UCCE Master Gardeners of San Joaquin County
University of California
UCCE Master Gardeners of San Joaquin County

Posts Tagged: herbicide resistance

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

Managing junglerice in corn

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.



Posted on Friday, June 13, 2014 at 4:28 PM

Weed Science postdoc position - research in CA rice

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.


Additional details and submission information is in the attached job description (link below the photo).

Posted on Tuesday, March 18, 2014 at 9:43 AM

Herbicide-resistant weeds a growing problem

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



Posted on Tuesday, February 25, 2014 at 11:12 AM

Herbicide resistance and its management

Prior to the latter half of the 20th century, weed control in agriculture was achieved almost exclusively through mechanical means (e.g. plowing, cultivating, disking, hoeing, and hand-pulling) (Timmons 1970). The earliest (mid-1800’s to mid-1900’s) investigations into chemical weed management focused, primarily, on the use of inorganic compounds such as sodium chloride, sulfuric acid, sodium arsenite, and copper- and iron-sulfate (Appleby 2005; Timmons 1970). Although some products provided an acceptable level of weed control efficacy, the extensive adoption of these chemicals was decidedly limited (Appleby 2005; Timmons 1970). 


The successful debut of 2,4-D (1946), an auxinic compound that allowed for the selective, effective and economic control of broadleaved weeds in cereal crops, helped to accelerate the search for novel, organic (e.g. carbon-containing), agricultural chemicals with specific activities against weeds (Timmons 1970). Many herbicides that are currently used in California agriculture, including diuron (introduced in 1954), propanil (1961), trifluralin (1963), paraquat (1966), glyphosate (1971), pendimathalin (1974), glufosinate (1981) and oxyfluorfen (1981), were developed and released in the decades directly following the advent of a “modern chemical weed control era” (Appleby 2005; Timmons 1970). According to recent EPA estimates (2011), herbicides account for almost half of the total amount if pesticide active ingredients applied in the US. 


Despite the widespread adoption and use of herbicides, weeds still persist in agricultural and horticultural systems. Weeds can escape chemical control for numerous reasons, including: incorrect herbicide or rate selection, improper sprayer calibration, clogged nozzles or otherwise malfunctioning equipment, weed size (e.g. too large for control), herbicide applications that are made under less-than-ideal environmental conditions (e.g. too cold, too windy, too wet or too dry), and the evolution and spread of herbicide-resistant weed species. The Weed Science Society of America (WSSA) defines herbicide resistance as “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide that is normally lethal”. There are two general mechanisms of herbicide resistance. Target site resistance occurs when the enzyme that is the target of the herbicide becomes "insensitive" to the herbicide that was applied. The loss of sensitivity is usually associated with a mutation in the gene that codes for the enzyme that the herbicide adheres to in the plant. These mutations lead to physical changes in the enzyme’s shape/structure, which prevents herbicide-binding - thus reducing or eliminating herbicidal activity. Alternately, mutations may affect other plant processes in ways that reduce the plant’s physiological exposure to the herbicide; these mutations primarily result in reduced uptake or translocation of the chemical, sequestration, or else more rapid degradation/detoxification. Because the resistance mechanism is not directly related to the herbicide target site, this type of occurrence is called non-target site resistance


The first occurrences of herbicide resistance were noted in 1957 in populations of spreading dayflower (Commelina diffusa) and wild carrot (Daucus carota), both of which were described as being insensitive to 2,4-D (Hilton 1957; Switzer 1957; Whitehead and Switzer 1963). The next recorded incidence of resistance occurred in 1970 when scientists reported the discovery of common groundsel (Senecio vulgaris) plants in a conifer nursery that were unable to be controlled by simazine (Radosevitch and Appleby 1973; Ryan 1970). Since these initial discoveries, the evolution (and detection) of herbicide-resistant weeds has increased, significantly. To date, more than 200 plant species, worldwide, have developed resistances to one or more herbicide modes of action (Heap 2013). Currently, there are more than 140 herbicide-resistant weed biotypes in the US; California, alone, has at least 20. The majority of California's confirmed resistances have developed in rice production systems, primarily in the Sacramento Valley. More recently, resistance to glyphosate has been discovered in species that are common to perennial cropping systems in California, including: horseweed (Conyza canadensis), hairy fleabane (conyza bonariensis), junglerice (Echinochloa colona), Italian ryegrass (Lolium multiflorum) and rigid ryegrass (Lolium rigidum).


Image credit: Brad Hanson


A list of Best Management Practices (BMPs) to reduce the risks of herbicide resistance was recently (2012) published in a special issue of the journal Weed Science (Norseworthy et al. 2012). According to the WSSA, managers should actively scout their fields and take careful notes about the density and distribution of the species they encounter - within individual growing seasons and across years. Record keeping allows growers to detect changes in weed population size and spread, evaluate herbicide performance and determine if weed-shifts are occurring due to a chemical selection pressure. If an herbicide-resistant species is identified and confirmed, escaped weed(s) should be kept from producing seed. Furthermore, field equipment should be cleaned, regularly to prevent spreading the biotype from one field/orchard/vineyard to another. Crop seed should be certified to ensure that weed pests are not entering production systems in planting materials. It is also important to remember that weed management must extend beyond the crop in both time and space; weeds that emerge post-harvest or along crop borders/roadsides can be as important as those actually in the field during the growing season. Any remaining individuals have the potential to reestablish or enhance the seedbank. In a nutshell, it is key to start clean and stay clean.


Rotation is an important strategy for reducing the intensity of herbicide selection pressure. (Note: it is important to remember that selection pressure, with respect to chemical control measures, is defined as the repeated use of a single herbicide or herbicide mechanism of action within a system. In the absence of additional control strategies, plants that are resistant to a herbicide will note be controlled; if allowed to set seed, their progeny could dominate the local seedbank over time.) Quite often, diversification is an early line of defense against the development of wide-spread herbicide-resistance. In annual production systems, the ability to vary crops, and, therefore, production practices can be an effective means for preventing or delaying herbicide-resistance. For example, the time of crop planting, herbicide availabilities and types of tillage can differ, substantially, among  crops; these differences can result in continuously changing environments that may be more difficult for weeds to adapt to. In perennial cropping systems, like orchards and vineyards, crop rotation is impractical. Rotations or tank mixes of herbicides with different modes of action should, therefore, be a part of the management plan to prevent the buildup of weeds that are resistant to a particular mechanism of action. Although herbicide rotations and mixtures are important tools for combating the development of herbicide-resistance in weeds, chemical products cannot be used indiscriminately so as to prevent additional resistances from developing. 


Vegetable cropping systems (which rely on older chemistries that often control only a limited numbers of weed species) depend on mechanical cultivation and hand weeding for weed management; these practices are promoted by the WSSA as an effective tool for reducing herbicide selection pressure. Although there has been a trend towards minimum tillage in agronomic cropping systems, many extension personnel have advocated for the use of physical disturbance to manage herbicide resistant weed populations. Cultivation equipment used in orchards and vine­yards includes narrow under-vine rototillers, small disk harrows, and a wide assortment of tools designed to cut weeds off just under the soil surface; this equipment must be properly set to make sure  roots or tree trunks are not damaged during mechanical weed control. Permanent sod strips are often used in vineyards and orchards to improve access and reduce dust. They can also prevent the establishment of some weedy species, but must be well-managed with mowing to prevent excessive water use.


Image credit: Lynn Sosnoskie


Resistance mitigation seeks to diversify crop production and weed control methods in order to delay the evolution process; this will occur by reducing the selection pressure exerted through the use of herbicides. Target site resistance is conferred by an alteration causing loss of plant sensitivity to herbicides with a specific mechanism of action. One way of dealing with this phenomenon is to switch to an equally effective herbicide that possesses a different mechanism of action. Unfortunately, non-target site resistance may be more difficult to predict or prevent. Because it can involve more ubiquitous biochemical processes that may be shared across herbicides (such as a common degradation route), the use of these chemicals in mixtures or sequences with each other may not be sufficient to avert the development of resistance. Although chemical diversity is an important component of any herbicide-resistance management program, the integration of non-chemical weed control methods (i.e. cultivation, mowing, tillage, crop rotation), whenever possible, is necessary to reduce herbicide-selection pressure. Careful observation and record-keeping can help growers evaluate the success (and failures) of their systems and allow for early detection and prevention of herbicide-resistant species. Herbicides are one of the most effective tools for weed management; however, they must be used judiciously. They should be ‘one of the many tools’ in a weed management toolbox rather than the only tool, or else we risk losing effective herbicides due to the evolution of herbicide-resistant weeds. 


Much of this information was originally prepared for a series of IPM articles by B. Hanson, A. Fischer, A. Shrestha, M. Jasieniuk, E. Peachey, R. Boydston, T. Miller, and K. Al-Khatib.



Appleby, A. P. 2005. A history of weed control in the United States and Canada: A sequel. Weed Sci. 53:762-768.

Hilton, H. W. 1957. Herbicide tolerant strains of weeds. Hawaiian Sugar Plant. Assoc. Annu. Rep. p 69.

Norseworthy et al. 2012. Reducing the risks of herbicide resistance: Best management practices and recommendations. Special Issue: 31-62.

Radosevich, S. R. and A. P. Appleby. 1973. Relative susceptibility of two common groundsel (Senecio vulgaris L.) biotypes to 6 s-triazines. Agron. J. 65:553-555.

Ryan, G . F. 1970. Resistance of common groundsel to simazine and atrazine. Weed Sci. 18:614-616.

Switzer, C. M. 1957. The existence of 2,4-D resistant strains of wild carrot. Proc. of the North Eastern Weed Control Conference. 11: 315-318.

Timmons, F. L. A history of weed control in the United States and Canada. Weed Sci. 2:294-307.

Whitehead, C. W. and Switzer, C. M. 1963. The differential response of strains of wild carrot to 2,4-D and related herbicides. Can. J. of Plant Sci. 43:255-262.

Posted on Friday, December 13, 2013 at 12:38 PM

Next 5 stories | Last story

Webmaster Email: mdhachman@ucdavis.edu