A tank mix of glyphosate and Velpar, or a rotation to Velpar and Gramoxone, was needed to adequately control all weed species at this location. To our knowledge there have been no documented cases of weed resistance in alfalfa during the first 3 years of RR alfalfa production in the United States. This is due to its perennial growth habit, its long stand life, and the potential for repeated use of a single herbicide over several years without crop rotation. Although some stands last 3 to 4 years, it is common in many areas of the United States to keep an alfalfa stand in production for 5 to 7 years or longer. If the rotation crop is not treated with an herbicide, an even longer period of time without herbicide diversity could occur. In this instance, weed populations could slowly return to preglyphosate composition, but the new species or resistant biotypes would not disappear. In areas where alfalfa is rotated with transgenic RR corn, cotton, or soybean varieties, thisagain could result in a prolonged time period where a single herbicide is used repeatedly. There are aspects of the alfalfa production system that both favor and discourage the development of weed shifts and the evolution of resistant weeds.First, grow rooms crop rotation opportunities with a perennial crop like alfalfa are significantly reduced compared with annual cropping systems.
Mechanical weed control, such as cultivation, is impractical in a solid seeded perennial crop like alfalfa, and hand weeding is not economical. Alfalfa is grown over extensive acreage in the United States and fields can be large in size; therefore, the overall weed flora available for selection of resistant traits or for weed shifts is plentiful. Perennials like alfalfa, if sprayed repeatedly with the same herbicide, are likely candidates for weed shifts and weed resistance.On the other hand, many weeds do not flourish in an alfalfa field due to its perennial nature and the competitiveness of the crop after establishment. Alfalfa is an aggressive competitor with most weeds, which fail to establish in alfalfa fields due to the crop’s vigorous growth and shading ability. In addition, many weed species do not tolerate the frequent cutting that occurs in alfalfa fields. The lack of soil disturbance once the alfalfa stand is established also reduces opportunities for germination of some weed species. Furthermore, the interval between alfalfa cuttings is short enough that seed production for many weeds is reduced compared with annual crops that allow completion of the weeds’ life cycles.Weed shifts or resistant weeds are unavoidable and will occur eventually with any herbicide used repeatedly, and the same is true with the use of glyphosate . Fortunately, resistance to glyphosate is not as common as resistance to many other herbicides, such as acetolactate synthase and acetyl-CoA carboxylase herbicides that have a single binding site and single target enzyme mechanisms of action .
The relatively low rate of resistance in weeds to glyphosate relative to the widespread use of this chemical has not been fully explained, but may be due to the number orfrequency of mutations that may be required to confer resistance to glyphosate. Two resistance mechanisms, a weak target site mutation, and a reduced glyphosate translocation mechanism have been documented in weed species that have evolved resistance to this herbicide . Regardless of the mechanism, weed resistance to glyphosate is not as common as resistance to other herbicides. However, cases of weed resistance to glyphosate have been documented and are increasing. There is a range of species across the world with documented resistance to glyphosate . Fortunately, most of these species are not common in alfalfa fields. Two weed species in particular have evolved resistant populations in California: Lolium spp. and Conyza sp. . The latter is not important in alfalfa, but ryegrass is frequently found in alfalfa fields. Glyphosate-resistant ryegrass is increasing in the Sacramento Valley and northern San Joaquin Valley of California and may become problematic during fall stand establishment of RR alfalfa. Weed shifts and/or weed resistance have occurred with the other transgenic RR crops released before RR alfalfa . Weed resistance is of greater concern than weed shifts and has occurred in RR soybean, cotton, and corn in less than a decade after their initial release . Alfalfa growers can learn from experience with these crops and in noncrop areas as a preemptive measure to avoid, or at least minimize, the problems with weed shifts and weed resistance. These problems are sure to occur in alfalfa if proper weed management practices are not followed.
Glyphosate-resistant crops have provided growers with an easy-to-use, low-cost, and effective weed management tool. However, the effectiveness of weed control systems using RR crops can make growers complacent in their weed control practices. Even though this technology is highly effective, growers must follow sound weed management principles to prevent short- or long-term weed shifts or weed resistance from occurring. This includes weed identification, crop rotation, attention to application rate, proper timing of application, herbicide rotation, and tank mixtures.Frequent Monitoring for Escapes It is difficult to detect an emerging weed shift or weed resistance problem if fields are not frequently monitored for weeds that escape current weed management practices. Identification and frequent monitoring can detect problem weeds early and guide management practices, including herbicide selection, rate, and timing. Herbicide Rate and Timing It is important to use the appropriate rate and timing for the weeds present. For example, some weeds that are considered somewhat tolerant to glyphosate can be controlled effectively in seedling alfalfa with glyphosate, provided the proper rate is used and the application is made when the weeds are very small. Research in Nebraska over a 7-year period demonstrated a rapid increase in lambsquarters when a low rate of glyphosate was applied, but a higher rate successfully controlled this weed. Just like with traditional weed management programs, the grower must be sure to use the recommended rate for the weed species present and treat at the appropriate time when the weeds are still small. Crop Rotation One of the most effective practices for preventing weed shifts and weed resistance is crop rotation, which allows growers to modify selection pressure imposed on weeds. Continuous alfalfa is not recommended for other agronomic reasons, but especially would be ill advised when it comes to management of resistance and weed shifts. Crops differ in their ability to compete with weeds; some weeds are a problem in some crops, while they are less problematic in others. Rotation therefore would not favor any particular weed spectrum. Crop rotation also allows the use of different weed control practices, such as cultivation and application of herbicides with different sites of action. As a result, no single weed species or biotype should become dominant. The effectiveness of crop rotation to manage weed shifts and resistance is substantially reduced if another RR crop is planted in rotation with RR alfalfa, since the same herbicide and selection pressure would likely occur. Agronomic Practices In addition to crop rotation, several management practices may have an impact on the selection of problem weed populations. If problem weeds germinate at a specific time of year, crop seeding date can be shifted to avoid these weed populations, allowing a vigorous alfalfa crop to develop that is capable of outcompeting weeds. Delaying irrigation after alfalfa cutting can reduce germination of certain summer annual weeds. However, this practice only works on some soil types, and water stress in alfalfa can reduce yields. Harvest management can, in some cases, assist in eliminating or suppressing problem weed populations, but harvests must occur before weed seed production to prevent weed proliferation.Rotation of Herbicides Weed shifts occur because herbicides are not equally effective against all weed species and herbicides differ greatly in the weed spectrum they control.
A weed species that is not controlled will survive and increase in density following repeated use of one herbicide. Therefore, pipp mobile storage systems rotating herbicides is recommended. Rotation of herbicides reduces weed shifts, provided the rotational herbicide is highly effective against the weed species that is not controlled with the primary herbicide. The grower should rotate to an herbicide with a complimentary spectrum of weed control, along with a different mechanism of action and therefore a different herbicide binding site. Weed susceptibility charts are useful to help develop an effective herbicide rotation scheme . In addition, publications on herbicide chemical families are available to assist growers in choosing herbicides with different mechanisms of action . Rotating herbicides is also an effective strategy for resistance management. Within a weed species there are different biotypes, each with its own genetic makeup, enabling some of them to survive a particular herbicide application. The susceptible weeds in a population are killed, while the resistant ones survive, set seed, and increase over time. Using an effective herbicide with a different mode of action from the one to which the weeds are resistant, however, controls both the susceptible and resistant biotypes. This prevents reproduction and slows the spread of the resistant biotype.Herbicide Tank Mixtures For the same reasons that rotating herbicides is effective, tank mixing herbicides is also recommended. The key is to select tank mix partners that have different target sites and that compliment each other so that, when combined, they provide complete or nearly complete weed control.The cost of RR alfalfa seed, including the technology fee, is generally twice or more than that of conventional alfalfa seed. Naturally, growers will want to recoup their investment as quickly as possible. Therefore, considerable economic incentive exists for the producer to rely solely on repeated glyphosate applications alone as a weed control program. Some producers may even be inclined to shave the rates to the minimum amount that would provide acceptable weed control. While relying solely on glyphosate and shaving rates may provide satisfactory results in the short term, it is a risky practice in the long run as it will accelerate weed species shifts and the evolution of resistant weeds. Sound weed management practices should be employed to maintain the effectiveness of the RR technology. Roundup Ready alfalfa is still a relatively new technology, so there has been limited field experience with it to date. The following are some suggestions to consider based upon proven resistance management strategies, our understanding of alfalfa production practices, and our initial experience with RR alfalfa. Ultimately, growers and pest control advisors hold the key to avoiding weed shifts and resistance by reducing selection pressure, which is accomplished by developing a weed management program that does not rely solely on the continuous use of glyphosate. Any management practice that reduces the selection pressure will help avoid weed species shifts and resistance. For Seedling Alfalfa, Use Glyphosate Alone or in a Tank Mix Combination Seedling alfalfa is most vulnerable to weed competition because weeds are often more vigorous and competitive than young alfalfa. Additionally, complete weed control in seedling alfalfa is often difficult to achieve and frequently requires tank mixes of different herbicides to control the broad spectrum of weeds found in an individual field. Yield and stand loss from weed competition, and injury from conventional herbicides, are usually far greater in seedling than in established alfalfa. Numerous field trials throughout the United States have proven the effectiveness of RR alfalfa for stand establishment. Superior weed control with no perceptible alfalfa injury has occurred in most studies. Therefore, it is only logical to use glyphosate for weed control in RR seedling alfalfa for the cost savings, improved weed control, reduced crop injury, superior stand establishment, and the elimination of the small percentage of alfalfa seedlings that do not carry the RR gene. Delayed removal of these nulls may cause weed control problems in the future by creating open spaces for weeds to grow. Ordinarily, 1.0 pound per acre active ingredient of glyphosate is sufficient for weed control during the seedling period. However, a higher rate may be needed if the field contains some of the more tolerant weeds listed in table 1. A tank mix may be advised if especially-difficult-to-control weeds are present. For example, a tank mix of glyphosate with imazamox or imazethapyr may be advised if burning nettle is present, or a tank mix with clethodim will be necessary if the field or surrounding area is known to have glyphosate-resistant ryegrass. Rotate or Tank Mix Herbicides at Least Once During the Life of an Alfalfa Stand Alfalfa stand life varies considerably throughout the western United States depending on the production area, grower practice, and the existence of profitable rotation crop options.