While working on articles for an upcoming volume of the California Weed Science Society Journal, I was reminded of a really nice UCANR publication Avoiding Weed Shifts and Weed Resistance in Roundup Ready Alfalfa Systems by S. Orloff, D. Putnam, M. Canevari, and T. Lanini. You can find this publication (#8362) as well as hundreds (probably thousands) of others here.
Roundup Ready (glyphoate-tolerant) alfalfa was introduced in 200,5 but new plantings were stopped in early 2007 due to court challenges. After another series of lawsuits, injunctions, decisions, etc., the decision of the Federal court was upheld in January 2012 and California growers could again plant Roundup Ready (RR) alfalfa.
Just like in other RR cropping systems, alfalfa growers have a new opportunity to selectively control weeds using a simple, reliable and effective strategy. However, also just like other crop producers, alfalfa producers need to be aware of the potential for weed shifts and selection for herbicide resistant weeds in RR alfalfa. The biggest risk for weed shifts and resistance will be if alfalfa growers use ONLY glyphosate-herbicides in these crops.
If alfalfa growers make the same mistake that soybean, cotton, and corn producers made and only use glyphosate, there will undoubtedly eventually be problems with resistance or shifts to tolerant species. In the Orloff et al. publication, weed shifts away from annual bluegrass and shepherd’s purse to burning nettle and sowthistle were documented.
The authors recommended using other, conventional herbicides as part of an integrated weed management program in RR alfalfa. I would emphasize that also and remind folks that RR technology should be used to EXPAND selective weed control choices NOT REPLACE all others; otherwise, a very useful weed management tool could have a very short useful lifespan.
Two very publications that I found very informative on this issue are:
- Avoiding Weed Shifts and Weed Resistance in Roundup Ready Alfalfa Systems
- Roundup Ready Alfalfa – What Have We Learned to Date?