UNITED STATES ENVIRONMENT AL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

MAR 1 0 2016 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

SUBJECT: Propoxycarbazone: Usage on Wheat and Proposed Herbicide Resistance 

Management Measures (PC # 122019). 

FROM: Carl Chen, Ph.D., Biologist GM.J.. ~ I'd:-_ 

William Chism, Ph.D., Senior Biologist . ~ (') (}. 

Jonathan Becker, Ph.D., Senior Science Advisor f'--- ~ ~ 

TllRU: 

Biological Analysis Branch flStephen Smearman, Economist k-.,J,.~~ / . Economic 

Biological 

Analysis 

and Economic 

Branch 

Analysis 

~~ 

Division (7503P) 

Colwell Cook, Ph.D., Acting Chief ~- /Jll.. I ~ Colu.et( Gx,J::_ 

Biological Analysis Branch r j f ~ 

Economic Analysis Branch v V\.. Timothy Kiely, Chief ~ \ 4 ·~ 

Biological and Economic Analy · Division ( · 50 ) 

TO: Marianne Mannix, Chemical Review Manager 

Kelly Sherman, Chief 

Risk Management and Implementation Branch III 

Pesticide Reevaluation Division (7508P) 

BEAD Product Review Panel date: February 24, 2016 

1 
SUMMARY 

The herbicide propoxycarbazone sodium is undergoing registration review in the Office of 

Pesticide Programs. In this memo, BEAD provides information on use and usage parameters to 

help characterize the modelled.ecological risks of propoxycarbazone sodium, including the 

extent of aerial applications, the number of multiple applications, and the application rates 

commonly used by growers. BEAD also provides information about the risks of development of 

weed resistance to propoxycarbazone sodium and proposes measures for resistance management. 

The Biological and Economic Analysis Division (BEAD) analyzed Market Research Data to 

determine the number of spring and winter wheat acres treated with propoxycarbazone sodiUm. 

BEAD determined that use of propoxycarbazone sodium on wheat is extremely small nationally 

with approximately 2% of all wheat acres being treated and that of those acres treated with 

propoxycarbazone sodium, less than 1 % of all wheat acres are treated aerially. Most aerial 

applications are made to winter wheat. Although propoxycarbazone sodium use is relatively low 

nationally, two states, Oregon and Washington, appear to rely on aerial applications. Oregon and 

Washington with 27% and 13% of their wheat acres treated aerially with propoxycarbazone 

sodium as a percent of total treated acres. On average, the annual application rates are well 

below the label maximum rate. However, for winter wheat, up to three applications may be used 

with annual rates that are close to the label maximum. Propoxycarbazone sodium may have 

some niche uses as being effective against certain primary weed pests. It is one of the commonly 

used cheatgrass herbicides in winter wheat (Market Research Data, 2010-2014). 

BEAD concurs with the registrant that the recommended droplet size for aerial application is 

"coarse" based on the limited information that is publically available on the efficaciousness of 

the product under scenarios where the droplet size is varied. 

To address the spread of herbicide resistant weeds the Agency is proposing weed resistance 

management elements during the registration review process. This is a tiered approach where 

the situations (i.e. the interaction of the mechanism of action, resistant weeds, and weed 

management strategies) with the least concern for herbicide resistant weeds will have the fewest 

number of resistance management elements and the situations with the highest concern will have 

the most resistance management elements. 

In addition to the propoxycarbazone sodium docket, a separate docket is being established for the 

Agency's herbicide resistant management approach. All stakeholders are encouraged to submit 

comments to both dockets. Public comments received from both the chemical specific docket 

will be considered prior to the Agency's final decision on propoxycarbazone sodium. 

Propoxycarbazone sodium is an inhibitor of acetolactate synthase (ALS) and the Weed Science 

Society of America (WSSA) has categorized it as mechanism of action (MOA) Group 2. This 

MOA is of high concern for weed resistance development because this mechanisms of action has 

a large number of confirmed herbicide resistant weeds species in the U.S., is used on a crop 

(wheat), which has many reported resistant weeds, and seven ALS herbicide resistant crops have 

been commercialized (conventionally bred or GM). 

2 
The Agency is proposing here a set of herbicide resistance elements for propoxycarbazone 

sodium. This includes changes to the label language and elements on reporting, education and 

stewardship. 

Background 

Propoxycarbazone sodium is now in the decision-making phase ofregistration review. The 

Agency is considering changes and restrictions in its use to mitigate potential ecological risks as 

mandated under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), which 

requires EPA to register pesticides before they can be distributed and used FIFRA Section 3(g) 

mandates that EPA re-evaluate the registrations at least once every 15 years. This periodic 

review is necessary in light of scientific advancements, changes in policy, and changes in use 

patterns that may alter the conditions underpinning previous registration decisions. FIFRA 

requires EPA to regulate the use of a pesticide to the extent necessary to prevent unreasonable 

adverse effects on the environment, including human health, where 'unreasonable' is defined as 

taking into account the economic, social, and environmental costs and benefits of the use of the 

pesticide. The Agency is considering options for mitigating risks to non-target plants that will be 

implemented through product label changes. 

OPP has completed risk assessments for propoxycarbazone sodium, an herbicide that is most 

commonly used in wheat production, but that may also be used on fallowed land, pasture and 

rangeland, and Conservation Reserve land. 

OPP's ecological risk assessments have identified risks to non-target plant species in areas 

adjacent to treated fields. Aerial applications will typically pose the greatest risks since drift is 

more likely and more widespread. Other main drivers in the risk assessment include application 

rates, frequency of application, droplet size, and boom height. 

Propoxycarbazone sodium 

Propoxycarbazone sodium inhibits acetolactate synthase (ALS), and is classified by the Weed 

Science Society of America (WSSA) as a WSSA Group 2 herbicide (WSSA, 2016). It is a 

selective herbicide and is absorbed by leaves and roots and then translocated. Propoxycarbazone 

sodium is characterized by a low rate of use and its efficacy is not affected by low air 

temperatures (Guarnone and Davi, 2012). The major agricultural use of propoxycarbazone 

sodium is in wheat. It is applied postemergence to the weeds through ground or aerial 

application equipment. It was applied to more than one million acres annually, approximately 

2% of all wheat acres in the United States, with a higher percentage in certain regions such as 

Washington. Propoxycarbazone sodium has an efficacy rating of "excellent" for cheatgrass 

control (TAMU, 2008), and is one of the commonly used cheatgrass herbicides in winter wheat 

(Market Research Data, 2010-2014). 

Alternatives to propoxycarbazone sodium in winter wheat include other ALS herbicides as well 

as synthetic auxins (WSSA Group 4). Alternatives for spring wheat are other ALS herbicides, as 

well as synthetic auxin, and inhibitors of ACCase (WSSA Group 1 ), and photosystem II 

3 
inhibitors (WSSA Group 6). Most of these alternatives have been used mainly postemergence, 

like propoxycarbazone sodium. 

Usage 

For the years 2010 to 2014, approximately 26,000 lb a.i. of propoxycarbazone sodium are 

applied annually on an average of 1, 100,000 acres. Nationally, approximately 2% of all wheat 

acres are treated with propoxycarbazone sodium. About 87% of the total pounds of 

propoxycarbazone sodium is used on winter wheat and the remaining 13% on spring wheat. In 

terms of area treated, about 72% of the acres treated are winter wheat and the remaining 28% is 

on spring wheat. (Table 1 ). 

Table 1. Propoxycarbazone sodium PCT, Average Annual Pounds Applied and Average 

A ,pp I' 1ca ti on R a t e. 

Average 

Average Average Percent Average Application 

Acres Base Acres Crop Annual lbs Rate 

Crop Grown Treated Treated Applied (lb ai/acre) 

Spring Wheat 14,575,400 310,922 2.1% 3,236 0.0104 

Winter Wheat 38,904,005 792,345 2.0% 22,135 0.0279 

Source: (Market Research Data, 2010-2014) 

Of the 2% of spring and winter wheat acres treated with propoxycarbazone sodium, 

approximately 2.5% of the spring wheat is treated aerially at an average rate of 0.0088 lb a.i. and 

approximately 5.7% of the winter wheat is treated aerially at an average application rate of 0.027 

lb a.i. (Table 2). 

T a bl e 2 . P rooo:x vcar b azone so d' ium Wh eat A cres T reate dA er1a . II 1y P er Y ear. 

Average 

Average Average Acres Percent Average lbs Application 

Total Acres Treated by Treated Applied by Rate 

Crop Treated Air Aerially Air (lb. ai/acre) 

Soring Wheat 310,922 7,654 2.5% 67 0.0088 

Winter Wheat 832,773 47,694 5.7% 1,290 0.0270 

Source: (Market Research Data, 2010-2014) 

Table 3 below presents the maximum single and average application rate as well as the rates used 

at the 99th percentile and the percentage of acres that are treated multiple time for spring and 

winter wheat. The label maximum single application rate for both spring and winter wheat is 

0.039 lbs a.i. per acre. 

The average application rate for spring wheat is 0.010 lbs a.i. per acre and 0.0279 lbs a.i. per acre 

for winter wheat. About 99% of the acres are treated at a rate of 0.0270 and 0.0390 lbs a.i. per 

acre or lower for spring wheat and winter wheat, respectively. Winter wheat receives multiple 

applications and that occurs on about 3.4% of wheat acres that are treated. 

4 
T a bl e 3 M . U If IP I A e • DD I' 1cations o f P ropoxycar b azone so d" 1um by Crop . *Labelled Average Application Rate 

Crop Maximum Single for 99 Percentile of 

Annlication Rate Application Rate Acres Treated 

Sprin2 Wheat 0.0394 0.0104 < 0.0270 lbs/acre 

Winter Wheat 0.0394 0.0279 < 0.0390 lbs/acre 

Source: (Market Research Data, 2010-2014) Numbers have been rounded. 

*EPA, EFED Label Data Report, 2014 

% of Acres 

Treated Multiple 

Times 

0% 

3.4% 

Table 4 shows the aerial treatments at the state level for states that reported propoxycarbazone 

sodium use on wheat. At the state level, the states with the highest percent of their wheat acres 

treated aerially with propoxycarbazone sodium as a percent of the total acres treated are Oregon 

(27%) and Washington (13%). 

Table 4. Average Wheat Acres Grown and the Percent of Total Wheat Acres Treated with 

P b d 0 

ropoxvcar azone so IUm A •PP r 1e d A . er1a II 1y. 

State Average Wheat Acres Percent of Total State Wheat Acres Treated 

Grown* Aerially with Propoxvcarbazone sodium 

Oregon 812,222 27% 

Washington 2,191,111 13% 

South Dakota 2,411,002 8% 

N. Dakota 7,004,446 2% 

Oklahoma 5,611,107 1% 

Source: (Market Research Data, 2005-2013) 

*Includes both winter and spring wheat acres based on Market Research Data. 

Number of Applications 

Table 5 below illustrates the number of spring and winter wheat acres that are treated once and 

the number of acres that are treated more than once. According Market Research Data for the 

years 2010 to 2014, 100% of the spring wheat was treated only once. For the same time period, 

94.5% of winter wheat was treated only once. However, Market Research Data reports that 

winter wheat had up to 3 applications. Approximately 6% of the winter wheat acres were treated 

twice and about 0.5% of the winter wheat acres had up to 3 applications. 

T a e bl 5 N b . um ero fWh ea tA cres T rea t e u dM If 1p1e I T" 1mes 

Spring Wheat Winter Wheat 

Acres Treated 310,922 792,345 

Acres Treated Once 310,922 765,705 

Lbs Aoolied 3,236 20,744 

A vera2e Ano Ii cation Rate (lbs ail acre 0.0104 0.0271 

Sprine: Wheat Winter Wheat 

Acres Treated Multiple Times 0 26,640 

Lbs Aoolied 0 1,391 

Average Application Rate Obs ail acre) NIA 0.0522 

Source: Market Research Data (2010-2014) 

5 
SPRAY DRIFT MITIGATION 

Several variables affect the magnitude and distance of drift during aerial application. These 

include: droplet size; speed; boom width; elevation of aircraft during application; nozzle size; 

optimal environmental conditions and chemical formulation, among several others. Several 

publications describe these, and other, variables in more detail and provide best management 

practices for spray drift (ASABE, 2012; O'Connor-Marer, 2011, and VT, 2015). 

Based on recent conversations with the registrant of propoxycarbazone sodium, the droplet size 

that they recommend for aerial application is "coarse". BEAD concurs with the registrant based 

on the limited information that is publically available on the efficaciousness of the product under 

scenarios where the drop size is varied. 

RESISTANCE MANAGEMENT 

The development and spread of herbicide resistant weeds in agriculture is a widespread problem 

that has the potential to fundamentally change production practices in US agriculture. While 

herbicide resistance weeds have been known since the 1950s, the number of species and their 

geographical extent, has been increasing rapidly. Currently there are 249 weed species 

worldwide with confirmed herbicide resistance, especially since the turn of the century. In the 

United States there are 155 weed species with confirmed resistance to one or more herbicides. 

Management of herbicide resistant weeds, both in mitigating established herbicide resistant 

weeds, and in slowing or preventing the development of new herbicide resistant weeds is a 

complex problem without a simple solution. Coordinated efforts of growers, agricultural 

extension, academic researcher, scientific societies, pesticide registrants, and state and federal 

agencies are required to address this problem. 

In September 2014, the Weed Science Society of America sponsored an international meeting, 

the Herbicide Resistance Summit II, hosted by the National Research Council (WSSA, 2014). 

This meeting was organized to facilitate a more unified understanding of the herbicide resistance 

issues across the country, understanding of differences of viewpoints, and approaches to 

solutions. The meeting was attended in person or via webinar by participants from 

approximately 100 locations across the United States, Australia, Canada and Germany, 

underscoring the significance and widespread nature of this problem and its impact on 

agricultural productivity. 

The Agency announced at this meeting that it would take a more proactive role in developing 

regulatory approaches for managing resistant weeds (Housenger, 2014). Shortly after this 

meeting, the Secretary of Agriculture announced USDA's herbicide resistance actions that were 

developed in collaboration with the Environmental Protection Agency (USDA, 2014). 

The EPA, Office of Pesticide Programs (OPP) is proposing measures for the pesticide registrants 

to provide growers and users with detailed information and recommendations to slow the 

development and spread of herbicide resistant weeds. This is part of a more holistic, proactive 

6 
approach recommended by crop consultants, commodity organizations, professional/scientific 

societies, researchers, and the registrants themselves. OPP's approach is measured, based on the 

inherent risk of weed resistance developing for a given herbicide, considering the target weeds 

and the agronomic practices of the registered crops. Situations with the least concern for the 

development of herbicide resistant weeds will have the fewest resistance management elements 

and the situations with the highest concern will have additional resistance management elements. 

Additional details are provide in the attached appendices, as follows: 

• Appendix I - Guidance for Pesticide Registrants on Pesticide Resistance Management 

Labeling (EPA, 2001) 

• Appendix II - Definition of Resistance and Likely Resistance 

• Appendix III-Best Management Practices for Herbicide Resistant Weeds (from the 

Herbicide Resistance Action Committee and the Weed Science Society of America 

Implementation Timeline and Opportunities for Public Comment on Herbicide Resistance 

Approach 

OPP is proposing to implement herbicide resistance measures for existing chemicals during 

registration review, and to implement herbicide resistance measures for new chemicals and uses 

at the time of registration. In registration review, proposed herbicide resistance elements will in 

every herbicide preliminary interim decision. Comments on these chemical specific measures 

are welcomed. 

All stakeholders are encouraged to comment on the herbicide resistance management approach, 

rationale, and proposed elements. 

Public comments received from both the chemical specific docket and the separate HR 

management docket will be considered prior to the final decision on this propoxycarbazone 

sodium. 

Propoxycarbazone sodium Specific Herbicide Resistance Measures 

Propoxycarbazone sodium acts to inhibit acetolactate synthase (ALS), a key enzyme in the 

biosynthesis of the branched-chain amino acids in plants. This herbicide is classified by the 

Weed Science Society of America (WSSA) as belonging to WSSA Mechanism of Action 

(MOA) Group 2 (WSSA, 2016). This MOA has a number of confirmed resistant weeds with 

over 150 confirmed cases of weed resistance worldwide and 48 cases of resistant weed species in 

the U.S. Of the 48 confirmed ALS resistant weeds in the US, approximately 15 have been 

identified and reported in wheat. Several target weeds in wheat have confirmed resistance to 

propoxycarbazone sodium. 

This MOA is of the highest concern for weed resistance development because this mechanism of 

action already has a large number of confirmed herbicide resistant weeds species in the U.S., is 

used on large acreage crop, wheat, with many reported resistant weeds, and Group 2 has seven 

commercialized herbicide resistant crops. Table 6 describes the elements of a resistance 

7 
management and stewardship plan. Table 7 identifies the elements needed for each level of 

concern. 

Table 6. Elements of a resistance management and stewardship plan 

Element Description 

List Mechanism of Action (MOA) Group Number. 

1 • This information is critical to allow the user to rotate between effective 

MOA's to reduce the buildup ofresistant pests. )- Registrant is responsible for placement on label. 

List seasonal and annual maximum number of applications and amounts. 

• This information is critical to allow the user to know how many 

2 applications and amounts can be applied in order to develop an effective 

IPM plan for the season and the entire year. 

)- Registrant is responsible for placement on label. 

Resistance Management language from PR Notice 2001-5, and/or 

Best Management Practices (appropriate to crop) from Weed Science Society of 

America (WSSA) & Herbicide Resistance Action Committee (HRAC), and/or 

3 HRAC proposed guidelines for herbicide labels. 

• This is an educational opportunity to remind users to look for and follow a 

resistance management plan. 

)- Registrant is responsible for placement on label. 

User should scout before and after application. 

• Reminding the user to scout can help insure that the proper pesticide is 

4 applied based on the weed species and growth stage and determine if the 

pesticide applied provided effective control. 

)- Registrant is responsible for placement on label. 

)- User is responsible for following the recommendations. 

Definition of Likely Resistance. 

• It can take up to five years to confirm herbicide resistance. By describing 

5 likely resistance users could proactively identify and attempt to control 

weeds in the early stages before they become widespread in their fields. 

)- Registrant is responsible for placement on label. 

User should report lack of performance to registrant or their representative. 

• Reporting lack of performance can help provide the user with additional 

resources for the identification and control of problem weeds. In some 

6 cases lack of performance could be an early indication of likely resistance 

and contacting the registrant can help insure the weed is controlled before 

resistance becomes widespread in their fields. 

)- Registrant is responsible for placement on label. 

)- User is responsible for following the recommendations. 

List confirmed resistant Weeds in a separate table and list effective or 

recommended rates for these weeds with the table. 

7 • This is an educational opportunity to clearly indicate which weeds are 

prone to resistance and remind the users to select the correct rates for their 

crop and site. 

8 
)oi- Registrant is responsible for placement on label. 

Registrant report new cases of likely and confirmed resistance to EPA and users 

yearly. This will be in addition to any adverse effects reporting. 

• This will allow the information likely and confirmed resistance to be 

8 available in a timely manner to users, consultants, extension, etc. so that 

they are aware of and proactively address the problem. 

• Provide weed species, crop or site, state, and herbicide used . )oi- Registrant is responsible for reporting. 

For sites of high concern provide growers with: 

D Resistance Management Plan 

0 Remedial Action Plan (to control resistant weeds this season or next season) 

0 Educational materials on resistance management. 

9 • Plans should be locally developed and easily modified. EPA recommends 

that registrants work with Extension, Consultants, Crop Groups, HRAC, & 

USDA. 

• This is an educational opportunity to remind users of the importance of 

resistance management. 

)oi- Registrant is responsible for creating or providing educational materials. 

For combination products with multiple MOA, list which herbicide is controlling 

which weed (a 3 way mixture may only have 1 effective MOA for some problem 

weeds). List minimum recommended rate if resistance is suspected. 

• Using a combination products with only one effective MOA can select for 

10 herbicide resistant weeds. 

• This will allow users to select herbicide combinations with multiple 

effective MOAs for the problem weeds on their fields. 

• Registrant is responsible for placing the list on label or otherwise providing 

the information. 

Any additional specific requirements (e.g. mandatory crop rotation, unique 

agronomic aspects, additional training, time limited registration, etc.). 

• During discussions with the Agency other elements may be deemed 

11 appropriate to help reduces the spread of resistance. 

• The elements may be on the label, the technical use agreement for the seed 

trait, or as a reporting requirement. 

)oi- Registrant is responsible. 

Footnote: Mechanism of Action Group number comes from the WSSA, definition of resistance 

and likely resistance, PR Notice 2001-5 and BMP language are in Appendix 1-3. 

Resistance elements for fungicides and insecticides. 

9 
Table 7. Herbicide Resistance Categories of Concern 

Low Concern Moderate Concern High Concern 

• Any new herbicide with a new or 

novel mechanism of action, or 

MOA's with no MOA's with a few resistant • Herbicide resistant crop( s) for 

resistance weed weed species in the U.S. that technology (conventionally 

species in the U.S. bred or GM), or 

• MOA's with highest number of 

resistant weeds in U.S. 

1. MOA on Label Elements 1 through 4 plus Elements 1 through 8 plus 

2. List seasonal and 5. Definition of likely and 9. Provide growers with: 

annual maximum confirmed resistance Resistance Management Plan, 

number of 6. Farmer should report Remedial Action Plan, 

applications and lack of performance to Educational materials on 

pounds registrant or its agent resistance management 

3. Resistance 7. List confirmed resistant 10. For combination products with 

management species in separate multiple MOAs, list which 

language from table and list effective herbicide is controlling which 

PRN 2001-5 or or recommended rates weed and minimum 

BMPs for these weeds with recommended rate 

4. Scout before and the table 11. Any additional specific 

after application 8. Registrant report new requirements (e.g. mandatory 

cases of likely and crop rotation, unique agronomic 

confirmed resistance to aspects, time limited 

EPA & users yearly registration, etc.). 

* If new resistant weed species are found a MOA may move to higher level of concern. 

REFERENCES 

ASABE. 2012. Best Management Practices for Boom Spraying. Approved January 2007; 

reaffirmed March 2012 as an American National Standard. American Society of 

Agricultural and Biological Engineers (ASABE). 

Environmental Protection Agency. BEAD Chemical Profile (BCP) for Registration Review: 

Propoxycarbazone sodium (122019) 2014. 

EPA. 200 I. Pesticide Registration (PR) Notice 2001-5. Guidance for Pesticide Registrants on 

Pesticide Resistance Management Labeling. Accessed February 2016 at 

http://www.epa.gov/pesticide-registration/prn-2001-5-guidance-pesticide-registrantspesticide-resistance-management. 

IO 
Guamone A, Davi R. 2012. Propoxycarbazone-sodium: new post-emergence herbicide for soft 

and durum wheat. Accessed February 2016 at 

http://www.cabdirect.org/abstracts/2014 3 03 983 5 .html;jsessionid=4 AC l 6B48BE96B 7367 

8 lEl 4841 F2D992B:jsessionid=B49EAC3672A5F442FOF04C95CB650BFC. 

Housenger, J. 2014. USDA Stakeholder Workshop on Coexistence March 12, 2015. Accessed 

March 2016 at: 

https://www.aphis.usda.gov/stakeholders/downloads/2015/coexistence/JackHousenger.pdf 

HRAC. 2016. Classification of Herbicides According to Site of Action. Herbicide Resistance 

Action Committee (HRAC). Accessed February 2016 at 

http://hracglobal.com/pages/classificationofherbicidesiteofaction.aspx 

Heap I. 2016. The International Survey of Herbicide Resistant Weeds. Accessed February 2016 

at http://www.weedscience.org/summary/ResistbyActi ve.aspx. 

Market Research Data (MRD) ( 1998 - 2014 ). Data collected and sold by a private market 

research firm. Data collected on pesticide use for about 60 crops by annual surveys of 

agricultural users in the continental United States. Survey methodology provides 

statistically valid results, typically at the state level. 

Norsworthy JK, Ward SM, Shaw DR, Llewellyn RS, Nichols RL, Webster TM, Bradley KW, 

Frisvold G, Powles SB, Burgos NR, Witt WW, Barrett M. 2012. Reducing the risks of 

herbicide resistance: Best Managment Practices and Recommendations. Weed Science 

Special Issue: 31-62. Accessed February 2016 at 

http://wssajournals.org/doi/pdf/1O.l614/WS-D-11-00155.1 

O'Connor-Marer, P. J. 2011. Aerial Applicator's Manual. A National Pesticide Applicator 

Certification Guide. National Assoeiation of State Departments of Agriculture Research 

Foundation. Accessed on March 7, 2016 at http://www.nasda.org/File.aspx?id=3786 

TAMU, 2008. Weed Control Recommendations in Wheat. Texas A&M University (TAMU) 

Extension. Accessed February 2016 athttp://amarillo.tamu.edu/files/2010/l l/B-6139-

2008-Weed-Control.pdf. 

USDA, 2014. USDA Announces Measures to Help Farmers Diversify Weed Control Efforts. 

United States Department of Agriculture. Accessed March 2016 at: 

www .usda.gov/wps/portal/usda/usdamediatb?contentid=2014/10/0227 .xml&printable=tr 

ue&contentidonly=true . 

VT. 2015. Nozzles: Selection and Sizing. Virginia Cooperative Extension Publication 442-032. 

Virginia Tech. Accessed November 2015 at https://pubs.ext.vt.edu/442/442-032/442-

032 pdf.pdf. 

WSSA. 2014. Herbicide Resistance Summit II. Weed Science Society of America. Accessed 

March 2016 at http://wssa.net/wssa/weed/resistance-summit-ii/. 

11 
WSSA. 2016. Classification of Herbicides According to Site of Action. Weed Science Society of 

America. Accessed February 2016 at 

http://weedscience.org/documents/showdocuments.aspx?DocumentID= 1193. 

12 
APPENDIX I. Guidance for Pesticide Registrants on Pesticide Resistance Management 

Labeling (EPA, 200 I) 

Herbicides 

1. The following general resistance management labeling statements are recommended for 

herbicide products containing only a single active ingredient or only active ingredients 

from the same group: 

a. "For resistance management, (name of product) is a Group (mode of action group 

number) herbicide. Any weed population may contain or develop plants naturally 

resistant to (name of product) and other Group (mode of action group number) 

herbicides. The resistant biotypes may dominate the weed population if these 

herbicides are used repeatedly in the same field. Other resistance mechanisms that are 

not linked to this mode of action but are specific for individual chemicals, such as 

enhanced metabolism, may also exist. Appropriate resistance-management strategies 

should be followed." 

For products containing active ingredients from different groups, the statement should be 

modified to reflect the situation, for example: 

b. "For resistance management, please note that (name of product) is both a Group (mode 

of action group number) and a Group (mode of action group number) herbicide. Any 

weed population may contain plants naturally resistant to Group (mode of action group 

number) and/or Group (mode of action group number) herbicides. The resistant 

individuals may dominate the weed population if these herbicides are used repeatedly 

in the same fields." 

2. The following additional resistance management labeling statements are recommended for 

herbicides, although each bulleted statement may not be appropriate or pertinent for every 

product label: · 

"To delay herbicide resistance: 

a. Rotate the use of (name of product) or other Group (mode of action group number) 

herbicides within a growing season sequence or among growing seasons with different 

herbicide groups that control the same weeds in a field. 

b. Use tank mixtures with herbicides from a different group if such use is permitted; Use the 

less resistance-prone partner at a rate that will control the target weed(s) equally as well 

as the more resistance-prone partner. 

c. Adopt an integrated weed management program for herbicide use that includes scouting 

and historical information related to herbicide use and crop rotation, and that considers 

tillage (or other mechanical control methods), cultural (e.g., higher crop seeding rates; 

13 
precision fertilizer application method and timing to favor the crop and not the weeds), 

biological (weed-competitive crops or varieties) and other management practices. 

d. Scout after herbicide application to monitor weed populations for early signs of resistance 

development. Indicators of possible herbicide resistance include: (1) failure to control a 

weed species normally controlled by the herbicide at the dose applied, especially if 

control is achieved on adjacent weeds; (2) a spreading patch of non-controlled plants of a 

particular weed species; (3) surviving plants mixed with controlled individuals of the 

same species. If resistance is suspected, prevent weed seed production in the affected 

area by an alternative herbicide from a different group or by a mechanical method such as 

hoeing or tillage. Prevent movement of resistant weed seeds to other fields by cleaning 

harvesting and tillage equipment when moving between fields, and planting clean seed. 

e. If a weed pest population continues to progress after treatment with this product, 

discontinue use of this product, and switch to another herbicide with a different target 

mode of action, if available. 

f. Have suspected resistant weed seeds tested by a qualified laboratory to confirm resistance 

and identify alternative herbicide options. 

g. Contact your local extension specialist or certified crop advisors for additional pesticide 

resistance-management and/or integrated weed-management recommendations for 

specific crops and weed biotypes. 

h. For further information or to report suspected resistance, contact (company 

representatives) at (toll free number) or at (Internet site)." 

14 
APPENDIX II. Definition of Resistance and Likely Resistance 

According to the Weed Science Society of America "Herbicide resistance is the inherited ability 

of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to 

the wild type. Jn a plant, resistance may be naturally occurring or induced by such techniques as 

genetic engineering or selection of variants produced by tissue culture or mutagenesis." 

"Herbicide tolerance is the inherent ability of a species to survive and reproduce after herbicide 

treatment. This implies that there was no selection or genetic manipulation to make the plant 

tolerant; it is naturally tolerant." (http://weedscience.org/documents/resistancecriterion.pdf). 

Indicators of likely herbicide resistance (called possible resistance in Norsworthy et al 2012, 

Page 39) include (1) failure to control a weed species normally controlled by the herbicide at the 

dose applied, especially if control is achieved on adjacent weeds; (2) a spreading patch of 

noncontrolled plants of a particular weed species; and (3) surviving plants mixed with controlled 

individuals of the same species. 

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APPENDIX 111. Best Management Practices for Herbicide Resistant Weeds (from the Herbicide 

Resistance Action Committee I Weed Science Society of America list of Best Management 

Practices) 

Crop Selection and Cultural Practices: 

1. Understand the biology of the weeds present. 

2. Use a diversified approach toward weed management focused on preventing weed seed 

production and reducing the number of weed seeds in the soil seed-bank. 

3. Emphasize cultural practices that suppress weeds by using crop competitiveness. 

4. Plant into weed free fields, keep fields as weed free as possible, and note areas where weeds 

were a problem in prior seasons. 

5. Incorporate additional weed control practices whenever possible, such as mechanical 

cultivation, biological management practices, crop rotation, and weed-free crop seeds, as part 

of an integrated weed control program. 

6. Do not allow weed escapes to produce seeds, roots or tubers. 

7. Manage weed seed at harvest and post-harvest to prevent a buildup of the weed seed-bank. 

8. Prevent field-to-field and within-field movement of weed seed or vegetative propagules. 

9. Thoroughly clean plant residues from equipment before leaving fields. 

10. Prevent an influx of weeds into the field by managing field borders. 

11. Fields should be scouted before application to ensure herbicides and application rates will be 

appropriate for the weed species and weed sizes present. 

12. Fields should be scouted after application to confirm herbicide effectiveness and to detect 

weed escapes. 

13. If resistance is suspected, treat weed escapes with an alternate mode of action or use nonchemical methods to remove escapes. 

14. Avoid outcrossing to weedy relatives, in crops that outcross. Control weedy relatives in 

surrounding field margins. Research has demonstrated that the pollen can move __ feet. 

Herbicide Selection: 

1. Use a broad spectrum soil applied herbicide with a mechanism of action that differs from this 

product as a foundation in a weed control program. 

2. A broad spectrum weed control program should consider all of the weeds present in the field. 

Weeds should be identified through scouting and field history. 

3. Difficult to control weeds may require sequential applications of herbicides with alternative 

mechanisms of action. 

4. Fields with difficult to control weeds should be rotated to crops that allow the use of 

herbicides with alternative mechanisms of action. 

5. Apply full rates of this herbicide for the most difficult to control weed in the field. 

Applications should be made when weeds are at the correct size to minimize weed escapes. 

6. Do not use more than two applications of "this herbicide" or any herbicide with the same 

mechanism of action within a single growing season unless mixed with another mechanism 

of action herbicide with overlapping spectrum for the difficult to control weeds. 

7. Report any incidence of non-performance of this product against a particular weed species to 

the representative (list contact information here). 

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