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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

 PC Code: 119901

DP Barcode: 429384

 Date: March 16, 2016

MEMORANDUM

Subject: Registration Review – Preliminary Problem Formulation for Ecological Risk,

Environmental Fate, and Drinking Water Assessments for Difenacoum

To: Ricardo Jones, Chemical Review Manager

Neil Anderson, Branch Chief

Risk Management and Implementation Branch 1

Pesticide Re-evaluation Division (7508P)

Office of Pesticide Programs

From: Nathan Miller, Biologist

Andrew Shelby, Physical Scientist

Environmental Risk Branch 6

Environmental Fate and Effects Division (7507P)

Office of Pesticide Programs

Reviewed: Michael Wagman, Biologist

William Eckel, Ph.D., Senior Science Advisor

Monica Wait, RAPL

Environmental Risk Branch 6

Environmental Fate and Effects Division (7507P)

Office of Pesticide Programs

Through: Mark Corbin, Branch Chief

Environmental Risk Branch 6

Environmental Fate and Effects Division (7507P)

Office of Pesticide Programs

The Environmental Fate and Effects Division (EFED) has completed the preliminary problem 

formulation for the ecological risk, environmental fate, and drinking water assessments to be 

conducted as part of the registration review of the rodenticide difenacoum. 

The problem formulation draws on information from both open literature and studies submitted 

by the technical registrants. This document is intended to provide an overview of what is 

currently known regarding the environmental fate and ecological effects associated with
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difenacoum and outlines uncertainties regarding attributes of the parent compound and its 

transformation products. It describes the preliminary ecological risk hypothesis and the 

processes that will be used during the completion of drinking water and ecological risk 

assessments in support of registration review. This document also recommends studies that 

should be included in a data call-in (DCI) to address uncertainties surrounding the environmental 

fate and potential ecological effects of difenacoum.
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Preliminary Problem Formulation for the Environmental 

Fate and Ecological Risk, Endangered Species, and Drinking 

Water Assessments in Support of the Registration Review of 

Difenacoum

Difenacoum

4-hydroxy-3-[3-(4-phenylphenyl)-1,2,3,4-tetrahydronaphthalen-1-yl]chromen-2-one

(CAS 56073-07-5)

(PC Code 119901)

Environmental Risk Branch VI

Environmental Fate and Effects Division

Nathan Miller, Biologist

Andrew Shelby, Physical Scientist

Reviewed by:

Michael Wagman, Biologist

William Eckel, Ph.D., Senior Science Advisor

Monica Wait, RAPL

Mark Corbin, Branch Chief

U. S. Environmental Protection Agency

Office of Pesticide Programs

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460
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Table of Contents

1 Executive summary................................................................................................................. 5

2 Introduction............................................................................................................................. 5

3 Use Characterization............................................................................................................... 6

4 Conclusions from Previous Risk Assessments....................................................................... 7

4.1 Endangered Species Assessments........................................................................ 7

4.2 Section 3 New Chemical Risk Assessment.......................................................... 7

4.3 Comparative Risk Assessment............................................................................. 7

4.4 Risk Mitigation Decision and Notice of Intent to Cancel .................................... 8

4.5 Drinking Water Assessments............................................................................... 8

5 Environmental Fate and Transport.......................................................................................... 9

5.1 Transformation Products.................................................................................... 10

6 Receptors............................................................................................................................... 10

6.1 Toxicity Summary.............................................................................................. 10

6.2 Secondary Toxicity ............................................................................................ 12

6.3 Incident Database Review.................................................................................. 15

7 Exposure Pathways of Concern ............................................................................................ 17

8 Analysis Plan ........................................................................................................................ 17

8.1 Stressors of Concern........................................................................................... 17

8.2 Measures of Exposure ........................................................................................ 17

8.3 Measures of Effect ............................................................................................. 23

9 Endangered Species Assessment .......................................................................................... 23

10 Endocrine Disruptor Screening Program.............................................................................. 24

11 Preliminary Identification of Data Gaps............................................................................... 24

11.1 Environmental Fate ............................................................................................ 24

11.2 Ecological Effects .............................................................................................. 26

12 References............................................................................................................................. 31

Appendix A: Difenacoum Fate/Chemistry and Ecotoxictiy Bibliography................................... 36

Appendix B. Screening Level Models to Evaluate Potential Importance of Drinking Water and 

Inhalation Exposure Pathways...................................................................................................... 42
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1 Executive summary

The Environmental Fate and Effects Division (EFED) has completed the preliminary problem 

formulation for the ecological risk, environmental fate, and drinking water assessments to be 

conducted as part of the registration review of difenacoum. The problem formulation describes 

the methods planned to be used during the completion of the ecological risk assessment in 

support of registration review and provides an overview of the environmental fate, ecological 

effects, and potential risks associated with the use of difenacoum as well as uncertainties unique 

to risk assessment of difenacoum. EFED does not anticipate conducting a drinking water 

assessment for difenacoum for use in the human health risk assessment for reasons that will be 

discussed in more detail below. This document also identifies additional studies that would be 

beneficial to the conduct of an ecological risk assessment. 

Data needs for ecological effects are:

• Avian acute oral toxicity test (850.2100), one passerine species

• Avian reproduction toxicity (850.2300), for an upland game bird (e.g., bobwhite quail 

(Colinus virginianus)) and waterfowl species (e.g., mallard duck (Anas platyrhynchos)).

• Earthworm Subchronic Toxicity Test (850.3100), one species (Eisenia fetida)

The test substance for each of these tests should be technical grade (TGAI). However, given that 

the available data indicate a differential in toxicity between difenacoum’s cis and trans isomers, 

the above requested toxicity studies should track the ratio of these isomers in the tested mixture. 

At this time, EFED does not anticipate needing to call in any additional environmental fate data 

for difenacoum.

2 Introduction

Difenacoum is a second generation coumarin anticoagulant rodenticide. It uncouples oxidative 

phosphorylation depressing hepatic synthesis of prothrombin and clotting factors II, VII, IX and 

X and interfering with the vitamin K1 cycle (Hayes and Laws, 1991).

Anticoagulants are vitamin-K antagonists that disrupt normal blood-clotting mechanisms and 

induce capillary damage. Typically, death is delayed for four to ten or more days after a lethal 

dose is ingested, and animals may continue to feed and move about until shortly before death. 

Death results from hemorrhage, and exposed animals may exhibit behavior that may make them 

more susceptible to predation (Cox and Smith, 1992). This may result in secondary exposure to 

predatory animals. Second generation anticoagulant rodenticides, like difenacoum, tend to be 

more acutely toxic in a single feeding and persistent in biological tissue than first generation 

anticoagulant rodenticides (Erikson and Urban, 2004).

EFED evaluated the most recent ecological risk assessments for difenacoum in association with 

the updated toxicity, exposure, and usage information to determine if sufficient data are available 

and if further updates are needed to support registration review. In addition, EFED considered 

the latest Agency science policies and risk assessment methodologies. The structure of 
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difenacoum as well as the chemical name and other identifiers can be found in the chemical 

identity table in Section 5 of this document. 

3 Use Characterization

There is no national level usage data for difenacoum (PC code 119901). Table 1 lists the 

maximum use patterns (application rates, number of reapplications, etc.) for the labeled use sites 

for difenacoum. The table is based on the Label Usage Report Package created by the Biological 

and Economics Analysis Division (BEAD) in support of Registration Review (USEPA, 2015). 

There are currently eight active Section 3 labels for difenacoum, two for technical difenacoum 

and six for end-use products. Difenacoum is formulated as ready-to-use pelleted bait. The target 

pests include: Norway rat (Rattus norvegicus), house mouse (Mus musculus), roof rats (Rattus 

rattus), Polynesian rats (Rattus exulans), cotton rats (Sigmodon hispidus), eastern harvest mice 

(Reithrodontomys humuli), golden mice (Ochrotomys nuttalli), meadow voles (Microtus 

pennsylvanicus), white-throated woodrats (Neotoma albigula), southern plains woodrats 

(Neotoma micropus), and Mexican woodrats (Neotoma mexicana). Concentrations in bait are 

0.005% difenacoum. All labeled uses carry the follow restrictions: 

“Do not place bait near to or inside of ventilation duct openings. Do not contaminate 

water, food feed, food or feed handling equipment, milk or meat handling equipment, or 

any area where there is a possiblility of contaminating food, feed, milk, or meat or 

surfaces that come in contact with food, feed, milk, or meat. All outdoor placements of 

this product must be in bait stations. Tamper resistant bait stations must be used for 

outdoor and indoor placements of this product if children, pets, non-target mammals, or 

birds may access bait. All placements of this product in USDA-inspected facilities must 

be in tamper-resistant bait stations. Do not broadcast. Wear water-resistant gloves when 

handling bait.”

Table 1. Label Use Information for difenacoum based on BEAD’s summary report

Use

Single Max 

Application Rate Application 

Methods

Agricultural/Farm Structures/Buildings and Equipment; 

Commercial/Institutional/Industrial/Equipment (Indoor); Commercial 

Transportation Facilities (All or Unspecified); Household/Domestic 

Dwellings Indoor Premises; Household/Domestic Dwellings Outdoor 

Premises; Public Buildings/Structures (Vert. Pest Control); Refuse/solid 

waste sites (indoor)

0.0001 lb/placement Bait station

Food processing plant premises (nonfood contact) 0.0001 lb/placement Bait station

Commercial Transportation Vehicles (All or Unspecified); Wide 

Area/General Indoor Treatment; Wide Area/General Outdoor Treatment 

(Public Health use)

0.0001 lb/placement Bait station
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4 Conclusions from Previous Risk Assessments

The Agency has conducted previous ecological risk assessments for difenacoum that serve as a 

basis for this problem formulation. They are briefly summarized below. 

4.1 Endangered Species Assessments

An endangered species risk assessment for San Francisco Bay Species was completed for 

difenacoum in 2012 (USEPA 2012) to address potential risk to the Salt marsh harvest mouse 

(SMHM) (Reithrodontomys raviventris), Alameda Whipsnake (AW) (Masticophis lateralis 

euryxanthus) and San Joaquin Kit Fox (SJKF) (Vulpes macrotis mutica) for which difenacoum

was alleged to be of concern in a complaint made by the Center for Biological Diversity against 

United States Environmental Protection Agency on May 30, 2007 (Case No: 07-2794-JCS).

Based on the best available information, the Agency made a “May Affect, and Likely to 

Adversely Affect” determination for the SMHM, AW, and SJKF from the uses of difenacoum. 

Additionally, the Agency determined that there was the potential for modification of designated 

critical habitat for the AW from the use of difenacoum.

4.2 Section 3 New Chemical Risk Assessment

The Section 3 new chemical ecological risk assessment for difenacoum was published in 2007

(USEPA, 2007). Difenacoum was conditionally registered in September 2007 in accordance 

with FIFRA section 3(c)(7)(C) provided that all difenacoum products are amended to comply 

with EPA’s Final Risk Mitigation Measures for registered products containing the other second 

generation anticoagulants, on the same time schedule as those similar rodenticides.

4.3 Comparative Risk Assessment

“Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative 

Approach” (USEPA 2004) evaluated primary and secondary exposure risks of several 

rodenticides to birds and nontarget mammals. This document did not evaluate difenacoum 

specifically, however it indicated that rodenticide baits are formulated to be lethal to small 

mammals, and they are not selective to nontarget species. The document stressed that all baits 

pose a high potential for primary risk to birds and nontarget mammals that eat bait. Avian and 

mammalian predators and scavengers are said to be at risk from feeding on animals poisoned 

with anticoagulant baits. The 2004 assessment also specified a number of factors contributing to 

uncertainty in assessing anticoagulant rodenticides. Those factors that contributed the most 

uncertainty were: (1) missing data, including acute, chronic, and secondary toxicity as well as 

data regarding retention of some active ingredients in the liver, blood, and other body tissues; (2) 

the variable quality and quantity of existing data on metabolism and retention times in rodents 

and non-target species; (3) specific use information by formulation, including typical amounts 

applied by use site, seasonally, and annually; distances applied from buildings; amounts used in 

rural versus urban areas; use by Certified Applicators versus homeowners and other non-certified 

applicators; and other such relevant information; (4) information on the number and species of 

birds and non-target mammals frequenting baited areas and the likelihood of their finding and 
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consuming bait or poisoned primary consumers in the various use areas; (5) methods to 

determine liver concentration(s) and total body burdens of rodenticide that would corroborate 

death or even if such a cause-effect relationship is appropriate (e.g., the “threshold of toxicity” 

concentration); (6) not accounting for the impacts of sub-lethal effects on reproduction and nontarget mortality (e.g., clotting abnormalities, hemorrhaging, stress factors including 

environmental stressors, such as adverse weather conditions, food shortages, and predation); (7) 

not accounting for bioaccumulation of repeated sub-lethal exposures to bait or poisoned rodents 

utilized as food by predators and scavengers; and (8) lack of incident reporting. 

4.4 Risk Mitigation Decision and Notice of Intent to Cancel

In 2008, EPA released the Risk Mitigation Decision for Ten Rodenticides (RMD) (EPA 2008, 

Document ID EPA-HQ-OPP-2006-0955-0764 available at www.regulations.gov). Difenacoum 

products were included in the list of ten rodenticides addressed in the 2008 document. The focus 

of the decision was:

1) To minimize children’s exposure to rodenticide products used in homes by requiring 

that all rodenticide bait products marketed to general and residential consumers be 

sold only with bait stations, with loose bait (e.g., pellets and meal) as a prohibited bait 

form. 

2) To reduce wildlife exposures and ecological risks, by requiring sale and distribution 

limits intended to prevent general consumers from purchasing residential use bait 

products containing four of the ten rodenticides that pose the greatest risk to wildlife 

(the second generation anticoagulants – brodifacoum, bromadiolone, difenacoum, and 

difethialone). Moreover, bait stations will be required for all outdoor, above-ground 

uses of these second generation anticoagulants.

EPA updated its previous risk assessment findings conducted in support of the May 2008 RMD 

through the application of additional effects and exposure data, use of additional exposure 

modeling, and quantitative risk assessment techniques for the four rodenticides subject to the 

Notice of Intent to Cancel (NOIC) (brodifacoum, difethialone, warfarin and bromethalin) and for 

two rodenticides that are likely alternatives (i.e., active ingredients included in products with 

RMD-compliant registrations for commensal rodent control, specifically, chlorophacinone and 

warfarin; bromethalin has both compliant and non-compliant registrations). To support the 

NOIC, EPA conducted an extensive assessment of the potential for primary and secondary 

effects of the rodenticides under evaluation. This work was brought before the Science Advisory 

Panel (SAP) in 2011 for review; meeting materials are available under docket number EPA-HQOPP-2011-0718 at www.regulations.gov. 

4.5 Drinking Water Assessments

Difenacoum is applied as a bait in bait stations; it tends to bind to organic matter, and little is 

expected to partition or runoff into drinking water sources; therefore, this route of exposure was 

not assessed during reregistration. Due to the limited use pattern (bait applications only) and the 

relatively low environmental exposure of difenacoum, no drinking water assessment is 
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anticipated during registration review unless use rates increase significantly or if a new use is 

proposed in the future.

5 Environmental Fate and Transport 

Table 2 summarizes the physicochemical properties of difenacoum based on available data and 

model estimates. Difenacoum is considered hardly mobile to immobile in the environment 

(based on FAO 2000 mobility classification and estimates from EPISuite v4.1) and persistent to 

all forms of degradation. Volatilization from moist soil surfaces is not expected to be an 

important route of dissipation given an estimated Henry's Law constant of 1.4X10-12 atmm3

/mole. Difenacoum is not miscible with water (1.7 mg/L) but is soluble enough for aquatic 

exposure to occur should it be released in clean water. However, if released in natural waters, 

difenacoum is expected to adsorb to suspended solids and sediment based upon the modeled 

KOC. Volatilization from water surfaces is not expected to be an important route of dissipation 

based upon this compound's estimated Henry's Law constant. An estimated bioconcentration 

factor (log BCF) of 3.8 suggests a potential for bioconcentration in aquatic organisms if exposure 

were to occur. With respect to abiotic routes of degradation, difenacoum is stable to hydrolysis 

but degrades rapidly due to aqueous photolysis with half-lives less than a day at all 

environmentally relevant pHs.

With respect to biotic routes of degradation, difenacoum is persistent with DT50s greater than 

108 days from registrant submitted studies. An EU study reports a half-life of 439 days.

Difenacoum end use products are formulated as a mixture of trans and cis isomers. Differential 

environmental fate data is not available for the separate isomers but the range of the cis isomer is 

50% to 80% of difenacoum and the trans isomer is 20% to 50% of difenacoum (EFSA, 2009).

No difenacoum aquatic monitoring data is currently available through NAWQA or California 

Department of Pesticide Regulation or evident through the open literature. Aquatic exposure is 

not expected to occur considering difenacoum bait is contained in bait stations and not exposed to 

precipitation. Furthermore, if bait were exposed to precipitation, it is expected that difenacoum 

would remain adsorbed to the bait considering its high adsorption coefficent. 

Table 2. Physical-chemical Properties of Difenacoum

Study Value and units Source

CAS Number 56073-07-5 (difenacoum)

151986-16-2 (trans isomer)

151986-15-1 (cis isomer)

PPDB1

EFSA, 20092

SMILES Code O=C\1c6c(OC(/O)=C/1C5c2ccccc2CC(c4ccc(c3

ccccc3)cc4)C5)cccc6

PPDB

Molecular weight 444.52 PPDB

 1 http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm

2 https://circabc.europa.eu/sd/a/27fea8d9-9eb2-4cf9-b16b-263500238206/Final-AR-difenacoumSep09public.pdf
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Table 2. Physical-chemical Properties of Difenacoum

Study Value and units Source

Hydrolysis Stable MRID 46750903

Photolysis 3.3 hrs at pH 5

8.1 hrs at pH 7

7.3 hrs at pH 9

PPDB

Solubility 1.7 mg/L at pH 7 MRID 46750903

Biodegradation DT50 > 108 days.

t1/2 = 439 days

MRID 46750914

PPDB

Organic carbon Partition 

Coefficient (Koc)

4,827,000 mL/g o.c. (MCI method)

46,020 mL/g o.c. (KOW method)

EPISuite v4.1

Vapor pressure 6.7*10-9 to 5.4*10-14 Pa EPISuite v4.1

Henry’s law constant 1.4X10-12 atm-m3/mole EPISuite v4.1

Octanol-water partition 

coefficient (Kow)

Log KOW = 7.6 EPISuite v4.1

Difenacoum has the potential to move offsite through food consumption by target species, which 

travel away from the application site. Other than direct consumption of bait and secondary 

poisoning, other routes of environmental exposure are expected to be minimal. Difenacoum is 

strongly sorbed to the bait formulation, and therefore movement of residues by means of 

leaching through the soil profile to an aquatic environment is not likely. Further, difenacoum is 

unlikely to volatilize, unlikely to move offsite due to method of application (bait formulation), 

and due to the relatively low application rates (maximum of 0.0001 lb a.i./placement), and 

unlikely to runoff or leach in substantial quantities.

5.1 Transformation Products

Difenacoum transformation products through abiotic, soil biotic, or sediment biotic degradation 

have not been identified due to the persistence of difenacoum and because those types of data are 

lacking.

6 Receptors

6.1 Toxicity Summary

Table 3 provides a summary of the taxonomic groups and the surrogate species tested to 

characterize the potential acute and chronic ecological effects of difenacoum. In addition, the 

table provides a preliminary overview of the potential acute toxicity of difenacoum by providing 

the acute toxicity classifications. No ecotoxicity data are available for any degradates of 

difenacoum (difenacoum has no known major degradates).

Difenacoum is moderately toxic to birds on an acute oral basis, very highly toxic to birds on a 

subacute dietary exposure basis, and very highly toxic to mammals on an acute oral basis. 

Sublethal effects noted in the acute oral study in live test birds that were euthanized and 

subsequently subjected to necropsy (MRID 46750922) included hemorrhaging and lethargy in all 

birds while mortality was 40% even at the lowest treatment group of 50 mg/kg bw. Post-mortem 
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examinations indicated evidence of hemorrhaging and blood clots in the liver and other organs 

for birds exposed to the ≥0.75 mg/kg treatment diets in the sub-acute dietary study (MRID 

46750926). A submitted study evaluating the chronic toxicity of difenacoum to Japanese quail

was found to be unacceptable due to high control mortality and husbandry concerns. 

Several studies were submitted on difenacoum’s (or its cis or trans isomers) effect on a number 

of mammalian species including the rat (Rattus norvegicus), house mouse (Mus musculus), rabbit 

(Oryctolagus cuniculus), domestic dog (Canis lupus familiaris), domestic cat (Felis sylvestris 

cattus), sheep (Ovis aries), domestic pig (Suc scrofa), guinea pig (Cavia porcellus) and hamster 

(Mesocricetus auratus). However, of the mammalian acute oral studies, only the rat studies were 

determined to be acceptable. 

Difenacoum is very highly toxic to mammals on an acute oral exposure basis based on the rat data 

(male acute oral LD50 = 1.8 mg/kg-bw, MRID 46750935). Additionally, MRID 46766206 shows 

that the cis isomer may be more toxic than the trans isomer and the house mouse may be more 

sensitive than the rat with male mice exposed to the cis isomer having an LD50 of 0.45 mg a.i./kgbw (female mice exposed to the cis isomer in the same study had an LD50 of 1.0 mg a.i./kg-bw), 

while the trans isomer was approximately 2-3 times less toxic (male and female LD50s of 1.18 and 

2.75, respectively). EFED typically averages the male and female LD50 values if a difference 

exists between them. In the risk assessment, male mouse data with the cis isomer will be used for 

risk characterization. 

No acceptable acute dietary or chronic mammalian data are available for difenacoum. In 

addition, data on the toxicity of difenacoum to terrestrial invertebrates and terrestrial plants have 

not been submitted to the Agency.

Technical grade difenacoum is classified as very highly toxic to freshwater fish and highly toxic 

to freshwater invertebrates on an acute exposure basis based on study-author reported toxicity. 

Acute estuarine/marine fish and invertebrate data as well as chronic exposure effects data to 

difenacoum for fish and invertebrates have not been submitted to the Agency. While no toxicity 

data for aquatic vascular plants were submitted to the Agency, toxicity data were submitted for a 

single species of aquatic nonvascular aquatic plant (green algae, Pseudokirchneriella 

subcapitata) and indicate that difenacoum is toxic to algae based on study-author reported 

toxicity.

Table 3. Most Sensitive Test Species Evaluated for Assessing Potential Ecological Effects of 

Difenacoum.

Taxonomic 

Group

Study 

Type Tested Species Toxicity Endpoint

(95% CI)

Acute Toxicity 

Classification/

NOAEC most 

sensitive 

endpoint(s)

MRID and 

Classification

Birds1

Acute oral 

(primary 

bait 

consumer)

Colinus 

virginianus

(Northern 

bobwhite)

LD50 = 67 mg a.i./kgbw (3.5—150.7). Slope 

= 1.22

Moderately toxic

46750922

(Acceptable)
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Table 3. Most Sensitive Test Species Evaluated for Assessing Potential Ecological Effects of 

Difenacoum.

Taxonomic 

Group

Study 

Type Tested Species Toxicity Endpoint

(95% CI)

Acute Toxicity 

Classification/

NOAEC most 

sensitive 

endpoint(s)

MRID and 

Classification

Dietary

Anas 

platyrhynchos

(Mallard)

LC50 = 14.1 mg a.i./kgdiet (6.9—88.2). Slope 

= 1.13

Very highly toxic 46750926

(Supplemental)

Mammals

Acute oral 

(single 

dose)

Laboratory Rat

LD50 = 1.8 mg a.i./kgbw

(1.5—2.1)

Very highly toxic

46750935

46750936

(Acceptable)

Chronic No data

Terrestrial 

invertebrates

Acute 

contact No data

Freshwater 

fish2 Acute

Rainbow trout 

(Oncorhynchus 

mykiss) 

96-hr LC50 0.064 mg

a.i./L

Very highly toxic 46750919 (In 

review)

Chronic No data

Freshwater 

invertebrates

Acute

Water flea 

(Daphnia 

magna)

48-hr LC50 0.61 mg/L Highly toxic 46750917 (In 

review)

Chronic No data

Estuarine/

marine fish

Acute and 

Chronic No data

Estuarine/

marine 

invertebrates

Acute and 

Chronic No data

Terrestrial 

plants

Seedling 

emergence

No data

Vegetative 

vigor No data

Aquatic 

plants and 

algae

Vascular No data

NonvascularGreen algae (P.

subcapitata) 72-hr EC50 0.32 mg/L Not applicable 46750929(In review) 1 Birds represent surrogates for terrestrial-phase amphibians and reptiles. 2 Freshwater fish are used as surrogates for aquatic-phase amphibians.

6.2 Secondary Toxicity

Secondary toxicity is considered exposure to non-target predatory animals that occurs as a result 

of their consuming other organisms (including target pests) that have eaten difenacoum treated 

baits. Secondary toxicity can also occur where nestlings and other young animals are fed 

exposed prey animals. As part of the litigation risk assessment for difenacoum for selected 

species in the San Francisco Bay area, a summary of available literature on secondary toxicity 

for both acute and chronic exposures were summarized (EPA, 2012). In the available studies, 

birds and mammals were exposed to difenacoum in whole or ground carcasses, which were 

usually those of rats or mice, or in fortified meat. Most prey animals were fed treated bait, 
Page 13 of 43

although some were dosed orally. Mortality was included as a measurement endpoint in all 

studies. 

Some studies also reported signs of toxicosis (e.g., bleeding, prolonged blood-coagulation time, 

abnormal behavior, or regurgitation) in surviving test animals. Although exposure scenarios, test 

species, and the number of test animals varied among the studies, collectively these studies 

provide useful information for characterization of secondary hazards from short-term exposure. 

Though these types of secondary exposure studies provide information on the potential effects 

from difenacoum exposure that may occur under field conditions, it is often not possible to 

quantify the level of exposure (rodenticide residues in primary consumers or food consumption 

by secondary consumers) or the sensitivity of the exposed animals to the rodenticide.

EPA will update the secondary toxicity analysis for difenacoum at the time of the registration 

review preliminary risk assessment.

6.2.1 Available Studies for Secondary Toxicity to Birds

Mendenhall and Pank (1980) compared secondary hazards of 6 anticoagulant rodenticides, 

including difenacoum, to barn owls (Tyto alba). For difenacoum, six owls were exposed for 

either 1, 3, 6, or 10 days to rats fed with bait (50 ppm a.i.). Exposed rats were offered free 

choice of bait (5 to 13 g daily) or laboratory chow for 10 days; body residues were not 

determined. None of the difenacoum-exposed owls died, but the 3 owls fed for 6 or 10 days 

hemorrhaged, 1 severely (Table 4). 

Table 4. Adverse Effects of Difenacoum to Barn Owls Exposed To Rats Fed Bait 

(adapted from Mendenhall and Pank 1980)

Days

exposed

No. tested/

no. dead

Sublethal effects 

Observed

Difenacoum (50 ppm) 1

3

6

10

1/0

2/0

1/0

2/0

the 3 owls offered rats for 6 or 10 days 

survived but all hemorrhaged 

(1 severely)

Wyllie (1995) and Newton et al. (1990) reported on toxic effects to barn owls fed mice exposed 

to four rodenticides, including difenacoum. The mice were fed bait (no choice) for a single day 

and allowed to die, which took 2 to 11 days. Residues of difenacoum in dead mice were present 

at greater concentrations in the liver than the rest of the carcass and were estimated to be 10-17 

μg difenacoum for a whole 35g mouse. Dead mice were then offered to the owls in 3 phases, 

each phase separated by a recovery period lasting at least 75 days. In phase I, each owl (6 per 

rodenticide) was offered 3 mice for 1 day only. Surviving owls were offered 6 mice each during 

a 3-day period in phase II and 12 mice each during a 6-day period in phase III. Mortality, 

evidence of external bleeding, and blood-coagulation times were monitored. The 6 owls fed 

difenacoum-dosed mice survived with no evidence of external bleeding, and blood-coagulation 

times returned to normal within 5 to 23 days. During phases II and III, both survivors bled from 
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the mouth, feet, and newly-grown feathers for up to 30 days, and blood-coagulation times did not 

reach normal until 16 to 78 days after treatment. 

The registrant also supplied a report by Gray et al. (1992; MRID 46750932), similar to the 

published paper by Gray et al. (1994) cited in EFED’s comparative risk assessment (USEPA, 

2004b). In this study, the authors examined effects in barn owls fed mice that were exposed to 

three rodenticides, including difenacoum. Batches of mice (6 per rodenticide) were fed 50 ppm 

wax block bait either in weighed amounts of 1.0 g – 5.0 g for 23 hours or ad libitum for 24 or 48 

hours. Representative mice from each batch were selected and analyzed for residues, and mice 

contained 0.95 – 5.20 mg/kg of residue depending on the rodenticide (specific data were not 

provided). Four owls per rodenticide were fed treated mice for 15 days, and the amount of 

rodenticide fed to the owls was varied by feeding mice that fed on more or less bait or by feeding 

them more or fewer mice. Details of the feeding scheme for each owl are not provided. After 

the 15-day treatment period the owls were fed untreated mice and observed for another 15 days 

or until death. One owl fed difenacoum died. Owls fed difenacoum ate 36 – 128 μg difenacoum 

per day, and the owl that died ate 101 μg/day. This owl ate the second highest dose of the four, 

but had the highest liver residue (Table 5). No other effects were described in the report. 

Susceptibility and possibly absorption and elimination varied between the owls, and the sample 

size was very small. The authors concluded that although the toxicity of difenacoum and the 

other rodenticides was high, the risk resulting from a baiting program was low due to the low 

incidence of treated mice likely to be present in a pulse-baiting program using a retrievable wax 

block bait available intermittently. These conclusions do not necessarily translate directly to the 

proposed use of difenacoum, however, since the pelleted end-use product is retrievable and can 

be used in permanent bait stations.

Table 5: Secondary Toxicity in Barn Owls (from Gray, 1992, 1994)

Residue Concentration (mg/kg)

Survival Status 

Anticoagulant 

consumed

(μg/day)

Cumulative 

consumption 

(mg/kg owl bw)

Liver Fat Muscle

Difenacoum (50 ppm):

Survived (n=3) 36 – 128 1.6 – 5.5 0.06 – 0.14 <0.01 0.01

Died (n=1) 101 3.7 0.25 0.01 0.01

6.2.2 Available studies for Secondary Toxicity to Mammals

Shore et al. (2003) present data on residues in polecats (Mustela putorius) accidentally killed on 

roads in England. Livers were analyzed for residues of second-generation anticoagulant 

rodenticides, including difenacoum, bromadiolone, flocoumafen, and brodifacoum. This work

was an extension of that presented in Shore et al. (1996) and Shore et al. (1999), which were 

discussed in EFED’s comparative risk assessment (USEPA, 2004b). Of 100 carcasses collected 

for all three studies between 1992 and 1999, 31 contained residues of at least one secondgeneration anticoagulant. Difenacoum was the most prevalent compound detected in 26 animals. 

Four polecats contained residues of multiple anticoagulants: two contained residues of 
Page 15 of 43

difenacoum and bromadiolone, and the other two contained residues of difenacoum, 

bromadiolone, and brodifacoum. Among these studies, the authors concluded that the residues 

resulted from hunting activities of the polecats on nearby farms in winter and that they were 

probably killed when they dispersed into the landscape at a later time. Since the terminal phase 

liver elimination half-life of most second-generation anticoagulants in mammals can exceed 100 

days, difenacoum and the other compounds can be detected for months after ingestion. 

Although the sampling scheme was unlikely to permit the collection of fatally poisoned animals, 

the maximum liver residue of difenacoum (0.917 ug/g) and bromadiolone (0.23 ug/g) 

approached that of a polecat that was poisoned by secondary exposure (Birks 1998, as cited in 

Shore et al. 2003) and a stoat that died after feeding on contaminated voles (Grolleau et al. 1989, 

as cited in Shore et al. 2003), respectively.

McDonald et al. (1998) tested for anticoagulant residues in stoats (Mustela erminea) and weasels 

(Mustela nivalis) shot or trapped on game farms in England. Nine out of 40 stoats and three out 

of 10 weasels contained residues of at least one rodenticide. Difenacoum was not found in any 

of the animals tested; however, in the area of England in which the study was conducted, 

coumatetralyl is used more widely, and was the most frequently detected anticoagulant. 

According to the authors, difenacoum and bromadiolone were the most widely used rodenticides 

in the rest of England, and these are more toxic than coumatetralyl. Thus, the authors concluded 

that the hazards of secondary poisoning may be more pronounced where difenacoum and 

bromadiolone anticoagulants are used. This conclusion is supported by the results of Shore et al. 

(2003). 

These studies indicate the widespread occurrence of secondary exposure resulting from rodent 

control activities. All three species are exclusively carnivorous and would have been very 

unlikely to consume bait directly. Polecats are known to prey on animals that are targets of 

rodent control programs. However, weasels and stoats feed on these animals much less often, so 

predators do not necessarily have to forage around buildings in order to be secondarily exposed 

(McDonald et al. 1998). 

6.3 Incident Database Review

A summary of the 5 difenacoum wildlife incidents reported in the Environmental Incident 

Information System (EIIS, version 2.1.1) database, with a certainty classification of “possible”, 

“probable”, or “highly probable” is included in Table 6. These classifications mean that, at a 

minimum, there was supporting evidence (e.g., hemorrhaging) and/or residue data to indicate 

that the wildlife involved in the incident were adversely affected by exposure to an anticoagulant 

rodenticide and/or an independent wildlife specialist, based on findings of a necropsy 

examination and toxicological analysis, identified an anticoagulant rodenticide as the cause of 

death in the incident report. The reported avian species include carnivore/scavenger birds that 

include the bald eagle, red-tailed hawk, and kite.

Although most of the reported wildlife incidents involve animals found dead, there are several 

rodenticide incidents that involve incapacitated animals (most of which died after being found). 

They demonstrate that animals exposed to rodenticides may be incapacitated in ways that would 

almost certainly make them more vulnerable to predators and accidental deaths (e.g., car or 

window strikes). If an animal dies due to predation or an accident, their death may not 
Page 16 of 43

necessarily be attributed to a pesticide, even if it is a contributing factor. Difenacoum has also 

not been registered for as long as other second generation anticoagulant rodenticides (e.g. 

brodifacoum), which have comparatively greater reporting of incidents associated with their use. 

The lack or low occurrence of reported incidents does not necessarily indicate the actual 

frequency with which incidents related to the use of difenacoum may occur. 

Table 6. Wildlife Mortality Incidents for Difenacoum Reported in the EIIS Database

Incident 

ID Year Legality Certainty State Land Use

Generic 

Species 

(Number 

Affected)

Comments

I016488-

005 1999 Undetermined Possible UK* N/R Kite - bird

(1)

Trace concentrations of 

difenacoum and brodifacoum 

detected in liver samples

I016488-

009 1999 Undetermined Probable UK* N/R Kite - bird

(1)

Brain hemorrhaging was linked 

to concentrations of difenacoum 

(0.2 ppm) and brodifacoum 

(0.04 ppm) detected in liver 

samples

I016488-

010

1997-

1999

Undetermined Possible UK* N/R Kite - bird

(9)

Five of nine samples tested were 

positive for either bromadiolone 

or difenacoum or both. Liver 

residues ranged from 0.05 to 

0.40 mg/kg; however, illegal 

poisoning with either 

alphachloralose or carbofuran 

was implicated in four of the 

five deaths

I023352-

001 2011 Undetermined Possible NE Rangeland/

pasture

Bald Eagle 

(1)

Less than 0.002 ppm difenacoum 

detected in liver samples along 

with less than quantifiable 

residues of six other 

rodenticides. Chlorophacinone 

was detected at 0.4 ppm.

I024829-

001 2009 Undetermined

Highly 

Probable NY N/R Red-tailed 

hawk (1)

Concentrations of difenacoum 

(0.02 ppm), bromadiolone (0.13 

ppm), and brodifacoum (0.05 

ppm) were detected in liver 

samples

*UK: United Kingdom. I016488-05,-09, -10 are incidents associated with difenacoum’s use that were reported in 

the open literature (Carter and Burn, 2000).

 N/R: Not reported

There were no aggregate incidents reported for difenacoum in the US EPA’s Office of Pesticide 

Programs Incident Data System as of December 21, 2015. EPA will update this review of 

reported difenacoum incidents at the time of the registration review preliminary risk assessment.
Page 17 of 43

7 Exposure Pathways of Concern

The environmental fate properties and use patterns of difenacoum indicate that non-target 

consumption of bait and consumption of individuals that have consumed bait represent potential 

transport mechanisms of difenacoum to terrestrial organisms. 

Drinking water and inhalation exposure pathways for birds and mammals were screened using 

the SIP (Screening Imbibition Program) and STIR (Screening Tool for Inhalation Risk) 

screening methods SIP and STIR are described in detail at: http://www2.epa.gov/pesticidescience-and-assessing-pesticide-risks/models-pesticide-risk-assessment#terrestrial. The drinking 

water pathway was found not to be an exposure pathway of potential concern for acute risks to 

birds due to low solubility of difenacoum relative to toxicity. However, the drinking water 

pathway is an exposure pathway of potential concern for acute risks to mammals. Chronic risks 

to mammals and birds cannot be excluded because there is insufficient data on chronic toxicity. 

The inhalation exposure pathway could not be explored using STIR because no mammalian 

inhalation toxicity data is available. However, inhalation is not expected to be an exposure 

pathway of concern for birds or mammals due to the low volatility of difenacoum. SIP results are 

presented in Appendix B.

8 Analysis Plan

8.1 Stressors of Concern

8.1.1 Ecological Risk Assessment

For this registration review case, the stressor of ecological concern for terrestrial organisms is the 

parent compound, difenacoum.

8.1.2 Drinking Water Assessment

A difenacoum drinking water assessment for use in the human health risk assessment is not

expected to be conducted during registration review because difenacoum baits are contained in 

bait stations, and therefore not available for runoff.

8.2 Measures of Exposure 

A previous risk assessment (USEPA, 2007) modeled aquatic exposure of difenacoum and found 

that negligible concentrations of difenacoum could occur in the water column relative to the 

toxicity to aquatic organisms. Model results indicated a peak concentration of 0.143 μg/L in 

surface water. This result is driven by difenacoum’s immobility within its bait formulations. The 

most sensitive aquatic endpoint is an LC50 of 64 μg a.i./L for the rainbow trout (Oncorhynchus 

mykiss). This endpoint is more than two orders of magnitude greater than the estimated 

environmental concentration (EEC). Additionally, outdoor bait is placed in bait stations and 

therefore not available for runoff, and spray drift is not expected for this chemical. For these 
Page 18 of 43

reasons, EFED does not expect to conduct a quantitative aquatic ecological risk assessment 

during registration review.

For the difenacoum registration review risk assessment, it will be assumed that terrestrial animals 

could be exposed in two different pathways. Animals may directly consume bait (primary 

consumption), or animals may consume contaminated prey either killed or scavenged by the 

consumer (secondary consumption). Both approaches and the expected exposure levels are 

detailed below. It is important to note that these methods differ from what is typically done in 

EFED risk assessments, but they are appropriate for difenacoum based on the use parameters and 

environmental exposures described previously and consistent with other EFED risk assessments 

for rodenticides. 

8.2.1 Estimating Primary Terrestrial Exposure

EFED’s exposure assessment for the rodenticides differs from that for most other pesticides. For 

a rodenticide, the bait itself is the potential food item of concern. Thus, the amount of active 

ingredient in the formulated bait is used as an estimated environmental concentration (EEC). 

This information is used to estimate the amount of bait that birds and mammals of various sizes 

need to consume to obtain a dose expected to be lethal to 50% of the individuals in the 

population (i.e., LD50). Estimates of food-ingestion rates (g dry matter per day) are determined 

from established allometric equations presented in the Wildlife Exposure Factors Handbook

(USEPA 1993). The concentration of difenacoum in grain bait is also used to estimate initial 

dietary exposure (mg a.i. per kg in bait) which in turn is used to calculate avian and mammalian 

dietary RQs. 

8.2.1.1 Expected Difenacoum Accumulation through a Single Feeding on 

Bait 

Exposure through bait consumption will be calculated using two methodologies. For the first 

method, difenacoum exposure is calculated as mg a.i./kg-bw, where kg-bw is the kilograms of 

the consuming individual for three standard weight classes of passeriforms and rodents. 

Exposure (food dry weight consumption) estimates were derived using allometric equations from 

USEPA (1993). The allometric equations for passeriform birds and rodent mammals were used 

as these would best approximate those individuals with high potential for consuming grain, and 

they would give the most conservative exposure estimates. Food dry weight was converted to 

wet weight assuming the bait contained 10% water, similar to the assumption that seeds for 

wildlife consumption contain 10% water (USEPA 1993). Formulas for calculation of dose 

estimates are provided in Table 7, and difenacoum exposure estimates (on a dose basis) are 

provided in Table 8. Risk quotients (RQs) are generated by dividing these exposure estimates of 

difenacoum (mg a.i./kg-bw) for a given weight class by the most conservative toxicity endpoint 

for the relevant taxa scaling adjusted for the default body weights. RQs using these exposure 

estimates will be generated for acute bird and mammal (using LD50 data) non-targets.
Page 19 of 43

Table 7. Formulas for Calculation of Daily Difenacoum Intake based on Consumption of Bait.

Passeriform bird food intake (g, dry weight): FI (g dry-wt/day) = 0.398 * Wt(g)0.850

Rodent mammal food intake (g, dry weight): FI (g dry-wt/day) = 0.621 * Wt(g)0.564

Difenacoum intake (mg a.i./kg-bw/day) = 

FI (g dry-wt/day) /bw (in kg)* mg a.i./kg-bait dry weight * 1kg/1000g

Where: Wt(g) = weight (in grams) of the bird or mammal consumer

mg a.i./kg-bait = 50 based on difenacoum concentration in registered bait

Table 8. Expected Daily Difenacoum Intake for Default Bird and Mammal 

Weights based on Consumption of Bait.

Weight 

(g)

Food intake 

(g drywt/day)

Difenacoum intake (mg a.i./kg-bw/day)

Bait concentration = 

50 mg a.i./kg-bait

Passeriform 

Birds*

20 5.1 12.8

100 19.9 10

1000 141.2 7.1

Rodent 

Mammals

15 2.9 9.7

35 4.6 6.6

1000 30.6 1.5

*surrogate for reptiles and terrestrial-phase amphibians

8.2.1.2 Expected Difenacoum Accumulation through Multiple Feedings on 

Bait 

Available data on the retention times for difenacoum are provided in Table 9. Vandenbroucke et 

al. (2008) orally dosed groups of mice with 12.88 μg a.i./mouse difenacoum, which is 

approximately one-half of the LD50 dose. Difenacoum was then assayed in liver tissue and blood 

plasma from euthanized mice at various times up to 21 days after dose administrations. The 

study authors estimated the elimination half-life of difenacoum in liver tissue to be 61.8 days and 

the half-life in blood plasma to be 20.4 days. 

The discrepancies between the half-lives estimated in Vandenbroucke et al. (2008) and Bratt 

(1987) may be a result of different difenacoum elimination rates among species. 
Page 20 of 43

Table 9. Retention of Difenacoum in Blood and Liver

Rodenticide Species Dose (mg 

a.i./kg) # doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference 

(MRID)

Difenacoum Mouse 12.88 

μg/mouse 1 20.4 61.8 (t1/2) Vandenbroucke et al.,2008

Difenacoum Rat 1.2 mg/kg 1 -- 118 (t1/2) Bratt, 1987 (46750957)

a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life

As indicated previously, difenacoum is known to persist in biological tissue. A compound that 

accumulates in body tissues may pose greater risk to primary and secondary consumers, 

especially if repeated sublethal exposure results in the accumulation of a lethal dose or if 

additional exposures occur before the lethal dose has taken its effect (i.e., leading to a dose that is 

beyond what is sufficient to cause mortality). 

Cumulative rodenticide doses for animals consuming bait for six consecutive days2 were also 

calculated using the formulas in Table 10. This analysis assumed that 100% of the daily diet of 

the animals is represented by the rodenticide bait and that they eliminate the difenacoum with a 

half-life of 118 days based on the highest definitive liver half-life value (Table 9). In the 

formula, D is the dose and Dt is the total dose. The variable “C” represents the concentration of 

the pesticide in the bait (mg/kg). The elimination rate constant (k) was calculated using the 

estimated difenacoum half-life (t1/2 = 118 days), and food intake (FI; mg/kg-bw/day) was 

calculated according to the allometric equation for each taxonomic group (Table 7). The 

resulting cumulative body burden concentrations of difenacoum are presented in Table 11. 

Different half-life assumptions based on available data may be used to further characterize risk in 

the registration review risk assessment.

 2 Average time to mortality between bait consumption and mortality for first and second generation anticoagulant 

rodenticides (risk assessment methodology from USEPA, 2011). 
Page 21 of 43

Table 10. Formulas for Calculation of Difenacoum Body Burden based on Consumption 

of Bait for Six Consecutive Days

DDtt = �DDnn

6

nn=1

∗ ee−kk(6−nn)

where

kk = ln(2)⁄tt1/2

Table 11. Calculated difenacoum body burden in avian and mammalian 

consumers of bait after six days of exposure

Body weight 

(g)

Bait concentration = 50 mg a.i./kg-bait

Passerine Generic Bird

20 75.1 67.3

100 59.0 38.4

1000 41.7 17.2

Rodent Generic Mammal

15 56.4 42.9

35 39.0 36.9

1000 9.0 20.3

8.2.1.3 Expected Difenacoum Exposure based on Bait Concentration

The second exposure method for primary consumption of bait results in a diet concentration of 

50 mg a.i./kg-bait (concentrations of difenacoum as packaged in bait). RQs will be calculated 

using this estimate of dietary concentration and the LC50 (mg a.i./kg-diet) available from dietary 

toxicity studies. RQs using these exposure estimates will be generated for acute bird and 

mammal (using LC50 data) non-targets.

8.2.2 Estimating Secondary Terrestrial Exposure

Secondary exposure analysis (from consumption of contaminated individuals) requires 

consideration of residues in tissues of target and non-target organisms that are commonly 

consumed by predators and scavengers. Moreover, it is important to know how long the residue 

persists in body tissues. Residue concentrations for non-target animals fed difenacoum are 

available in open literature (Gray et al., 1992, 1994). Additionally, a number of laboratory tests 

using avian and mammalian predators and scavengers are available to assess mortality from 

secondary exposure resulting from consumption of prey animals that have been exposed to

difenacoum. Design and methodology vary among studies, adding unknown variability to the 

results and analysis. Pending development of additional methods and testing requirements for 

such studies, these tests provide the best data available. 
Page 22 of 43

The determination of difenacoum intake by individuals consuming difenacoum poisoned animals 

or carcasses is calculated in a manner similar to the approach for individuals consuming bait 

(Section 8.2.1.2). Empirical residue data are used instead of bait concentration of difenacoum 

(Table 11). Difenacoum body burdens in carcasses collected from field and laboratory studies 

were determined in mammals, after exposure to difenacoum bait. Data are available for a variety 

of small- and medium-sized mammalian granivores and omnivores. No data were identified for 

other exposed taxonomic groups (e.g., birds and reptiles). For all studies, it was assumed the 

concentrations were reported using wet weights of the mammals. 

In laboratory studies, mean difenacoum residues in mammalian carcass ranged from 0.52 to 0.74

mg a.i./kg-bw and are summarized in Table 12. 

Table 12. Difenacoum Residue Levels in Mammalian Primary Consumers.

mg ai/kg 

bait

Target 

species Site Sample size Days 

exposed

Mean wholecarcass residue 

(mg a.i./kg-bw)

Reference (Ecotox 

Number)

25 Rat Laboratory 10 per strain 5 Susceptible 

strain (TAS)

0.52 ± 0.038 

(SE)

Atterby et al. 2005

(E079379)

Resistant strain 

HH

0.63 ± 0.045

(SE)

Resistant strain 

HB

0.68 ± 0.062

(SE)

25 Rat Laboratory 10 per strain 10 Resistant strain 

HH

0.55 ± 0.055

(SE)

Atterby et al. 2005

(E079379)

Resistant strain 

HB

0.64 ± 0.032

(SE)

25 Rat Laboratory 10 20 Resistant strain 

HB

0.74 ± 0.04 (SE)

Atterby et al. 2005

(E079379)

50 ppm Mouse Gray et al. 1994

Difenacoum exposure through carcass consumption is calculated as mg a.i./kg-bw, where kg-bw

is the kilograms of the consuming individual for three weight classes of birds and mammals. For 

this analysis, bird weight classes of 50, 1000, and 2000 g individuals and mammal weight classes 

of 50, 1000, and 3000 g individuals are used to better represent the larger size of carnivores and 

scavengers relative to the full range of bird and mammal weights. The measured maximum body 

burden in the prey is then used to estimate rodenticide exposure in the secondary consumer using 

the general food intake allometric equations described in Section 8.2.1 above. Secondary 

consumers are assumed to be mammals weighing 50g, 1000g, or 3000g, or birds weighing 100g, 
Page 23 of 43

1000g, or 5000g. For example, American kestrels range from 100-150g, red-tailed hawks range 

from 900-1000g and bald eagles range from 4000-5000g (Dunning 1984). Exposure (food dry 

weight consumption) estimates are derived using the generic bird and mammal allometric 

equations from (EPA 1993). Food dry weight was converted to wet weight assuming the 

consumed prey/carcass contained 68% water (USEPA 1993). 

8.2.3 Estimating Additional Environmental Exposures

Screening level calculations suggest that the drinking water pathway and inhalation pathways 

will not pose significant risks to birds and mammals (Appendix B). If additional information 

indicates there may be concerns, these pathways will be re-evaluated at the time of the 

registration review risk assessment. 

No monitoring data in surface water, in groundwater or in air were found from the USGS 

NAWQA program (http://water.usgs.gov/nawqa), the California Department of Pesticide 

Regulation CDPR (http://www.cdpr.ca.gov/docs/emon/surfwtr/surfcont.htm) or from the USEPA 

STORET program. Water monitoring programs such as these generally monitor for agricultural 

pesticides and typically do not include analysis for rodenticides such as difenacoum. If water 

monitoring data are identified at the time the risk assessment is conducted, those data will be 

used for additional exposure characterization.

8.3 Measures of Effect 

Toxicity data presented in Section 6 of this problem formulation will be used to calculate risk 

quotients. Any additional information submitted by the registrant or found in the open literature 

prior to conduct of the risk assessment will also be considered. The open literature studies are 

identified using EPA’s ECOTOXicology database (ECOTOX) (USEPA, 2007), which employs a 

literature search engine for locating chemical toxicity data for aquatic life, terrestrial plants, and 

wildlife. The evaluation of both sources of data can also provide insight into the direct and 

indirect effects of pesticides on biotic communities from loss of species that are sensitive to the 

chemicals and from changes in structure and functional characteristics of the affected 

communities.

9 Endangered Species Assessment

Consistent with EPA’s responsibility under the Endangered Species Act (ESA), the Agency will 

evaluate risks to federally listed threatened and endangered (listed) species from registered uses 

of pesticides in accordance with the Joint Interim Approaches developed to implement the 

recommendations of the April 2013 National Academy of Sciences (NAS) report, Assessing 

Risks to Endangered and Threatened Species from Pesticides. The NAS report outlines 

recommendations on specific scientific and technical issues related to the development of 

pesticide risk assessments that EPA and the Services must conduct in connection with their 

obligations under the ESA and FIFRA. EPA will address concerns specific to difenacoum in 

connection with the development of its final registration review decision for difenacoum. 
Page 24 of 43

In November 2013, EPA, the U.S. Fish and Wildlife Service, National Marine Fisheries (the 

Services), and USDA released a white paper containing a summary of their joint Interim 

Approaches for assessing risks to listed species from pesticides. These Interim Approaches were 

developed jointly by the agencies in response to the NAS recommendations, and reflect a 

common approach to risk assessment shared by the agencies as a way of addressing scientific 

differences between the EPA and the Services. Details of the joint Interim Approaches are 

contained in the November 1, 2013 white paper, Interim Approaches for National-Level 

Pesticide Endangered Species Act Assessments Based on the Recommendations of the National 

Academy of Sciences April 2013 Report.

Given that the agencies are continuing to develop and work toward implementation of the 

Interim Approaches to assess the potential risks of pesticides to listed species and their 

designated critical habitat, this ecological problem formulation supporting the Preliminary Work 

Plan for difenacoum does not describe the specific ESA analysis, including effects 

determinations for specific listed species or designated critical habitat, to be conducted during 

registration review. While the agencies continue to develop a common method for ESA analysis, 

the planned risk assessment for the registration review of difenacoum will describe the level of 

ESA analysis completed for this particular registration review case. This assessment will allow 

EPA to focus its future evaluations on the types of species where the potential for effects exists, 

once the scientific methods being developed by the agencies have been fully vetted. Once the 

agencies have fully developed and implemented the scientific methods necessary to complete 

risk assessments for listed species and their designated critical habitats, these methods will be 

applied to subsequent analyses of difenacoum as part of completing this registration review.

10 Endocrine Disruptor Screening Program

Difenacoum was not included in the first group or the second group of chemicals issued an order 

to conduct Tier 1 EDSP testing. For additional information the EDSP program, visit

http://www2.epa.gov/endocrine-disruption/endocrine-disruptor-screening-program-edspoverview. 

11 Preliminary Identification of Data Gaps 

11.1 Environmental Fate

Since difenacoum’s use patterns and product formulations in bait boxes are not expected to 

result in exposure to soil, water, or air, many OCSPP 835 environmental fate guideline studies 

have not been submitted or are fulfilled in summary form. If a new use with increased 

environmental exposures is proposed in the future, additional environmental fate guideline 

studies may be necessary for a better understanding of difenacoum’s persistence and mobility.
Page 25 of 43

Table 13. Available Environmental Fate Data for Difenacoum.

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies (MRID)

Study 

Classification

Are data needed 

for risk 

assessment?

Current Additional Data 

Need

835.2120 Hydrolysis 46750910 Unreviewed No None

835.2240 Aqueous 

photolysis

46750928 Unreviewed 

summary

No A guideline study has not 

been submitted, but a

summary with physical 

chemical modeling 

estimates has been 

submitted. This summary 

fills data needs for the 

current use pattern given 

aquatic exposure is not 

expected.

835.2410 Soil photolysis (None) (N/A) No Data have not been 

submitted to fulfill this 

guideline. A data gap was 

not identified as these data 

are not necessary for risk 

assessment at this time.

835.4100 Aerobic soil 

metabolism

46750928 Unreviewed 

summary

No A guideline study has not 

been submitted, but a data 

summary has been 

submitted. This summary 

fills data needs for the 

current use pattern given 

aquatic exposure is not 

expected.

835.4200 Anaerobic soil 

metabolism

(None) (N/A) No Data have not been 

submitted to fulfill this 

guideline. A data gap was 

not identified as these data 

are not necessary for risk 

assessment at this time.

835.4300 Aerobic aquatic 

metabolism

(None) (N/A) No Data have not been 

submitted to fulfill this 

guideline. A data gap was 

not identified as these data 

are not necessary for risk 

assessment at this time.

835.4400 Anaerobic 

aquatic 

metabolism

46750916 Unreviewed No Unreviewed study 

conducted under ISO 11734 

835.1230

835.1240

Adsorption/ 

desorption and 

leaching

46750907

46750908

Unreviewed No Unreviewed studies 

conducted under OECD 

guideline 121 and BBA Part 

IV/4-2

835.6100 Terrestrial field 

dissipation

(None) (N/A) No Data have not been 

submitted to fulfill this 
Page 26 of 43

Table 13. Available Environmental Fate Data for Difenacoum.

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies (MRID)

Study 

Classification

Are data needed 

for risk 

assessment?

Current Additional Data 

Need

Storage Stability 

Study (TFD)

(None) (N/A) No guideline. A data gap was 

not identified as these data 

are not necessary for risk 

assessment at this time.

850.1730 Fish 

bioconcentration

(None) (N/A) No Data have not been 

submitted to fulfill this 

guideline. A data gap was 

not identified as these data 

are not necessary for risk 

assessment at this time.

Environmental

Chemistry

Method: Water

850.6100

Analytical 

method in water 

and independent 

laboratory 

validation 

(None) (N/A) No

Environmental

Chemistry

Method: Soil

Analytical 

method in soil 

and independent 

laboratory 

validation

(None) (N/A) No

11.2 Ecological Effects

Tables 15 and 16 identify submitted ecological effects studies by MRID that offer data for each 

guideline requirement, as well as study classifications and whether or not further data are needed 

in order to support risk assessment. 

Currently, inadequate data are available to further characterize the differential toxicity among 

bird species. In addition, there is evidence of different or multiple modes-of-action of 

rodenticides in different taxa. Additional toxicity data on a wider range of species or data 

exploring mode-of-action would be beneficial to better understand the natural variation and to 

improve confidence in the assessment of risk to non-target species. 

No data are available to assess the effects of long-term, low-dose exposure of difenacoum on 

avian reproduction (OCSPP 850.2300) or survival. Avian reproduction data available for another 

registered anticoagulant rodenticide, chlorophacinone, highlights the potential for effects from 

long-term, low-dose exposure. For chlorophacinone, mallard ducks exposed to dietary 

concentrations for five days had an LC50 = 172 mg/kg-diet (MRID 41513103); however, nine of 

16 (56%) adult females died exhibiting symptoms of anticoagulant poisoning during the avian 

reproduction study eating a diet containing only 0.96 mg a.i./kg-diet (MRID 489904002). EFED 

recommends that additional data on effects of long-term low-dose exposure of difenacoum to 

birds through extended exposure dietary studies and avian reproduction studies be requested. 

No data are available to assess of the toxicity of difenacoum to terrestrial invertebrates. EFED 

does not have concerns for difenacoum exposure to pollinators such as bees and butterflies, 
Page 27 of 43

because expected exposures would be low. Difenacoum is not applied as a spray; therefore, 

expected exposure on plants, including flowers, would be low. Pollinators would not be expected 

to feed on live animals or carcasses that may have been contaminated with difenacoum. 

Difenacoum exposure to terrestrial invertebrates may occur if individuals feed on the bait, feed 

on live animals contaminated with difenacoum residues, or feed on carcasses with difenacoum

residues. No terrestrial invertebrate incidents have been reported for difenacoum.

The earthworm LC50 values for bromadiolone, a second generation rodenticide, at 24 h and 48 h 

were 145.70 μg/cm2 and 25.03 μg/cm2, respectively, and at 14 d and 28 d were 449.958 mg/kgsoil and 105.593 mg/kg-soil, respectively (Liu et al. 2015). Incidences of terrestrial invertebrates 

consuming bait containing an anti-coagulant rodenticide and subsequently excreting it have been 

documented (Fisher et al. 2007; Hernandez-Moreno et al. 2013). Toxicity of difenacoum to 

terrestrial invertebrates is unknown; however, there is evidence to suggest that toxicity may 

occur and that the exposure pathway for secondary exposure is complete. EFED is not requesting 

the full suite of pollinator protection studies as outlined in recent guidance, at this time 

(http://www2.epa.gov/pollinator-protection/pollinator-risk-assessment-guidance). However, 

EFED does request an earthworm toxicity study for difenacoum and any existing data that 

demonstrate toxicity, or lack of toxicity, to difenacoum for terrestrial invertebrates. 

Given that the available data indicates that the cis isomer is more toxic than the trans isomer, 

additional toxicity studies that report the ratio of these isomers in the tested material would be 

helpful. 

Although they are technically considered data gaps, EFED does not recommend requesting 

terrestrial plant, aquatic plant, chronic fish, or chronic aquatic invertebrate toxicity data for 

difenacoum at this time since these data would not be expected to change the conclusions of the 

registration review risk assessment. There remains uncertainty in the level of risk for each of 

these taxonomic groups given the missing data; however, the expected exposures for the 

currently registered use patterns will be relatively low:

• Terrestrial plants - No data are available to assess of the toxicity of difenacoum to 

terrestrial plants; however, exposure to plants is also expected to be low since there is no 

registered spray application of difenacoum, and the material is applied as a formulated 

bait in bait stations. Difenacoum binds strongly to organic material (i.e., the formulated 

bait) and little is expected to be available for runoff or leaching. No plant incidents have 

been reported for difenacoum.

• Aquatic plants – No data are available to assess the toxicity of difenacoum to aquatic 

vascular plants, and only one study is available for nonvascular aquatic plants which 

demonstrates toxicity potential; however, aquatic exposure is also expected to be low 

since there are no registered spray or aerial baiting applications, and the material is 

applied as a formulated bait in bait stations. Difenacoum binds strongly to organic 

material (i.e., the formulated bait) and little is expected to be available for runoff or 

leaching. No aquatic incidents have been reported for difenacoum.
Page 28 of 43

• Chronic fish and aquatic invertebrates – No data are available to assess the chronic 

toxicity of difenacoum to fish and aquatic invertebrates; however, aquatic exposure is 

expected to be low since there are no registered spray or aerial baiting applications and 

the material is applied as a formulated bait in bait stations. Difenacoum is very highly

acutely toxic to fish and highly toxic to freshwater invertebrates, and chronic effects 

would not be unexpected. However, difenacoum binds strongly to organic material (i.e., 

the formulated bait) and little is expected to be available for runoff or leaching for this 

proposed use. No aquatic incidents have been reported for difenacoum.

Table 14. Available ecological effects data for terrestrial organisms exposed to difenacoum and 

remaining data gaps. 

OCSPP 

Guideline Data Requirement Submitted Studies (MRID)

Study 

Classification

Are data 

needed for 

risk 

assessment?

Current Additional 

Data Need

850.2100 Avian oral toxicity 46750922 acceptable

Yes

Data are requested for 

avian acute oral with 

passerine species. Due 

to the delayed toxicity of 

anticoagulant 

rodenticides, the 

observation period for 

this study should be 

extended beyond the 

standard 850.2100 

guideline 

recommendation of 14 

days (i.e. it should be 

long enough to capture 

effects of difenacoum 

poisoning and continue 

until evidence of toxicity 

has subsided). The study 

should track the ratio of 

difenacoum’s cis and 

trans isomers in the 

tested material.

46750921 acceptable

850.2200 Avian dietary toxicity 46750926 supplemental

46750923 supplemental No --

46766204 supplemental

850.2300 Avian reproduction 46799101 unacceptable Yes Data are requested for 

avian reproduction 

toxicity testing an 

upland game bird 

species and a waterfowl 

species. The study 

should track the ratio of 

difenacoum’s cis and 

trans isomers in the 

tested material
Page 29 of 43

Table 14. Available ecological effects data for terrestrial organisms exposed to difenacoum and 

remaining data gaps. 

OCSPP 

Guideline Data Requirement Submitted Studies (MRID)

Study 

Classification

Are data 

needed for 

risk 

assessment?

Current Additional 

Data Need

NonguidelineAvian inhalation (None) (N/A) No Data have not been 

submitted to fulfill this 

guideline. These data are 

not necessary for risk 

assessment at this time

850.3020 Honey bee acute 

contact toxicity

(None) (N/A) No Exposures of honey bees 

to difenacoum residues 

are not expected since 

they are unlikely to feed 

on bait blocks and 

residues are not 

expected to be present in 

pollen and nectar. 

Therefore, no honey bee 

data is requested at this 

time, but data on other 

invertebrate species may 

be useful for risk 

assessment

850.3030 Honey bee residue on 

foliage

(None) (N/A) No

Data have not been 

submitted to fulfill these 

guidelines. These data 

are not necessary for 

risk assessment at this 

time

OECD TG 

213

Acute oral toxicity to 

adult honey bees

(None) (N/A) No

OECD TG 

237

Acute oral toxicity to 

larval honey bees

(None) (N/A) No

NonguidelineChronic oral toxicity 

to adult honey bees

(None) (N/A) No

NonguidelineChronic oral toxicity 

to larval honey bees

(None) (N/A) No

NonguidelineField trial of residues 

in pollen and nectar

(None) (N/A) No

OECD 

Guidance 75

Semi-field testing for 

pollinator

(None) (N/A) No

850.3040 Field testing for 

pollinator

(None) (N/A) No

850.3100 Earthworm 

Subchronic Toxicity 

Test 

(None) (N/A) Yes Data are requested for 

earthworm toxicity

(Eisenia fetida). The 

study should track the 

ratio of difenacoum’s cis

and trans isomers in the 

tested material.

850.4100 Seedling Emergence 

and Seedling Growth

(None) (N/A) No
Page 30 of 43

Table 14. Available ecological effects data for terrestrial organisms exposed to difenacoum and 

remaining data gaps. 

OCSPP 

Guideline Data Requirement Submitted Studies (MRID)

Study 

Classification

Are data 

needed for 

risk 

assessment?

Current Additional 

Data Need

850.4150 Vegetative Vigor (None) (N/A) No Data have not been 

submitted to fulfill this 

guideline. These data are 

not necessary for risk 

assessment at this time

Table 15. Available ecological effects data for aquatic organisms exposed to difenacoum and 

remaining data gaps. 

OCSPP

Guideline

Data 

Requirement

Submitted 

Studies 

(MRID)

Study 

Classifications

Are data 

needed to 

conduct 

risk 

assessment?

Current Additional Data 

Need

850.1010 Freshwater 

invertebrate 

acute toxicity

46750917 In review Submitted 

data are in 

review

Submitted data are in review

46750918 In review

850.1025

850.1035

850.1045

850.1055 

Saltwater 

invertebrate 

acute toxicity 

(None) (N/A) No Data have not been submitted 

to fulfill these guidelines. 

These data are not necessary 

for risk assessment at this time

850.1075 Freshwater fish 

acute toxicity 

46750919 In review Submitted 

data are in 

review

Submitted data are in review

46750920 In review

850.1075 Saltwater fish 

acute toxicity 

(None) (N/A) No Data have not been submitted 

to fulfill this guideline. These 

data are not necessary for risk 

assessment at this time

850.1300 Freshwater 

invertebrate 

life cycle 

(None) (N/A) No Data have not been submitted 

to fulfill this guideline. These 

data are not necessary for risk 

assessment at this time

850.1350 Saltwater 

invertebrates 

life cycle 

(None) (N/A) No Data have not been submitted 

to fulfill this guideline. These 

data are not necessary for risk 

assessment at this time

850.1400 Freshwater fish 

early-life stage

(None) (N/A) No Data have not been submitted 

to fulfill this guideline. These 

data are not necessary for risk 

assessment at this time

850.1400 Saltwater fish 

early-life stage 

(None) (N/A) No Data have not been submitted 

to fulfill this guideline. These 

data are not necessary for risk 
Page 31 of 43

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relationship between the pharmacokinetics and pharmacodynamics of 4-

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850.1500 Fish life cycle (None) (N/A) No Data have not been submitted 

to fulfill this guideline. These 

data are not necessary for risk 

assessment at this time

850.4400 Aquatic plant 

Toxicity Test 

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(None) (N/A) No Data have not been submitted 

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Page 36 of 43

Appendix A: Difenacoum Fate/Chemistry and Ecotoxictiy

Bibliography

161-1 Hydrolysis

MRID Citation Reference

46750910 Lewis, C. (1992) Difenacoum: Hydrolysis Study (Final Report). Project Number: 

7031, 38/166. Unpublished study prepared by Hazleton Uk. 96 p.

46750928 Harrison, E. (2005) Summary of Environmental Fate and Ecological Effects 

Studies Conducted with Difenacoum, A New Rodenticide Active Ingredient. 

Unpublished study prepared by Woodstream Corp. 15 p.

46766202 Lewis, C. (1992) Difenacoum: Hydrolysis Study: Amendment to Final Report. 

Project Number: 38/166R, 7031. Unpublished study prepared by Hazleton Uk. 6 

p.

161-2 Photodegradation-water

MRID Citation Reference

46750928 Harrison, E. (2005) Summary of Environmental Fate and Ecological Effects 

Studies Conducted with Difenacoum, A New Rodenticide Active Ingredient. 

Unpublished study prepared by Woodstream Corp. 15 p.

162-1 Aerobic soil metabolism

MRID Citation Reference

46750928 Harrison, E. (2005) Summary of Environmental Fate and Ecological Effects 

Studies Conducted with Difenacoum, A New Rodenticide Active Ingredient. 

Unpublished study prepared by Woodstream Corp. 15 p.

162-4 Aerobic aquatic metabolism

MRID Citation Reference

46750928 Harrison, E. (2005) Summary of Environmental Fate and Ecological Effects 

Studies Conducted with Difenacoum, A New Rodenticide Active Ingredient. 

Unpublished study prepared by Woodstream Corp. 15 p.
Page 37 of 43

163-1 Leach/adsorption/desorption

MRID Citation Reference

46750928 Harrison, E. (2005) Summary of Environmental Fate and Ecological Effects 

Studies Conducted with Difenacoum, A New Rodenticide Active Ingredient. 

Unpublished study prepared by Woodstream Corp. 15 p.

835.1110 Activated sludge sorption isotherm

MRID Citation Reference

46750907 Hogg, A. (2003) Difenacoum: Physico-Chemical Testing with Difenacoum: 

Estimation of Adsorption Coefficient. Project Number: 21677, 341765, 4176A. 

Unpublished study prepared by Inveresk Research International. 47 p.

835.1220 Sediment and soil absorption /desorption isotherm

MRID Citation Reference

46750908 Lewis, C. (1992) (Carbon 14)-Difenacoum: Aged Soil Leaching: Final Report. 

Project Number: 38/168, 7066, P6732D. Unpublished study prepared by 

Hazleton Uk. 62 p.

835.2110 Hydrolysis as a function of pH

MRID Citation Reference

46750910 Lewis, C. (1992) Difenacoum: Hydrolysis Study (Final Report). Project Number: 

7031, 38/166. Unpublished study prepared by Hazleton Uk. 96 p.

46766201 Lewis, C. (1992) (Carbon 14)-Difenacoum: Aged Soil Leaching: Amendment to 

Final Report. Project Number: 38/168, 7066. Unpublished study prepared by 

Hazleton Uk. 7 p.

46766202 Lewis, C. (1992) Difenacoum: Hydrolysis Study: Amendment to Final Report. 

Project Number: 38/166R, 7031. Unpublished study prepared by Hazleton Uk. 6 

p.
Page 38 of 43

835.2210 Direct photolysis rate in water by sunlight

MRID Citation Reference

509 Hall, B.; Jackson, R.; Priestley, I. (1992) Difenacoum: Photolysis in Buffered 

Aqueous Solutions. Project Number: 8704, 381614. Unpublished study prepared 

by Inveresk Research International. 123 p.

835.3300 Soil biodegradation

MRID Citation Reference

46750914 Lewis, C. (1992) (Carbon 14)- Difenacoum: A Study of the Degradation in Two 

Soils: Final Report. Project Number: 38/167, 6927. Unpublished study prepared 

by Hazleton Uk. 78 p.

46766203 Lewis, C. (1992) (Carbon 14)-Difenacoum: A Study of the Degradation in Two 

Soils: Amendment to Final Report. Project Number: 38/167R, 6927. Unpublished 

study prepared by Hazleton Uk. 7 p.

835.3400 Anaerobic biodegradability of organic chemicals

MRID Citation Reference

46750916 Penwell, A. (2004) Difenacoum: Determination of Anaerobic Biodegradability. 

Project Number: BL7788/B. Unpublished study prepared by Astrazeneca Uk Ltd. 

24 p.

850.2100 Avian acute oral toxicity test

MRID Citation Reference

46750921 Nolan-Smith, S. (1997) Difenacoum: Acute Oral Toxicity to Mallard Duck: 

Final Report. Project Number: 355/39/1007. Unpublished study prepared by 

Covance Laboratories, Ltd. 60 p.

46750922 Ross, D.; ROberts, N.; Fairley, C. (1980) The Acute Oral Toxicity (LD50) of 

Difenacoum to the Bobwhite Quail. Project Number: ICI/309/WL/8076, 

CTL/C/932. Unpublished study prepared by Huntingdon Life Sciences, Ltd. 20 

p.

71-2 850.2200 Avian Dietary Toxicity
Page 39 of 43

MRID Citation Reference

46750924 Nolan-Smith, S. (2000) Difenacoum: Acute Dietary Toxicity to Mallard Duck: 

Final Report. Project Number: CLE/355/40, CLE/355/40/D1141, DF/959/0077. 

Unpublished study prepared by Covance Laboratories, Ltd. 67 p.

46750925 Ross, D.; Roberts, N.; Fairley, C. (1980) The Subacute Dietary Toxicity 

(LC50) of Difenacoum to the Bobwhite Quail. Project Number: 56/79, 

ICI/305/791197. Unpublished study prepared by Huntingdon Life Sciences, 

Ltd. 16 p.

46750923 Nolan-Smith, S. (1998) Difenacoum: Acute Dietary Toxicity to Bobwhite Quail: 

Final Report. Project Number: 355/41/1007. Unpublished study prepared by 

Covance Laboratories, Ltd. 60 p.

46750926 Ross, D.; Roberts, N.; Fairley, C. (1980) The Subacute Dietary Toxicity 

(LC50) of Difenacoum to the Mallard Duck. Project Number: CTL/C/888, 

ICI/304/791198. Unpublished study prepared by Huntingdon Life Sciences, 

Ltd. 19 p.

46766204 Nolan-Smith, S.; Sharples, R. (2000) Difenacoum: Acute Dietary Toxicity to 

Ring-necked Pheasants: Final Report. Project Number: 355/42, 

355/42/D1142. Unpublished study prepared by Covance Laboratories, Ltd. 63 

p.

850.2300 Avian reproduction test

MRID Citation Reference

46799101 Linder, T. (2006) Avian Reproduction Study with Difenacoum in the Japanese 

Quail (Coturnix coturnix japonica): Final Report. Project Number: 04012. 

Unpublished study prepared by Genesis Laboratories, Inc. 205 p.

71-5 Simulated or Actual Field Testing

MRID Citation Reference

40077207 Hoppe, A.; Krambias, A. (1982) The response of captive Chukar partridges to 

the ingredients and anticoagulant poisons used in ro- dent baits in Cyprus. P. 

639-646 in Proceedings of a conference on the organization and practice of 

Vertebrate Pest Control; Aug 30-Sept 3, 1982, Elvetham Hall, Hampshire, 

England. 

850.1075 Fish acute toxicity test, freshwater and marine

MRID Citation Reference
Page 40 of 43

46750919 Wyness, L. (1995) Difenacoum: Acute Toxicity to Oncorhynchus mykiss: Final 

Report. Project Number: 355/17/1018. Unpublished study prepared by 

Corning Hazleton (Europe). 32 p.

46750920 Wyness, L. (1995) Difenacoum: Acute Toxicity to Lepomis macrochirus: Final 

Report. Project Number: 355/23/1018. Unpublished study prepared by 

Corning Hazleton (Europe). 32 p.

850.1010 Aquatic invertebrate acute toxicity, test, freshwater 

daphnids

MRID Citation Reference

46750917 Wyness, L. (1995) Difenacoum: Acute Toxicity to Daphnia magna: Final 

Report. Project Number: 355/18/1018. Unpublished study prepared by Corning 

Hazleton (Europe). 38 p.

46750918 Kent, S.; Tapp, J.; Sankey, S.; et. al. (1991) Difenacoum: Acute Toxicity to 

Daphnia magna. Project Number: BL4314/B, V480/B. Unpublished study 

prepared by Imperial Chemical Industries, Ltd. 19 p.

850.5400 Algal toxicity, Tiers 1 and II

MRID Citation Reference

46750929 Wyness, L. (1995) Difenacoum: Inhibition of Growth to the Alga Selenastrum 

Capricornutum: Final Report. Project Number: 355/19/1018, 355/19. 

Unpublished study prepared by Corning Hazleton (Europe). 37 p.

46766205 Smyth, D.; Tapp, J.; Sankey, S.; et. al. (1991) Difenacoum: Toxicity to the 

Green Alga: Selenastrum capricornutum. Project Number: V480/A, BL4307/B, 

FT51/91. Unpublished study prepared by Imperial Chemical Industries, Ltd. 21 

p.

NON Guideline Selections

35806 Mendenhall, V.M.; Pank, L.F. (1979) Secondary Poisoning of Owls by 

Anticoagulant Rodenticides. (U.S. Fish and Wildlife Service, Patuxent and 

Denver Wildlife Research Centers, unpublished study) 

80242 Mendenhall, V.M.; Pank, L.F. (1979) Secondary Poisoning of Owls by 

Anticoagulant Rodenticides. (U.S. Fish and Wildlife Service, Patuxent Wildlife 

Research Center; unpublished study; CDL: 245704-M) 

94694

40077202 

on DER

Mendenhall, V.M.; Pank, L.F. (1980) Secondary poisoning of owls by 

anticoagulant rodenticides. Wildlife Society Bulletin 8(4):311- 315. 

(Also~In~unpublished submission received Feb 7, 1982 under 12455-34; 

submitted by Bell Laboratories, Madison, Wis.; CDL: 246741-C) 
Page 41 of 43

46750931 Mendenhall, V.; Pank, L. (1980) Secondary Poisoning of Owls by Anticoagulant 

Rodecticides: (Difenacoum, Bromadiolone, Brodifacoum, Diphacinone, 

Fumarin, Chlorophacinone). The Wildlife Society Bulletin 8(4): 311-315.

46750932 Gray, A.; Eadsforth, C.; Dutton, A.; et. al. (1992) Toxicity of Second Generation 

Rodenticide to Barn Owls: (Brodifacoum, Difenacoum, Flocoumafen). Brighton 

Crop Protection Conference Pests and Diseases: 781-786. 

46750933 Newton, I.; Wyllie, I.; Freestone, P. (1990) Rodenticides in British Barn Owls: 

(Difenacoum, Brodifacoum). Environmental Pollution 68: 101-117.

46750934 Buckle, A. (2004) Expert Review of Literature on the Ecotoxicology of 

Difenacoum and Wildlife in the European Union. Unpublished study prepared by 

Alan Buckle Consulting, Ltd. 239 p.

46750961 Croucher, A. (1998) Analysis of Difenacoum in Rat Carcasses: Validation of a 

Method and Analysis of Samples: Amended Final Report. Project Number: 

CLE/355/45/D2140, 355/45, CLE/355/45/02R. Unpublished study prepared by 

Covance Laboratories, Ltd. 31 p.

46750928 Harrison, E. (2005) Summary of Environmental Fate and Ecological Effects 

Studies Conducted with Difenacoum, A New Rodenticide Active Ingredient. 

Unpublished study prepared by Woodstream Corp. 15 p.
Page 42 of 43

Appendix B. Screening Level Models to Evaluate Potential 

Importance of Drinking Water and Inhalation Exposure 

Pathways

SIP (Screening Imbibition Program, v1.0) Inputs and Conclusions

Table 1. Inputs

Parameter Value

Chemical name Difenacoum

Solubility (in water at 25oC; mg/L) 2

Mammalian LD50 (mg/kg-bw) 1.8

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian 

species

Mammalian NOAEL (mg/kg-bw)

Mammalian test species

Body weight (g) of "other" mammalian 

species

Avian LD50 (mg/kg-bw) 67

Avian test species northern bobwhite quail

Body weight (g) of "other" avian species

Mineau scaling factor 1.15

Mallard NOAEC (mg/kg-diet)

Bobwhite quail NOAEC (mg/kg-diet)

NOAEC (mg/kg-diet) for other bird species

Body weight (g) of other avian species

NOAEC (mg/kg-diet) for 2nd other bird 

species

Body weight (g) of 2nd other avian species

Table 2. Mammalian Results

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 0.3440 0.3440

Adjusted toxicity value (mg/kg-bw) 1.3845 0.0000

Ratio of exposure to toxicity 0.2485 0.0000

Conclusion*

Exposure through 

drinking water alone is a 

potential concern for 

mammals

Due to insufficient data, 

risk cannot be precluded

Table 3. Avian Results

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 1.6200 1.6200

Adjusted toxicity value (mg/kg-bw) 48.2688 0.0000

Ratio of exposure to acute toxicity 0.0336 0.0000
Page 43 of 43

Conclusion*

Drinking water exposure 

alone is NOT a potential 

concern for birds

Due to insufficient data, 

risk cannot be precluded

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. Therefore, 

when aggregated with other routes (i.e., diet, inhalation, dermal), pesticide exposure through drinking water 

may contribute to a total exposure that has potential for effects to non-target animals.