To design effective resistance management strategies for the long term, UC and other scientists are conducting basic research on weed biology and on ecological and evolutionary processes in weed populations. In a few cases, the mechanisms that confer resistance to herbicides have altered the fitness of resistant plants, as compared with susceptible plants of the same species in the absence of herbicide treatment. Differential plant fitness among biotypes can affect the rate at which herbicide resistance can spread. For example, if resistant and susceptible plants have equal fitness, the number of resistant plants in the population would not change relative to the number of susceptible plants during periods when the herbicide was not being applied . In contrast, if resistant plants are less fit than susceptible plants, the number of resistant plants may decrease during periods when herbicide is not applied. Fitness is usually evaluated by growing resistant and susceptible plants in direct competition with one another, or with the crop of interest, and comparing relative productivity or fecundity. Similar to efforts for other invasive weeds, insects and disease pathogens, surveys are sometimes used to delineate the extent of population growth or the expansion of new herbicide-resistant weed biotypes. Because there often are a few escaped weeds in herbicide-treated fields, pipp mobile storage systems herbicide resistance may not be recognized until the resistant biotype makes up a significant portion of the local population .
Surveys can help inform growers of emerging herbicide-resistant weed populations while they are still localized; surveys are also often used to encourage adoption of resistance mitigation measures to minimize economic and environmental impacts. Further, surveys combined with population genetic research can determine the evolutionary and geographic origins, and routes of spread, of resistance across an agricultural landscape . Herbicide resistance in California Herbicide resistance has been an important management concern in California flooded rice production for several years . Weeds with resistance to the ALS inhibitors , thiocarbamates and ACCase inhibitors are the dominant weed management problems in most of the Sacramento Valley rice production region. In orchards and vineyards, herbicide resistance is a more recent development and is dominated by resistance to the broad-spectrum postemergence herbicide glyphosate. This herbicide is, by far, the most widely used herbicide in the state in perennial crop production systems, as well as in many roadsides, canal banks and residential and industrial areas. Glyphosate-tolerant cotton, alfalfa and corn are becoming widely adopted in the state, which will further increase selection pressure for additional glyphosate-resistant and -tolerant species. Herbicide resistance in flooded rice. Most California rice is produced in monoculture systems due to impeded soil drainage, which limits rotation to other upland crops .
Rice fields are kept under continuous flood conditions during the growing season, primarily for the control of grass weeds . Although this system favors sedges and other water-tolerant weeds, selective herbicides such as molinate and bensulfuron provided highly effective weed control for several years. However, in the early 1990s, after repeated use, resistance to the ALS-inhibiting herbicide bensulfuron became widespread among weedy species in rice. By 2000, several additionalweed biotypes with resistance to ALS inhibitors, thiocarbamates or ACCase inhibitors had evolved and were causing significant weed management, economic and environmental issues in the rice cropping system. UC researchers, extension personnel and industry partners have devoted considerable efforts to understanding and managing herbicide-resistant weeds in rice. Smallflower umbrella sedge and California arrowhead resistance to ALS-inhibiting herbicides was first reported in California rice fields in 1993 following repeated use of bensulfuron . Field research has shown that California arrowhead is a fairly weak competitor in rice systems and that the ALS-resistant biotypes can be adequately controlled with other registered herbicides. Recently, small flower umbrella sedge biotypes with multiple resistance to the PSII herbicide propanil and to several ALS-inhibiting herbicides were identified in the Sacramento Valley , and research is ongoing to elucidate the mechanisms of resistance and any cross resistance to other rice herbicides. Eared redstem and ricefield bulrush resistance to ALS inhibitor herbicides in rice was reported in 1997. Redstem research has focused on intraand interspecific competition in an effort to develop agronomic solutions to reduce its competition with rice .
Studies have shown that California populations of ricefield bulrush are resistant to most registered ALS inhibitors, whereas populations from other regions are resistant only to one chemical family, the sulfonylureas, in the ALS inhibitor group . Recently, rice field bulrush biotypes with multiple resistance to propanil and bensulfuron were identified in the Sacramento Valley . Late watergrass populations resistant to ACCase inhibitors, ALS inhibitors and the thiocarbamate herbicides in rice systems were reported in 1998 . This resistance to multiple herbicides within an individual plant indicated that using herbicides with different modes of action would be unlikely to provide satisfactory control of the species in the long term. Further complicating the situation in rice, populations of late watergrass and barnyardgrass with resistance to both ACCase inhibitors and thiocarbamates, and thus exhibiting multiple resistance, were reported in 2000. Later research confirmed that the mechanisms of multiple resistance to several herbicide classes are due to metabolic degradation of these compounds . Smooth crabgrass resistance to the synthetic auxin herbicide quinclorac was reported in 2002. Detailed research into the mechanisms of resistance suggested that the cause was an altered sensitivity in the auxin response pathway, leading to ACCase activity, ethylene synthesis and enhanced ability to detoxify cyanide . Although crabgrass is not an important rice weed, quinclorac is used in rice systems for control of other weeds, and resistance to it has been reported in Echinochloa species of rice in California and from other regions. Most importantly, the observed changes in ethylene synthesis and production of toxic byproducts may also relate to the plant’s ability to tolerate abiotic stress. Two implications of this finding include the possibilities that quinclorac-resistant smooth crabgrass has the potential to invade a more diverse range of habitats and become an important weed of rice; and adaptation to the abiotic stress of the flooded environments may predispose Echinochloa phyllopogon or other major rice weeds to evolve resistance to quinclorac in the future. Herbicide resistance in orchard and vineyard cropping systems. The first herbicide-resistant weed in orchard cropping systems was perennial ryegrass, Lolium perenne , reported in 1989 . This ALS inhibitor–resistant biotype was selected on roadsides by the use of sulfometuron and, thus far, has not been a major problem in orchards or vineyards because relatively little of this class of herbicides is used in these crops. However, several ALS inhibitors, including rimsulfuron, penoxsulam, halosulfuron and flazasulfuron, are becoming more widely used in tree and vine crops, and selection pressure for ALS inhibitor resistance may increase in the future. The first case of glyphosate resistance in California was reported in a population of rigid ryegrass in 1998 . However, most confirmed glyphosateresistant ryegrass populations have been identified as Italian ryegrass . Glyphosate-resistant ryegrasses have become widespread and are a major weed problem in orchards, vineyards and roadsides of Northern California .
Research indicated that resistance in ryegrass is not due to metabolism of the herbicide and is instead due to an altered EPSPS enzyme . Glyphosate resistance in these areas has been largely driven by decreases in grower use of other herbicides, especially those under increasing regulatory pressure because of pesticide contamination of ground or surface water. The use of glyphosate-based herbicide programs also increased when the patent on Roundup expired in 2000 and low-cost, generic glyphosate herbicides became readily available. Today, drying and curing buds glyphosate accounts for over 60% of all herbicide-treated acreage in California orchard and vineyard systems . Glyphosate-resistant horseweed, or mare’s tail , was reported in 2005 and is one of the dominant weeds in and around raisin and tree fruit production areas of the San Joaquin Valley, as well as on roadsides and canal banks in the region that would be required for these larger vehicles and different use cases . In conjunction, the research team assessed likely charging behavior that would be typical of small business in the subject categories. Again, the goal has been to better understand how vehicle use case, charging behavior, and assumptions around the grid, with a particular focus on marginal emissions, may affect the relative pros and cons of EVs as a substitute for the incumbent vehicle technology . A secondary goal of this phase of the effort is to develop guidance and tools to assist stakeholders in understanding the implications of the use case scenarios and the simulation outputs. It is the team’s intent that while the subject of this study has been granular and necessarily regional, the final report and findings can be disseminated to other regions with great effect. In addition, technology transfer activities are anticipated to share these NCST developed tools with practitioners and decision-makers more broadly, so as to maximize the effectiveness of public and private investments in charging infrastructure. The guidance may also provide strategies to businesses seeking to deploy and/or invest in Electric Vehicles, as well as in electric power more broadly. This convergence research has revealed important findings relative to the comparative emissions impact of vehicle charging during various times of the day. Whereas Phase I findings are valuable to an individual vehicle owner, the findings of Phase 2 are of much greater interest to businesses that operate fleets comprised of light-duty pickup trucks, vans, medium-duty delivery/moving trucks, as well as refuse trucks. The pros and cons of replacing a conventional ICEV within the context of larger fleet vehicles that are operated by small and medium public or private businesses are similar in nature, but much greater in magnitude when compared to privately owned and operated cars. Interestingly, this has multiple dimensions including economic as well as environmental . To characterize the differences, the team compares results under five unique emission assumptions, each with its own relevance to the future state of the grid. As an example, the study investigates rather extreme scenarios wherein a specific generation resource is assumed to be dispatched to meet a specific marginal EV charge. In this case, the “effective emissions rate” of that EV charging session is tied directly to a single generation type . For multiple extreme cases, we observe that CO2 emissions can more than double when charged in the early afternoon compared to an identical charging event during the overnight . This finding suggests that it will be essential to adjust and/or coordinate charging schedules to reduce the environmental impacts of EVs. More specifically, to the extent emissions impacts are prioritized among other objectives, individuals and policymakers should be encouraged or incentivized to charge when marginal emissions are lowest whenever possible. This idea also has important implications for the location, type, and ownership models for tomorrow’s charging infrastructure. This not only includes charging equipment, but it also has significant implications on the generation mix, as well as transmission and distribution networks. Translating and operationalizing this type of guidance will require a combination of education, access to rigorous and clear resources, signals between stakeholders , risk management analyses, and behavioral change. In this way, our two studies, taken together can shed light on the critical nature of assumptions involved with serving incremental new electric power demand to charge vehicles. As in Phase I, this study is aimed at comparative analyses that provide insights into how a marginal assumption for CO2 emissions compares. As before, marginal CO2 assumptions generally yield higher CO2 impacts, sometimes a lot higher, than identical simulations that assume weighted average emissions. This variance is broad, ranging from 46% lower to 24% greater, depending on a host of case-sensitive factors. This study manifests the reality that weighted average emissions in the U.S. remain on a declining trajectory due to coal retirements, and the scale-up of combined cycle natural gas plants and renewables. In the Southeast, 2023 is also experiencing the commissioning of new base load nuclear generation, and Georgia is the only state in the country to add new nuclear since 2000. The overall environmental impact of these trends is favorable. However, EV growth as a demand sector for electric power has mixed outcomes because low carbon baseload will be consumed by current demand sectors, and renewables are generally considered non-dispatchable. As such, grid operators will generally deploy flexible generating resources to meet incremental loads like EVs.