The potential for weeds to develop resistance to specific herbicides is always a concern when herbicides are used, 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 low-cost herbicide programs such as glyphosate alone, a real potential exists for the evolution of glyphosate-tolerant weeds. Researchers in California have already identified and confirmed glyphosate-resistant ryegrass (Lolium rigidum), horseweed and hairy fleabane (Conyza spp.), and junglerice (Echinochloa colona). Other weeds are becoming more difficult to control in other Roundup Ready cropping systems.
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.
Avoid continuous Roundup Ready crops (e.g., Roundup Ready corn, sugarbeets, or cotton), since the continuous use of glyphosate is likely to result in weed shifts or resistant weeds. For more information on transgenic alfalfa, see HERBICIDE-TOLERANT VARIETIES AS A WEED MANAGEMENT STRATEGY.
No matter which type of production system is used (standard or Roundup Ready), a well-balanced, long-term weed management approach is important. Such a program will incorporate resistance management strategies, including crop rotation, rotation of herbicides that have different mode-of-action group numbers, herbicide tank mixes with complimentary modes of action, and control of escaped weeds by tillage or hand removal in order to delay or prevent the occurrence of resistant weeds or significant weed shifts.
Tank mixing glyphosate with a soil-active herbicide is often an effective practice to reduce the likelihood of herbicide resistance or weed shifts and to improve weed control. It is important to avoid rotation with other Roundup Ready crops whenever possible (such as Roundup Ready cotton, corn, or sugarbeets), to avoid the potential for overuse of glyphosate and possible problems with glyphosate-tolerant volunteer crops.
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 versus 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 with the technology fee cost effective?
- Are your buyers willing to accept transgenic alfalfa?
For more information on the management of weed shifts and resistance, see Avoiding Weed Shifts and Weed Resistance in Roundup Ready Alfalfa Systems, UC ANR Publication 8362.