The use of short-residual herbicides also reduces selection pressure for herbicide resistance. In addition, tank-mixing of herbicides with different modes of action can inhibit the evolution of resistance, but the combinations used should broaden the spectrum of weeds controlled along with controlling the weed species of major concern. If two herbicides have nearly the same weed control spectrum, you would do better to rotate between them rather than tank-mix the two compounds; little additional control will be achieved by adding the second material. Though unlikely, it is possible in theory that a weed population will simultaneously be selected for resistance to both herbicides. While weeds have traits that enhance their potential to evolve resistance, they also have traits that reduce that potential. Weed species with seeds that remain dormant in the soil for several years will maintain a population of susceptible plants within the seed bank. By maintaining susceptible plants in the population, a grower can dilute the resistance trait. If there is a fitness cost to resistance, removing the herbicide at some point in the crop rotation cycle will allow competition between the resistant and susceptible plants, further diluting the gene pool for the resistance trait. Besides the practice of crop rotation, certified seed, equipment sanitation, cultivation, and hand-weeding all impede resistance evolution. Resistance problems usually go undetected until land managers or farmers observe about 30 percent weed control failure for a particular species. If you can identify these resistant weed patches early, curing marijuana before their populations increase, you can employ management practices that prevent their spread.
If weed escapes appear in patterns such as distinct strips, or if several species normally controlled by the herbicide are present in these strips, the problem probably is associated with a calibration or application error. However, patches made up of only one escaped species and showing no distinctive pattern may indicate a herbicide-resistant population. Suspicious areas should be brought to the attention of a Farm Advisor or Extension Specialist, especially if weed populations reoccur in subsequent years after use of the same herbicide.However, weed species shifts and the selection for glyphosate-resistant weeds can result from the increased use of this technology if the crop is not managed properly from the outset. Aspects of the alfalfa production system both favor and discourage the occurrence of weed shifts and the evolution of resistant weeds. Alfalfa is a competitive perennial crop that is cut multiple times per year, making it difficult for most weeds to become established. On the other hand, the RR alfalfa system may be vulnerable to weed shifts and resistant weeds for several reasons: tillage typically only occurs between crops, alfalfa is produced over a wide geographical area and in large fields with a great diversity of weeds, and there is potential for long-term repeated use of a single herbicide because it is a perennial crop. In this publication we recommend an integrated weed management system designed to prevent the proliferation of tolerant or resistant weeds. Elements include crop rotation, rotations with herbicides of different modes of action , tank mixtures, and irrigation and harvest timing. Successful adaptation of these concepts into production systems would assure the long-term effectiveness and sustainability of the Roundup Ready system in alfalfa.
A preemptive approach is warranted; these strategies should be employed before weed shifts and weed resistance occur.Alfalfa, the queen of forages, is the principal forage crop in the United States and frequently the third most important crop in value. It is a vital component of the feed ration for dairy cows and is a principal feed for horses, beef cattle, sheep, and other livestock. Because animal performance depends upon the palatability and nutritional value of alfalfa, livestock managers, especially those in the dairy and horse industry, expect high-quality hay. Although many factors influence quality, the presence of grassy and broadleaf weeds plays a significant role in reducing the feeding value of hay throughout the United States. Weeds that accumulate nitrates or are poisonous to livestock are also a major concern in alfalfa, since poisonous weeds sicken or kill animals every year . Most livestock producers demand weed-free alfalfa for optimum quality and maximum animal performance. Weed-free alfalfa can be difficult to achieve, whether using non-chemical methods or conventional herbicides. Typically, no single herbicide controls all weeds present in a field, and some weeds—especially perennials—are not adequately controlled with any of the currently registered conventional herbicides. Cultural practices such as modifying harvest schedules, grazing, time of planting, and use of nurse crops such as oats help suppress weeds; however, these practices are almost never entirely effective and some of them suppress alfalfa seedling growth. In addition to being difficult to achieve, complete weed control in alfalfa is costly. Alfalfa growers continually seek ways to enhance the level of weed control while minimizing costs.Glyphosate is generally considered the most effective broad spectrum post-emergence herbicide available. The first commercially available glyphosate-resistant crops were soybean, canola, cotton, and corn, which were released in 1996, 1997, 1997, and 1998, respectively. Glyphosate-resistant or Roundup Ready alfalfa was developed through biotechnology in late 1997 and became commercially available in the fall of 2005.
This technology imparts genetic resistance to glyphosate by inserting a single gene from a soil bacterium into alfalfa. These biotechnology-derived alfalfa plants have an altered enzyme that allows them to tolerate a glyphosate application while susceptible weeds are killed. Glyphosate resistance is the first commercially available, genetically engineered trait in alfalfa. This technology was a major development in alfalfa weed control, providing growers with a useful weed management tool and a means to deal with some of the most difficult-to-control weed species. Researchers have evaluated its effectiveness as a weed control strategy . The advantages and disadvantages of this technology have been reviewed . Glyphosate was found to be especially effective for weed control in seeding alfalfa . Glyphosate typically causes no perceptible crop injury, is much more flexible and less restrictive in application, and provides superior weed control across a range of weed species when compared with other currently used herbicides. One of the greatest advantages of this technology is that it provides a tool for suppressing perennial weeds such as dandelion , yellow nutsedge , bermudagrass Pers., and quackgrass Nevski that have not been adequately controlled with conventional practices. After deregulation of this trait in 2005, over 300,000 acres of RR alfalfa were planted in the United States, about 1.4 percent of U.S. acreage. However, in the spring of 2007, further plantings were suspended pending the outcome of a legal challenge and further environmental analysis by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service . There were two key issues in this process: the possibility of contamination of organic and conventional alfalfa through the adventitious presence of the gene, and the possibility of a greater level of weed resistance due to the adoption of the Roundup Ready technology in alfalfa . Grower experience in commercial fields following deregulation confirmed many of the benefits that early research had suggested in terms of the efficacy and safety of the RR system . Growers have generally found that this technology is easy to use and provides superior weed control and improved forage quality in many cases compared with conventional herbicides. However, no new technology is a panacea, and, pipp mobile storage like other weed control strategies, RR alfalfa has its limitations. An important limitation of this new weed-management system is the potential for weed shifts and weed resistance. This publication discusses techniques that are available to manage the possibility of weed shifts and weed resistance occurring in Roundup Ready alfalfa weed control systems.Change in weed populations as a result of repeated use of a single herbicide is not a new phenomenon. Such changes result from shifts in the weeds present from susceptible to tolerant species, or conversion of a population within a species to resistant individuals, as a consequence of selection pressure .In the case of chemical weed control, no single herbicide controls all weeds, as weeds differ in their susceptibility to an herbicide. This allows inherently tolerant weed species to remain, which often thrive and proliferate with the reduced competition. As a result, there is a gradual shift to tolerant weed species when practices are continuously used that are not effective against those species.
A weed shift does not necessarily have to be a shift to a different species. For example, with a foliar herbicide without residual activity like glyphosate, there could also be a shift within a weed species to a late-emerging biotype that emerges after application. In the case of weed shifts, the total population of weeds does not necessarily change as a result of an herbicide or an agronomic practice; these practices simply favor one species over another.In contrast to a weed shift, weed resistance is a change in the population of weeds that were previously susceptible to an herbicide, turning them into a population of the same species that is no longer controlled by that herbicide .While weed shifts can occur with any agronomic practice , the evolution of weed resistance is only the result of continued herbicide application. The use of a single class of herbicides continually over time creates selection pressure so that resistant individuals of a species survive and reproduce, while susceptible ones are killed.A weed species shift is far more common than weed resistance, and ordinarily takes less time to develop. If an herbicide does not control all the weeds, the tendency is to quickly jump to the conclusion that resistance has occurred. However, a weed shift is a far more likely explanation for weed escapes following an application of glyphosate. See table 1 for a list of weeds sometimes found in alfalfa fields that are tolerant to or difficult to control with glyphosate.A common misconception is that weed resistance is intrinsically linked to genetically engineered crops. However, this is not correct. The occurrence of weed shifts and weed resistance is not unique to genetically engineered crops. Weed shifts and resistance are caused by the practices that may accompany a GE crop , not the GE crop itself. Similarly, some people believe that herbicide resistance is transferred from the GE crop to weed species. However, unless a crop is genetically very closely related to a naturally-occurring weed, weed resistance cannot be transferred from crop to weed. In the case of alfalfa, there are no known wild plants that cross with alfalfa, so direct transfer of herbicide resistance through gene flow to weedy species will not occur. However, the glyphosate-tolerant genes from RR alfalfa can be transferred to feral alfalfa plants if cross pollination occurs.resistance is transferred from the GE crop to weed species. However, unless a crop is genetically very closely related to a naturally-occurring weed, weed resistance cannot be transferred from crop to weed. In the case of alfalfa, there are no known wild plants that cross with alfalfa, so direct transfer of herbicide resistance through gene flow to weedy species will not occur. However, the glyphosate-tolerant genes from RR alfalfa can be transferred to feral alfalfa plants if cross pollination occurs.Transgenic herbicide-resistant crops do, nonetheless, have greater potential to foster weed shifts and resistant weeds since a grower is more likely to use a single herbicide repeatedly in herbicide-resistant crops such as RR alfalfa. Additionally, the accumulation of acreage of different RR crops could increase the potential for weed shifts or weed resistance in cropping systems utilizing RR crops. This is because the probability of repeated use of the same herbicide is higher and the potential applied acreage is greater. Fortunately, there are simple methods available to prevent weed shifts and weed resistance from occurring. In studies conducted in San Joaquin County, California, weeds shifts were found to occur during the first few years of use when glyphosate-tolerant weeds were present . Annual bluegrass and shepherd’s purse were adequately controlled with glyphosate, whereas chickweed control was about 80 percent and burning nettle andannual sowthistle were not adequately controlled with any of the glyphosate rates . During the 3 years of this field trial, when glyphosate was used repeatedly, there was a gradual weed species shift away from annual bluegrass and shepherd’s purse to higher populations of burning nettle and annual sowthistle .