California due to their direct and indirect costs to agriculture, the environment and society. Pimentel et al. estimated that weeds cost U.S. crop producers and pasture managers over $30 billion in control-related expenses and reduced productivity. Although specific data are not available for California’s portion of these losses, weed management costs for the state’s 40 million acres of crop and grazing lands, as well as the remaining 60 million acres of land area, amount, undoubtedly, to several billion dollars annually. In addition to the direct cost of weed control and lost agricultural productivity, weeds also affect ecosystem quality and function, reduce recreational access and degrade aesthetics in natural areas, change wild land fire regimes and severity, and impede water flow through rivers and canals, among other negative impacts.
Although crop weeds are seldom considered as being “invasive” in the traditional sense, novel biotypes can develop, spread and subsequently occupy a greater proportion of crop acreage than might normally be expected. For example, when a weed population evolves resistance to an herbicide or any other control measure, a “routine” pest can become a new and serious problem. The first case of an herbicide-resistant weed in California was reported in 1981 by UC scientists ; in recent years, additional species have evolved resistance to various herbicide chemistries used in some of California’s signature cropping systems, including flooded rice, orchards and vineyards as well as nearby non-crop areas.Environmental factors and production practices influence species composition at any location, a phenomenon known as selection pressure. Under constant conditions, the weed community will become dominated by species that thrive under those conditions. If this steady state is upset by a change in management practices, a weed shift may occur, resulting in a community dominated by different species adapted to the new conditions . This weed shift can be caused by agronomic and horticultural practices or by the use of herbicides, which are very strong selective agents. Some species will be less susceptible than others to any management practice, and repeated use of the same control strategy can shift weed populations to become dominated by naturally tolerant species . Herbicide resistance, on the other hand, implies that a genetic change has caused a formerly susceptible population of a species to become resistant to an herbicide. Herbicide resistance arises from the process of adaptive evolution, whereby mutations change the physiology of plants in such a way that the herbicide is less effective. Under the continued selection pressure exerted by the herbicide, resistant plants with the new genotype are not controlled, and their offspring build up in the population .
Depending on the initial frequency and genetic basis of resistance, the regularity and rate of herbicide applications, and the reproductive system of the weed, it may take from a few to many generations for resistance to become. The strongest selection pressure for herbicide-resistant weeds tends to be in modern, high-intensity agricultural cropping systems due to a high reliance on herbicides. According to the International Survey of Herbicide Resistant Weeds , since the first confirmed report of a resistant biotype in 1957, herbicide-resistant weed biotypes have been reported in at least 60 countries and include more than 400 unique species-herbicide group combinations . The United States has more herbicide-resistant biotypes than any other country , and California accounts for 21 of these . Worldwide, resistance to acetolactate synthase –inhibiting herbicides and photo system II –inhibiting herbicides are the most commonly occurring among weedy species. However, in recent years, glyphosate resistance and multiple resistances have also emerged as major problems in some cropping systems. Interestingly, while herbicide resistance in the United States as a whole is primarily found in broadleaf weeds, California has more herbicide-resistant grasses or sedges than broadleaf species . Due to the extensive use of preplant and in-season tillage in some agronomic crops in California, drying rack for weed along with the use of pre- and postemergence herbicides, herbicide resistance is not as widespread as it is in other parts of the country where no-till and minimum-till systems have been widely adopted. Reduced tillage systems are heavily reliant on a few herbicide modes of action and have correspondingly larger problems with herbicide resistance . In contrast to the rest of the United States, where herbicide resistance problems are centered on agronomic crops, the greatest problems with herbicide resistant weeds in California are in orchards, vineyards, flooded rice, roadsides and irrigation canal banks. Herbicideresistant weeds have become especially challenging problems in California’s signature cropping systems, which are characterized by little or no crop rotation due to soil limitations or long cropping cycles and relatively few opportunities for mechanical weed control. Although large by specialty crop standards, the approximately 3 million acres devoted to orchard, vineyard and rice production in California is a small market for herbicide manufacturers; thus, herbicide options are somewhat limited. Combined, these factors have led to a high degree of selection pressure for herbicide-resistant weed biotypes as well as weed population shifts to naturally tolerant species .In order to combat complex issues such as herbicide resistance, organized collaborations between weed scientists and other agricultural researchers with a wide array of expertise are required. This includes the activities of UC Cooperative Extension farm advisors and specialists, Agricultural Experiment Station faculty, support scientists, research staff and graduate students, as well as faculty from other universities and agricultural industry representatives .
Current herbicide-resistant weed management efforts range from applied research and extension efforts to basic plant biology and evolutionary ecology studies. Although the specifics vary, these efforts can be grouped into three general areas: applied management of herbicide-resistant plants, physiology and mechanisms of resistance and biology, ecology and evolution of herbicide resistance. Applied management of herbicide resistant plants. Many cases of herbicide resistance in weeds are identified after growers, land managers or pest control advisers observe weed control failures with treatments that were once effective. These weeds are generally brought to the attention of local or statewide Cooperative Extension personnel. If the herbicide application method is ruled out as the cause of poor weed control , researchers often conduct field or greenhouse tests to verify and quantify the level of resistance. Plants from the suspected herbicide-resistant population are treated with the herbicide of interest at rates ranging from below normal doses to doses well above those legally allowed in the field . The response of the putative resistant population is then compared with the response of the known susceptible, or wild-type, population. Resistance is confirmed if the herbicide affects the two populations of the same species in markedly different ways with respect to plant growth and survival. In many cases, an estimate of the level of resistance also is made from these data. For example, if the susceptible population is controlled at one-half the field rate, but the resistant population survives at twice the field rate, it would be described as having a fourfold level of resistance. Physiology and mechanisms of herbicide resistance. Identifying and verifying herbicide resistance and developing alternative management strategies provides short-term solutions for weed managers. Researchers often conduct further studies to determine the underlying molecular and physiological causes of resistance and to compare the biology, growth and competitive ability of herbicide-resistant species and biotypes. The mechanism and fitness costs of herbicide resistance can have important ramifications on the selection, spread and competitive ability of herbicide-resistant biotypes, in addition to directly impacting their management. The goal of these efforts is to help growers and pest control advisers recognize the importance of taking a proactive approach to preventing the evolution of a resistant population, rather than a reactive approach to managing herbicide resistance after it occurs. Target-site resistance occurs when the enzyme that is the target of the herbicide becomes less sensitive, or fully insensitive, to the herbicide, often due to a physical change in the target enzyme’s structure. These physical changes can impair the ability of the herbicide to attach to a specific binding site on the enzyme, thus reducing or eliminating herbicidal activity. Target-site resistance is sometimes evaluated at the tissue level using portions of plants such as leaves, leaf disks or roots . In some cases, a functioning target enzyme can be extracted and its function evaluated in laboratory in vitro experiments in the presence or absence of the herbicide. Recently, overproduction or enhanced activity of the target enzyme has been shown to confer herbicide resistance in certain cases . Several mechanisms of non-target-site resistance confer resistance to herbicides in plants without involving the target sites of the herbicides. This can result in unpredictable resistance to unrelated herbicides . Of these, the best-known cases involve resistance in which herbicide-resistant plants have an enhanced ability to metabolically degrade the herbicide to less- ornontoxic forms. Many processes can be involved in metabolic resistance, but the most well-understood cases are due to changes in three groups of isozymes and changes in ATP-binding cassette transporters . This type of resistance is most commonly evaluated using nonherbicidal inhibitors of the various isozymes in the presence or absence of the herbicide and comparing metabolic degradation of the herbicide in laboratory or greenhouse assays. Biology, ecology and evolution of herbicide resistance.