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Posts Tagged: herbicide resistance

California Agriculture - Review of herbicide resistant weed issues in CA (and other pest problems)

The journal California Agriculture recently published a special issue on research and extension efforts related to the University's Endemic and Invasive Pests and Diseases strategic initiative. 

The subtitle of the special issue is: How UC and it's collaborators detect, contain, and manage (pests and diseases).  There are six articles under three headings: Excluding Pests and Pathogens, Managing Newly Established Pests, and Maintaining Long-Term Management.  Table of contents available HERE.

A group of weed scientists reviewed the California status of herbicide-resistant weeds and described research approaches in an article, entitled "Herbicide-Resistant Weeds Challenge Some Signature Cropping Systems" (Direct Link to PDF HERE).  We focused the discussion on orchard and rice production systems and also includes two interesting counter-point sidebars about the lack of herbicide resistance in natural areas and vegetable production systems.


I was also happy to see that a photo of weed science Professor Dr. Marie Jasieniuk and PhD candidate Libby Karns cross pollinating herbicide-resistant ryegrass was used for the cover of this issue as well. 


Take care,




CalAg 68 4 cover Dec2014
CalAg 68 4 cover Dec2014

Posted on Thursday, February 5, 2015 at 8:58 AM

WSSA press release and fact sheet about "Superweed" fallacies

Below is a recent press release from the Weed Science Society of America (WSSA) announcing a new fact sheet addressing misconceptions about the so-called "superweeds".

I alluded to this few weeks ago in my post "Can herbicide resistance move from crops to weeds?" when I discussed the potential for gene flow from crops to weeds and how that might or might not affect the development of herbicide-resistant weeds.  That earlier post was basically the working draft of background information provided to the WSSA committee that developed this new fact sheet. 

[As an aside, at one point in time, I lived and breathed gene flow research.  My PhD dissertation at the University of Idaho was on pollen-mediated geneflow among wheat cultivars and from herbicide-resistant weed to a related weed species called jointed goatgrass.  It was fun to revisit some of those issues for this project]

The test of the WSSA press release is below and here is the direct link to the fact sheet "Dispelling Common Misconceptions about Superweeds"



Weed Scientists Uproot Common “Superweed” Myths

LAWRENCE, KANSAS – OCTOBER 8, 2014 – Today the Weed Science Society of America (WSSA) issued a new fact sheet to uproot common misconceptions about “superweeds”– a catchall term used by many to describe weeds resistant to herbicides. The paper explores the truth behind two widespread fallacies.

  • Fallacy 1: Superweeds are a product of rampant gene transfer from genetically modified field crops.  The truth: 

WSSA scientists say gene transfer from some crops to certain weed species can happen, but it has not been a factor in the development of herbicide resistance across large acreages. The true culprit, they say, is overreliance on a single class of herbicides, resulting in selection for weeds that can survive the products in that class.  

“Resistance to pesticides is not new or unique to weeds,” says Brad Hanson, Ph.D., a member of WSSA and Cooperative Extension weed specialist at the University of California at Davis. “Overuse of any compound class, whether antibiotic, antimicrobial, insecticide, fungicide or herbicide, has the potential to lead to reduced effectiveness. Although weeds resistant to herbicides were first reported more than a half century ago, integrated weed management strategies that included more tillage, more hand weeding and multiple herbicides kept them in check to a large degree.  Today, however, it has become common in some cropping systems for farmers to repeatedly use a single class of herbicides to the exclusion of other weed control methods, and this has led to the growing problem with herbicide-resistant weeds.”

  • Fallacy 2:  Superweeds have supercharged abilities to muscle out competing plants in new and more aggressive ways.  The truth: 

Many believe today's herbicide-resistant superweeds exhibit properties unlike anything we've ever seen before. But WSSA scientists say bully-like weed behavior isn't new.  In the absence of herbicides, resistant weeds are no more competitive or ecologically damaging than their non-resistant relatives. 

All weeds – herbicide resistant or not – can outcompete other more desirable plants for water, nutrients, sunlight and space. They grow by leaps and bounds and can be prolific seed producers. A single Palmer amaranth plant, for example, can produce hundreds of thousands of seeds.

Scientists say the key to keeping weeds from causing dramatic changes in crop production is to adopt effective management strategies.

According to Andrew Kniss, Ph.D., WSSA board member and University of Wyoming faculty member, "Nearly any weed species can be economically devastating if left uncontrolled. It is important to incorporate a variety of weed management practices and not rely exclusively on herbicides for weed control. Monitoring weed populations is also important. Early recognition of resistant populations and rapid intervention can help reduce the impact these weeds have.”

The full WSSA paper on superweeds is posted online at http://wssa.net/weed/wssa-fact-sheets. The same website contains a variety of best management practices recommended by WSSA to combat herbicide resistance – from proactive steps to reduce the number of weed seeds in the soil to the use of well-established cultural practices to suppress weeds through crop competition.  

About the Weed Science Society of America The Weed Science Society of America, a nonprofit scientific society, was founded in 1956 to encourage and promote the development of knowledge concerning weeds and their impact on the environment. The Society promotes research, education and extension outreach activities related to weeds, provides science-based information to the public and policy makers, fosters awareness of weeds and their impact on managed and natural ecosystems, and promotes cooperation among weed science organizations across the nation and around the world. For more information, visit www.wssa.net.

Posted on Wednesday, October 8, 2014 at 8:12 AM

Herbicide Resistance Summit II, in Washington DC (and webcast) September 10th

Meeting announcement from the Weed Science Society of America.  Brad

To Those Concerned About the Explosion of Herbicide-Resistant Weeds,

Attached you will find more information regarding the upcoming 2nd Herbicide Resistance Summit.  In addition to this information, a few notes:

  • For those who will not be able to attend the meeting in Washington, a live webcast will be available.  Check the WSSA website for the link to this webcast at www.wssa.net .
  • For those participating via webcast, there will be an email address provided for submission of comments.  That information will be sent out in two weeks.
  • A block of rooms has been reserved for the Summit; deadline for using this block is August 9.
  • Registration is free, but required to attend the Summit.  Go to the WSSA website to register.
  • This Summit is designed to provide a call to action for every participant, whether it be weed managers, federal and state agencies, academics, industry, stakeholder organizations, or advocacy groups.
  • A display table will be provided for those with information on herbicide resistance management.  However, to display you must contact David Shaw in advance with the information you plan to exhibit.

Please disseminate this information broadly to your constituencies.  Attendance is open to anyone, and we need diverse thoughts represented at the Summit if it is to be successful.

Look forward to seeing you September 10! 

David Shaw, Chair

2nd Herbicide Resistance Summit Planning Committee

Posted on Friday, August 8, 2014 at 8:23 AM

Can herbicide resistance move from crops to weeds?

The risks of GMO herbicide-resistant crops as a source for resistance traits in weeds has garnered recent attention in discussions of so-called “superweeds”. [I've commented previously on my general disagreement with the term "superweed" when talking about herbicide resistance].  Some media reports and online sources have suggested that herbicide resistance can be caused by resistance “jumping” from the crops into weeds. In fact, at least one online dictionary defines the problem in these terms:

World English Dictionary
superweed  (ˈsuːpəˌwiːd)
  a hybrid plant that contains genes for herbicide resistance: produced by accidental crossing of genetically engineered crop plants with wild plants

This topic deserves some scrutiny and clarification to point out areas where this is (or could be) accurate and where it is inaccurate or unlikely.  To start with, though, I would argue that either: 1) the above definition of the problem is simply wrong or 2) we don't have many "super weeds" problems and should not refer to most of our herbicide-resistant weeds using this term.  Here's why:

First, there are very few cases of herbicide-resistant weeds that are resistant because of a trait that originated in a crop (GMO or conventional) - most came about the old-fashioned way through selection of naturally occurring biotypes. Outcrossing species such as creeping bentgrass and canola have been shown to be the source for resistance in a few regionally important weeds.   However, to date no herbicide-resistant weeds in corn, cotton, or soybean production regions appear to have become resistant due to traits moving from the crop.   There are, however, many cases where the adoption of herbicide-tolerant crops (either GMO or developed through conventional breeding) has led to significant changes in herbicide use patterns and repeated use of the same herbicide or mode of action group has led to selection of resistant weeds.   This is not primarily a GMO crop issue; instead this is a lack-of-herbicide-rotation issue.

Second, the idea of resistance traits “jumping” among species is a bit imprecise and sensationalistic. The only known mechanism by which a crop trait could move into weeds (or vice versa) is through pollen-mediated gene flow – basically, sexual crossing between the crop and the weed. This can happen if the crop and the weed are the same species or very close relatives. Hybridization among more distantly related plant species is rarer; often due to one or more physiological barriers such as pollen incompatibility, different number of chromosomes, ploidy differences, etc.  This also is not a GMO crop issue either – sexual compatibility among crop and weed species is not known to depend on whether the crop is a GMO, conventional, or even organic.  

Opportunity for crop-weed gene flow also depends on the proximity of compatible species in a production region. In North America, many of our major field crops do not have significant problems with weedy relatives while others do; these scenarios present vastly different chances for this type of gene flow.   For example, soybean which is primarily inbreeding, does not have compatible relatives in North America thus risk of gene flow is very low. On the other hand, crops like sunflower, rice, and canola have some degree of outcrossing and compatible relatives (eg, wild sunflower, red rice, and a number of Brassica weeds, respectively) in their major production areas. Importantly, gene flow from a crop to a weed is VERY specific to the compatible species only – not to all weeds in a field. A good example here could be rice and red rice (both Oryza sativa) where gene flow among the wild and cultivated types is a real risk but that risk doesn't extend to other non-Oryza spp. weeds that may be in the same field.

Gene flow between crops and related, non-crop plants has been used accidentally or purposely by plant breeders to introduce adaptive traits into crops. Typically, this is done to get traits from wild plants into the crop plant, although the reverse can also happen. Breeding crop cultivars this way is a multi-generation process of crossing and backcrossing that must be conducted in order to get a line that is mostly like the crop parent except for the “new” trait. In the past, “crop” traits were generally considered to provide little benefit to weeds because attributes that are beneficial to crops (large seeds, low seed dormancy, uniform ripening, low shattering) generally are not very good “weed” traits. However, crop traits that could increase the range, reproductive ability, or fitness of a weed (such as tolerance to disease, insects, herbicide, or harsh environmental conditions) could lead to a greater weed problem and should be taken into consideration.

In a few cases where sexually compatible crops and weeds are grown in the same area, there is a possibility of herbicide resistance traits moving from a crop into weeds via hybridization. This phenomena depends on species compatibility and proximity and is not new or specifically related to GMO crops. Generally, the risk of herbicide resistance genes moving from crops to weeds in this way is low and is largely overshadowed by the risks of resistance imposed by selection pressure from non-diverse herbicide programs.

(note: the original draft of the above text was prepared as background information for a Weed Science Society of America press release on the topic of superweeds).


Posted on Monday, July 28, 2014 at 11:16 AM

Links to recent UC weed articles in CAPCA Advisor

I thought I'd make a quick post today to share links to several recent articles in the trade publication "CAPCA Advisor".  This magazine is targeted towards Pest Control Advisors (CAPCA = California Association of Pest Control  Advisors) and is published every two months.  Most issues of CAPCA Advisor have at least one article written by UC Cooperative Extension pest control researchers. 

The magazine has a nice web interface with the last 12 issues of the magazine and here are links to four reports by UC weed science researchers:

April 2014 - Richard Smith, UCCE Monterey County
- Automated thinner/weeder for lettuce production

Oct. 2013 - Marcelo Moretti et al., UC Davis
- Degree of glyphosate and paraquat resistance in hairy fleabane changes with time of year

June 2013 - Richard Smith, UCCE Monterey County
- Weed control options for dry bulb onions

December 2012 - Richard Smith, UCCE Monterey County
- Weed control options for fresh market spinach


Take care,



Posted on Thursday, June 19, 2014 at 8:34 AM

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