Posts Tagged: herbicide resistance
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:
|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).
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
From the May 2014 Tulare County UC Cooperative Extension "Field Crop Notes" newsletter
Managing Junglerice in Corn
by Steve Wright and Carol Frate
Introduction. The summer annual grass weed junglerice (Echinocloa colona) has become a difficult problem to control in corn fields in the southern San Joaquin Valley, especially minimum till fields, as well in other crops. Glyphosate products do not easily kill this weed unless the grass is quite small. Seed continues to germinate throughout the summer so even if junglerice seedlings are killed by a post-emergent herbicide, new seedlings can emerge the next day or next irrigation.
Junglerice identification. Seedling leaves are grayish or dull green in color. Often leaves are banded with purplish-red stripes across the blade but this feature can be absent. Mature plants are prostrate or erect and 2-3 ft tall. Leaves are rolled in the stem before emerging. Leaf blades are flat and usually the upper surface is hairless. Stems are hairless except at the nodes. There are no ligules or auricles. Purple banding on the leaves is the easy way to distinguish junglerice from barnyardgrass. There are more photographs and details on identification at the UC IPM website: http://www.ipm.ucanr.edu/PMG/WEEDS/junglerice.html.
A major concern is the development of glyphosate (Roundup) resistance in junglerice in California. Rotating glyphosate-resistant corn with other glyphosate-resistant crops such as cotton or alfalfa will only increase this problem. To help prevent the development of herbicide-resistant weeds and prevent weed shifts from occurring, it is important to incorporate tillage into your weed management practices, as well as alternating or tank-mixing herbicides that have different chemical modes of action.
Research Results. Research conducted in the SJV in 2011- 2013 by S. Wright and C. Frate with Matrix (rimsulfuron) and Laudis (tembotrione) demonstrated excellent junglerice control could be achieved when these materials are applied according to the labels. Both herbicides will enhance control of broadleaves, grasses, and glyphosate-resistant weeds, while also reducing glyphosate induced weed shifts. Matrix can be applied either preemergent to the corn and junglerice or postemergent to the corn. In the first case, corn is planted dry, the herbicide is applied and then followed by an irrigation to germinate the corn and activate the herbicide. The other approach is to preirrigate, plant or strip till and then plant. After weeds emerge treat postemergent to corn and junglerice. The most consistent results have been observed with a tank mix of glyphosate and Matrix. Matrix can be applied postemergent up to 12 inch corn but weeds must be small. “Steadfast”, a combination of Accent plus Matrix, applied postemergent has also demonstrated effective on control of young junglerice.
Laudis (tembotrione) also adds to the options available for corn growers to control junglerice. Laudis is for postemergence use. Best results are obtained when it is applied to young actively growing weeds. According to the label, Laudis can affect weeds that are larger than the recommended height; however applications of Laudis when weeds are taller than 4 to 5 inches in height may result in incomplete weed control activity. Broadcast applications of Laudis may be made to corn from emergence up to the V8 stage of growth. A second post-emergence application is allowable on corn but it must be a minimum of 14 days from the first application. According to the label, cultivation can help remove suppressed weeds or multiple flushing weeds. However, don't cultivate within 7 days of an application of Laudis as this could decrease the effectiveness of weed control due to disruption of herbicide translocation in the plant.
See the attached position advertisement for a postdoctoral research position at the Rice Experiment Station in Biggs, CA.
Project Background and Position Description:
We offer a post-doctoral position to work with the RES Rice Breeding Program in screening and evaluating tolerance of rice germplasm and available mutant populations to herbicides for weed control in rice. The successful candidate will be responsible for developing and testing screening protocols for different herbicides in the lab, greenhouse and rice field, as well as develop additional mutant populations using chemical or physical mutagens. If applicable, the incumbent will develop mapping population if the search for herbicide tolerance is found. Activities will be conducted in conjunction with the activities of the breeding program, the DNA marker laboratory, and other activities in cooperation with weed research by UC Davis at RES.
Salary and Duration:
The position will be available immediately and with the project duration of 3 years. The salary is competitive and will depend upon qualifications and experience.
Job qualifications include a PhD in Weed Science, Plant Ecology, Plant Physiology or related field with knowledge of plant ecology, and physiology. Strong background in weed ecology, ecophysiology, herbicide tolerance, and experience in research is required. Working knowledge of DNA markers and rice genetics is an advantage. Evidence pertaining to ability to work in and excellent verbal and written communication skills is essential.
Todd Fitchette wrote the article "Herbicide-resistant weeds a growing problem" for the Western Farm Press (Feb. 18, 2014). Here's the link: http://westernfarmpress.com/management/herbicide-resistant-weeds-growing-problem