Guidelines for Weed Management in Roundup Ready Alfalfa
Roundup Ready Alfalfa
- Roundup Ready alfalfa is resistant to glyphosate, an herbicide that can be used to provide weed control.
- Roundup Ready alfalfa can provide high-quality, weed-free forage with excellent crop safety and minimal harvest restrictions.
Importance of Weed Management
- To maximize alfalfa production for yield and quality, weed management should be addressed. The most critical time for weed management in alfalfa is during establishment.
- Yield longevity of an alfalfa stand depends on successful initial establishment of the crop since all subsequent harvests are dependent on initial stand density.
Stand Establishment
- To maximize the benefits of Roundup Ready alfalfa, glyphosate should be applied to seedling alfalfa at the three to five trifoliate stage when weeds are under 4 inches tall. If weed problems persist, an additional application of glyphosate can be made up to five days prior to harvest.
- An initial glyphosate application is necessary at the three to five trifoliate stage to remove the small percentage of glyphosate-susceptible alfalfa plants that are present in the new seeding.
Stand Removal
- Removal of Roundup Ready alfalfa prior to crop rotation is similar to that of conventional alfalfa varieties. Growth regulator herbicides such as 2,4-D and dicamba are the most common herbicides for removing alfalfa prior to crop rotation in no-till conditions. Deep tillage is also effective for removing established alfalfa stands.
Alfalfa (Medicago sativa) is an important perennial forage crop used around the world. In terms of acreage, alfalfa is the fourth largest crop grown in the United States, behind corn, soybeans, and wheat. Unlike annual cropping systems, alfalfa management differs greatly due to its perennial habit of growth. The alfalfa crop will remain in the field for several growing seasons and be harvested several times each season. Therefore, management practices will affect not only the first harvest of the crop but also subsequent harvests within the life of the alfalfa stand.
What Is Roundup Ready Alfalfa?
The introduction of Roundup Ready alfalfa provides another option for weed management in alfalfa. Roundup Ready alfalfa incorporates genetic resistance to the herbicide glyphosate (Roundup, Credit, Gly Star, etc.) into the alfalfa plant. Similar to other Roundup Ready crops on the market today, such as corn and soybean, a single bacterial gene that modifies 5-enolpruvylshikimate-3-phosphate (EPSP) synthase has been inserted into the alfalfa plant to allow resistance to glyphosate.
EPSP synthase is an enzyme present in most plants that is essential for protein synthesis and normal plant growth. Glyphosate applied to susceptible plants blocks EPSP synthase, preventing the production of essential amino acids and causing the plant to die. However, in Roundup Ready plants, a modified EPSP synthase is unaffected by glyphosate and allows the plant to continue growing. There is little or no crop injury associated with glyphosate application and Roundup Ready alfalfa.
Importance Of Weed Management
Alfalfa forage composition as affected by weeds can have significant impacts on protein content and overall feed quality (Cords 1973). Weed management in alfalfa is critical during stand establishment to ensure successful plant population. Alfalfa seedlings establish slowly and are very sensitive to competition for limited resources. Weed competition during establishment will reduce alfalfa seedling vigor and potentially reduce the alfalfa plant population, which can affect crop yield and quality throughout the life of the stand. In Pennsylvania, Stout et al. (1992) demonstrated that controlling weeds during the establishment year reduced stress on alfalfa, increased seedling weight and leaf numbers, and ultimately increased yields the following year. Leaf number and leaf content in alfalfa hay are direct indicators for digestibility, crude protein, and relative feed value (Kuehn et al. 1999). In addition, successful weed seed production during alfalfa establishment will potentially increase weed seed reserves in the soil, contributing to future infestations.
Once alfalfa is established and has successfully entered the forage production period (after the first harvest), wellmanaged alfalfa stands are generally vigorous and aggressive enough to compete well with later emerging weeds. In fact, over 95 percent of weed control in a healthy established alfalfa crop can be attributed to alfalfa's competitive nature. Thus, careful weed management during the establishment period of alfalfa is essential for maximum returns from an alfalfa crop over the life of the stand.
Where Does Roundup Ready Alfalfa Fit?
Roundup Ready alfalfa is a potential solution for historically weedy fields that are to be planted to alfalfa. Fields with perennial weed problems may also be good candidates. Table 1 compares the relative effectiveness of glyphosate and other herbicides on common individual alfalfa weeds. Ratings are based on labeled application rates and weed size or growth stage. Results may differ with variation in weed size, temperature, rainfall, soil moisture, soil type, and soil pH. A crop injury rating of 1 is rarely significant. Ratings were collected from trials conducted in Pennsylvania or neighboring states. Roundup Ready alfalfa may also help in the establishment of no-till alfalfa stands; however, additional research is still needed on effectively using notill in Roundup Ready alfalfa.
Spring Alfalfa Establishment | Glyphosate | Maestro* | Butyrac | Pursuit | Raptor |
---|---|---|---|---|---|
Foxtail, giant** | 9+ | — | — | 9 | 9 |
Ragweed, common | 9+ | 9 | 9 | 8 | 8+ |
Lambsquarters, common | 9+ | 9+ | 8+ | 7 | 9 |
Velvetleaf | 9 | 8 | 9 | 9 | 9 |
Pigweed, smooth | 9 | 8 | 8+ | 9 | 9 |
Crop injury | 0 | 2+ | 1+ | 1 | 1 |
Late Summer Alfalfa Establishment | Glyphosate | Maestro* | Butyrac | Pursuit | Raptor |
Lambsquarters, common | 9 | 9 | 9 | 8 | 8+ |
Dock, curly | 9 | N | 6 | 8 | 8 |
Chickweed, common | 9+ | 6 | 6 | 8+ | 8+ |
Shepherd’s purse | 9 | 9 | 9 | 8+ | 9 |
Crop injury | 0 | 2+ | 1+ | 1 | 1 |
Weed control rating: 10 = 95–100%; 9 = 85–95%; 8 = 75–85%; 7 = 65–75%; 6 = 55–65%; 5 = 45–55%+ = upper end of rating scale; — = not available. Crop injury rating: 0 = excellent; 1 = good; 2 = fair; 3= poor. | |||||
*Labeled for seedling alfalfa only. | |||||
**Select and Poast Plus are also labeled for grass control in alfalfa. |
Although Roundup Ready alfalfa can be an excellent option for weed control in alfalfa, it may not be appropriate for all situations. In particular, alfalfa-grass mixtures and alfalfa seeded with companion or nurse crops are generally not suitable. In addition, fields that tend to have low weed populations may not need the weed control that glyphosate can provide. Finally, fields that are consistently planted to other Roundup Ready crops where glyphosate is routinely used should not be planted to Roundup Ready alfalfa to avoid the selection for glyphosate-resistant weeds.
In the Northeast, alfalfa is often seeded with a grass in a mixed stand. Preliminary research has shown that it is possible to attain good weed control in mixed alfalfa-grass stands using glyphosate and Roundup Ready alfalfa, but strict management guidelines must be followed. At Penn State, glyphosate was used for weed control during early establishment of Roundup Ready alfalfa, while the grass was seeded after herbicide application (four to five weeks after seeding alfalfa). Glyphosate must be applied before any grass emergence and cannot be used beyond establishment because of its effectiveness on grasses. Preliminary results indicate that the alfalfa stand is increased while weed density is reduced in Roundup Ready alfalfa/grass mixtures and that these effects may persist into the life of the stand. Currently, there are no herbicides labeled for use in new alfalfa-grass seedings, so Roundup Ready alfalfa may be an option.
How Do I Use Roundup Ready Alfalfa?
Roundup Ready alfalfa management is identical to conventional alfalfa management in all respects except weed control. Roundup Ready alfalfa can be planted in the spring or late summer using standard alfalfa establishment practices. Roundup Ready alfalfa should be seeded at typical rates, while the seedbed may be prepared through tillage or, if suitable, no-till.
Once the alfalfa reaches the three to five trifoliate stage (typically about four to six weeks after planting), a glyphosate application should be made (Table 2). The importance of this herbicide application is twofold. First, and most important, effective weed control should be implemented at this stage so that crop yield losses do not occur from weed competition. Applying glyphosate prior to the three to five trifoliate stage may be too early for effective weed control since many of the weeds have not emerged by this time (depending on seeding date). If the glyphosate application is made after the three to five trifoliate stage, some yield loss may occur due to the competitive effects of the weed on the crop—in other words, the damage has already been done.
Table 2. Recommended glyphosate rates and timings for weed control in Roundup Ready alfalfa.
Alfalfa | Application | Rate (lb ai or ae per acre) | Product Rate (per acre) | Additional Information |
---|---|---|---|---|
Stand establishment | Three to five trifoliate alfalfa stage | 0.75 to 1.5 | 22 to 44 fl oz Roundup PowerMax 4.5S 32 to 64 fl oz 3S formulations |
Remove livestock before application and wait a minimum of five days after application before grazing, or cutting and feeding forage or hay |
Established stands | After the first cutting, apply up to five days prior to harvest | 0.75 to 1.5 | 22 to 44 fl oz Roundup PowerMax 4.5S 32 to 64 fl oz 3S formulations |
Remove livestock before application and wait a minimum of five days after application before grazing, or cutting and feeding forage or hay |
A second reason for glyphosate application at this time is to control the alfalfa "nulls." Up to 10 percent of the Roundup Ready alfalfa seedlings may not be resistant to glyphosate due to the genetic diversity of alfalfa; these susceptible plants are referred to as "nulls." If an appropriate glyphosate application is not made early in the life of the alfalfa stand, such as the recommended three to five trifoliate application, later applications will control the susceptible nulls, leaving gaps or holes in the canopy of the alfalfa. However, when glyphosate is applied to seedling alfalfa, removal of the nulls has no impact on the future production of the stand.
The three to five trifoliate stage recommendation applies to alfalfa crops seeded in the spring and late summer. Late summer seedings may have glyphosate applied in the fall or the spring to control weeds. Some late summer seedings may require an additional herbicide application if weeds are present in the spring. If necessary, the spring application should be made when weeds are actively growing but under 4 inches tall.
If weed problems persist after the initial application, a second application of glyphosate may be applied up to five days prior to first harvest. After the first harvest and in subsequent years, glyphosate may be applied up to five days prior to harvest and when annual weeds are under 4 inches tall. For perennial weed problems, glyphosate should be applied during early fall. To reduce the potential development of glyphosate-resistant weeds, glyphosate should only be applied when conditions warrant weed control.
Glyphosate Stewardship
Glyphosate resistance management should always be implemented when using glyphosate-resistant crops such as Roundup Ready alfalfa. Repeated applications of any herbicide, including glyphosate, may result in herbicideresistant weeds. Once herbicide-resistant weeds become established within a field, the herbicide becomes ineffective against those weeds that are resistant. Measures to reduce the potential of developing glyphosate-resistant weeds can include crop rotation, herbicide rotation, proper herbicide rate, proper herbicide timing, and alternative weed control methods such as tillage and mowing. Rotation to non-Roundup Ready crops using nonglyphosate herbicides after Roundup Ready crops is also effective in reducing the potential for glyphosate-resistant weeds.
Economic Considerations
When considering the value of this technology, the higher seed cost compared to conventional varieties should be amortized over the life of the stand, and the value of glyphosate herbicide should be compared to other competitive products.
How Do I Remove Roundup Ready Alfalfa?
When rotating into other crops, some additional consideration should be made for the removal of Roundup Ready alfalfa. If properly planned, Roundup Ready alfalfa is no more difficult to remove than conventional alfalfa varieties. Glyphosate, of course, is not effective in the control of Roundup Ready alfalfa; however, glyphosate alone is also not an effective means for control of conventional alfalfa. Alfalfa is often controlled by a combination of tillage and herbicide. Roundup Ready alfalfa should also be removed by similar methods. Currently, the best herbicides for control of alfalfa include 2,4-D, dicamba (Clarity), and clopyralid (Stinger). Greater than 95 percent control can be achieved with a combination of 2,4-D and dicamba without tillage when used prior to no-till corn. Specifically, a combination of 1 pint per acre of 2,4-D plus 1 pint per acre of dicamba (0.5 lb ae per acre of each) is the most effective control for Roundup Ready alfalfa; the combination of 2,4D and dicamba is more reliable and effective than either herbicide alone. 2,4-D and dicamba can be applied in the fall or the spring prior to corn, but they should be applied to alfalfa that is actively growing with at least 10 inches of spring growth or 6 inches of postharvest regrowth. Do not use dicamba prior to planting soybeans or other susceptible crops. Table 3 compares the relative effectiveness of herbicides for control of Roundup Ready alfalfa. Ratings are based on labeled application rates and alfalfa size or growth stage. Results may differ with variations in alfalfa size, temperature, and rainfall. Ratings were collected from trials conducted in Pennsylvania and from neighboring states.
Table 3. Roundup Ready alfalfa stand removal prior to no-till corn.
Herbicide(s)* | Rate | Alfalfa Control |
---|---|---|
2,4-D LV4 | 1 pt/acre | 7+ |
dicamba | 1 pt/acre | 8 |
2,4-D LV4 + dicamba | 1 + 1 pt/acre | 9 |
2,4-D LV4 + dicamba | 1 + 0.5 pt/acre | 8+ |
2,4-D LV4+ dicamba | 0.5 + 1 pt/acre | 8 |
2,4-D LV4 + dicamba | 0.5 + 0.5 pt/acre | 8 |
Clopyralid (Stinger) | 8 oz/acre | 9 |
Alfalfa control rating: 10 = 95–100%; 9 = 85–95%; 8 = 75–85%; 7 = 65–75%; 6 = 55–65%; 5 = 45–55%+ = upper end of rating scale. | ||
*Only 2,4-D may be applied prior to soybean planting. Follow label guidelines. Herbicide should be applied to alfalfa with at least 10 inches of spring growth or after 6 inches of alfalfa regrowth. |
References
- Cords, H.P. "Weeds and alfalfa hay quality." Weed Science 21 (1973): 400-401.
- Kuehn, C. S., H. G. Jung, J. G. Linn, and N. P. Martin. "Characteristics of alfalfa hay quality grades based on the relative feed value index." Journal of Production Agriculture 12 (1999): 681-84.
- Stout, W. L., R. A. Byers, K. T. Leath, C. C. Bahler, and L. D. Hoffman. "Effects of weed and invertebrate control on alfalfa establishment in oat stubble." Journal of Production Agriculture 5 (1993): 349-52.
Prepared by Bryan L. Dillehay, former graduate student; William S. Curran, professor emeritus of weed science; and Dwight Lingenfelter, extension weed scientist.