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Guidelines for Weed Management in Roundup Ready Alfalfa

Alfalfa (Medicago sativa) is an important perennial forage crop used around the world. Alfalfa is the fourth largest crop, in terms of acreage, grown in the United States behind corn, soybeans, and wheat.

Roundup Ready Alfalfa

  • Roundup Ready alfalfa is resistant to glyphosate herbicide which 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 3 to 5 trifoliate stage when weeds are less than 4 inches tall. If weed problems persist, an additional application of glyphosate can be made up to 5 days prior to harvest.
  • An initial glyphosate application is necessary at the 3 to 5 trifoliate stage to remove the small percentage of glyphosate-susceptible alfalfa plants that are present in the new seeding.

Stand Removal

  • Roundup Ready alfalfa removal prior to crop rotation is similar to 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. Alfalfa is the fourth largest crop, in terms of acreage, 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 will be harvested several times each season. Therefore, with alfalfa, management practices will not only affect the first harvest of the crop, but will also affect subsequent harvests within the life of the alfalfa stand.

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 is successfully into the forage production period (after the first harvest), well managed alfalfa stands are generally vigorous and aggressive enough to compete well with later emerging weeds. In fact, over 95% of weed control in a healthy established alfalfa crop can be attributed to the 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.

What Is roundup ready alfalfa?

The introduction of Roundup Ready alfalfa provides a new option for weed management in alfalfa. Roundup Ready alfalfa incorporates genetic resistance to the herbicide glyphosate (Roundup, Touchdown, Glyphomax, 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 present in most plants and is an enzyme essential for protein synthesis and normal plant growth. When glyphosate is applied to susceptible plants, glyphosate blocks EPSP synthase preventing the production of essential amino acids and the plant dies. However, in plants that are Roundup Ready, 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.

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. The effectiveness of glyphosate and other herbicides on common alfalfa weeds are described in Table 1. Roundup Ready alfalfa may also help in the establishment of no-till alfalfa stands; however, additional research is still needed on effectively using no-till in Roundup Ready alfalfa.

Table 1. Effectiveness of selected POST herbicides for weed control in seedling and/or established Roundup Ready alfalfa.
Spring Alfalfa Establishment Glyphosate Buctril1 Butyrac Pursuit Raptor
Foxtail, giant2 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 Buctril1 Butyrac Pursuit Raptor
Lambsquarters, common 9 9 9 8 8+
Dock, curly 9 N 6 8 8
Chickweed, common 9+ 6 6 8+ 8+
Shepherds purse 9 9 9 8+ 9
Crop injury 0 2+ 1+ 1 1

Table compares the relative effectiveness of herbicides on individual 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. Crop injury rating of 1 or less is rarely significant. Ratings were collected from trials conducted in Pennsylvania or neighboring states.

Weed Control Rating

10 = 95-100%
9 = 85-95%
8 = 75-85%
7 = 75-65%
6 = 65-55%
5 = 55-45%

Crop Tolerance

0 = Excellent
1 = Good
2 = Fair
3 = Poor

1 Labeled for seedling alfalfa only.
2 Select and Poast Plus 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 (4 to 5 weeks after seeding alfalfa). Glyphosate must be applied before any grass emergence and can not be used beyond establishment because of it’s 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, through no-till.

Once the alfalfa reaches the 3 to 5 trifoliate stage (typically about 4 to 6 weeks after planting) a glyphosate application should be made (Table 2). The importance of this herbicide application is two-fold. First and most importantly, 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 3 to 5 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 3 to 5 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/acre) Product Rate per acre Additional Information
Stand Establishment 3 to 5 trifoliate alfalfa stage 0.75 to 1.5 22 to 44 fl. oz Roundup Weathermax 4.5S or 32 to 64 fl. oz 3S formulations Remove livestock before application and wait a minimum of 5 days after application before grazing, or cutting and feeding forage or hay
Established Stands After the first cutting, apply up to 5 days prior to harvest 0.75 to 1.5 22 to 44 fl. oz Roundup Weathermax 4.5S or 32 to 64 fl. oz 3S formulations Same as above

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 3 to 5 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, the removal of the nulls has no impact on the future production of the stand.

The 3 to 5 trifoliate stage recommendation applies to both spring and late summer seeded alfalfa crops. Late summer seedings may have glyphosate applied in the fall or spring to control weeds. Some late summer seedings may require an additional herbicide application in the spring if weeds are present. If necessary, the spring application should be made when weeds are actively growing but less than 4 inches tall.

If weed problems persist after the initial application, a second application of glyphosate may be applied up to 5 days prior to first harvest. After the first harvest, and in subsequent years, glyphosate may be applied up to 5 days prior to harvest and when annual weeds are less than 4 inches tall. For perennial weed problems, glyphosate should be applied during early fall. Glyphosate should only be applied when conditions warrant weed control as to reduce the potential development of glyphosate-resistant weeds.

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 herbicide-resistant 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

Like other Roundup Ready crops, a Technology Use Agreement will be necessary as well as a “Line Item Technology Royalty Fee” in addition to the cost of the seed. When considering the value of this technology, the Royalty fee 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 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 (Banvel), and clopyralid (Stinger). Greater than 95% 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 pt/acre of 2,4-D plus 1 pt/acre of dicamba (0.5 lb ae/acre of each) is the most effective control for Roundup Ready alfalfa; the combination of 2,4-D and dicamba is more reliable and effective than either herbicide alone (Table 3). 2,4-D and dicamba can be applied in the fall or spring prior to corn but should be applied to alfalfa that is actively growing with at least 10 inches of spring growth or 6 inches of post-harvest regrowth. Do not use dicamba prior to planting soybean or other susceptible crops.

Table 3. Roundup Ready alfalfa stand removal prior to no-till corn1.
Herbicide(s)2 Rate Alfalfa

Table 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.

Alfalfa Control Rating

10 = 95-100%
9 = 85-95%
8 = 75-85%
7 = 75-65%
6 = 65-55%
5 = 55-45%

1 Only 2,4-D may be applied prior to soybean planting. Follow label guidelines.
2 Herbicide should be applied to alfalfa with at least 10 inches of spring growth or after 6 inches of alfalfa regrowth.

2,4-D LV4 1 pt/A 7+
dicamba 1 pt/A 8
2,4-D LV4 + dicamba 1 + 1 pt/A 9
2,4-D LV4 + dicamba 1 + 0.5 pt/A 8+
2,4-D LV4 + dicamba 0.5 + 1 pt/A 8
2,4-D LV4 + dicamba 0.5 + 0.5 pt/A 8
Clopyralid (Stinger) 8 oz/A 9

References

  • Cords, H.P. 1973. Weeds and alfalfa hay quality. Weed Science 21:400–401.
  • Kuehn, C.S., H.G. Jung, J.G. Linn, and N.P. Martin. 1999. Characteristics of alfalfa hay quality grades based on the relative feed value index. Journal of Production Agriculture 12:681–684.
  • Stout, W.L., R.A. Byers, K.T. Leath, C.C. Bahler, and L.D. Hoffman. 1992. Effects of weed and invertebrate control on alfalfa establishment in oat stubble. Journal of Production Agriculture 5:349–352.

Prepared by Bryan L. Dillehay, Ph.D. candidate, and William S. Curran, professor of weed science.

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Title

Guidelines for Weed Management in Roundup Ready Alfalfa

Series

Agronomy Facts 65

Code

UC193

Cost

Free

This publication is available in alternative media on request.

Contact Information

William S. Curran
  • Professor of Weed Science
Phone: 814-863-1014