How to Manage Pests

UC Pest Management Guidelines


Transgenic Herbicide-Tolerant Alfalfa

(Reviewed 11/06, updated 11/06)

In this Guideline:

Glyphosate-tolerant alfalfa (Roundup-ready alfalfa) is genetically engineered to tolerate over-the-top applications of glyphosate. Roundup-ready alfalfa allows growers a weed management timing option that allows effective control of many difficult-to-control weeds. Glyphosate controls most winter and summer annual weeds associated with alfalfa and suppresses or controls some problematic perennial weeds such as bermudagrass, quackgrass, and dandelion that are not currently well controlled with conventional herbicide systems.

Roundup-ready alfalfa varieties became commercially available in the fall of 2005. At the current time there are only a few Roundup-ready varieties available, especially for non-dormant varieties. However, the number of commercial alfalfa varieties with the glyphosate-resistant trait should increase in coming years.

While glyphosate controls a broader weed spectrum than the conventional herbicides used on alfalfa, it does not have residual control. Therefore, it is important to spray glyphosate after most of the weeds have emerged or the crop canopy is sufficient to out-compete later-emerging weeds. On the other hand, do not delay application so long that weeds become large and difficult-to-control. Generally, the best time to treat seedling alfalfa is when the alfalfa is between the three- and six-trifoliolate leaf stage. The soil is usually bare with earlier applications allowing subsequent weed emergence. With later applications, weeds may be too large for adequate control or with heavy weed infestations, alfalfa stand density or vigor may already be affected. Sometimes it may be necessary to make two applications of glyphosate during alfalfa stand establishment. Glyphosate treatments can be used between cuttings in established alfalfa to control summer annual grasses, such as yellow and green foxtail (Setaria spp.) and barnyardgrass (Echinochloa crus-galli). The number of applications depends on the geographic area and the infestation level.

Weed Resistance and Weed Shift Concerns
Potential for weeds to develop resistance to specific herbicides is always a concern with herbicide programs, but with Roundup-ready alfalfa, weed resistance is of greater concern because of the tendency to use a single herbicide repeatedly for several years.

Glyphosate is an herbicide that controls many weeds, including hard-to-control species. Because many growers may choose to use less expensive herbicides such as glyphosate, a real potential exists for the development of weeds that are resistant to it. Researchers in California have already identified and confirmed glyphosate-resistant ryegrass (Lolium rigidum) and horseweed (Conyza spp.) and other weeds are becoming more difficult to control.

A weed shift is another potential outcome of relying on a single herbicide or control strategy to manage weeds. A weed shift occurs when populations of tolerant weed species increase. While glyphosate controls most weeds, there are some tolerant species including cheeseweed (Malva parviflora), burning nettle (Urtica urens), filaree (Erodium spp.), and purslane (Portulaca oleracea). The prevalence of these weeds and others may increase if glyphosate alone is used repeatedly.

No matter which type of production system is used (standard or Roundup-ready varieties) a well-balanced, long-term weed management approach will incorporate resistance management strategies, including crop rotation, rotation of herbicides that have different mode of action group numbers, and control of escaped weeds by tillage or hand removal in order to delay or prevent development of resistant weeds or significant weed shifts.

Take the following factors into consideration when making a decision whether or not to use Roundup-ready varieties:

  • What are the dominant weed species present (annual vs. perennial) and how are they controlled with conventional herbicides versus glyphosate?
  • What is the density and extent of the perennial weed infestation?
  • Is the price of the seed cost effective?
  • Is the information regarding the agronomic suitability of the transgenic variety adequate for your area?
  • Are your buyers willing to accept transgenic alfalfa?


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

General Information

S. B. Orloff, UC Cooperative Extension, Siskiyou County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
W. M. Canevari, UC Cooperative Extension, San Joaquin County

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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