UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C.  20460

	OFFICE OF CHEMICAL SAFETY

	AND POLLUTION PREVENTION

  SEQ CHAPTER \h \r 1 MEMORANDUM

May 12, 2010					

SUBJECT:	Diquat Dibromide   Petition for the Establishment of Permanent
Tolerances for New Use on Canola. Summary of Analytical Chemistry and
Residue Data.

PC Code:  032201	DP Barcodes:  D375811

Decision Nos.:  423144	Registration Nos.:  100-1061

Petition Nos.:  9F7639	Regulatory Action: Section 3 

Risk Assessment Type:  NA	Case No.:  0288

TXR No.:  NA	CAS No.:  85-00-7

MRID No.:  See MRID Summary Table	40 CFR:  180.226

		              									

FROM:	Becky Daiss,   SEQ CHAPTER \h \r 1 Biologist

		Risk Assessment Branch 4

		Health Effects Division (7509P)

THROUGH:	Susan Hummel, Senior Scientist

		Risk Assessment Branch 4

		Health Effects Division (7509P)

TO:		  SEQ CHAPTER \h \r 1   SEQ CHAPTER \h \r 1 Bethany Benbow, Risk
Manager Reviewer

		Herbicide Branch

		Registration Division (7508P)

	

	

MRID Summary Table

MRID No.	Study Type	Comments

47910101	860.1500 Canola	Summarized in new DER; 47910101.der.doc 

47910102	860.1500 Canola	Summarized in new DER; 47910101.der.doc 

47910103	860.1340 Canola	Summarized in new DER; 47910103.der.doc 



Executive Summary

Diquat dibromide (6,7-dihydrodipyrido(1,2-a:2', 1'-c)pyrazinediium
dibromide) is a non-selective contact herbicide, desiccant, and plant
growth regulator primarily registered as a general herbicide for the
control of broadleaf and grassy weeds on terrestrial non-crop and
aquatic areas.  Diquat dibromide is also registered as a preharvest
desiccant in potato and crops grown for seed including alfalfa, carrot,
clover, radish, sorghum, soybean, and turnip.  Special local need
registrations have recently been granted for the use of diquat dibromide
as a postharvest desiccant on cantaloupe, cucumber, pepper, squash,
tomato, and watermelon.  The diquat dibromide formulations registered
for use on aquatic areas and food/feed crops are the 0.2 and 2 lb
cation/gal soluble concentrate/liquid (SC/L).  On crops, these
formulations may be applied as preplant or preemergence broadcast
applications or as preharvest/postharvest desiccants using ground or
aerial equipment.  On noncrop aquatic areas, the formulations registered
may be applied as surface, sub-surface, or bottom placement treatments
[Source: REFS search, 8/7/01].

The established tolerances for residues of diquat dibromide in/on raw
agricultural commodities and in animal products are presently expressed
in terms of the diquat cation [Source: 40 CFR §180.226 (a) and (b)]. 
Tolerance levels of 0.05-0.1 ppm are established for crop commodities
and 0.02 ppm for animal commodities.   

Under PP#9F7639, Syngenta Crop Protection Inc is supporting the use of
diquat dibromide as a harvest aid dessicant on canola.  Syngenta has
submitted the label Reglone Desiccant (EPA Reg. No. 100-1061).  Reglone
Dessicant is a soluble concentrate containing 3.73 lbs diquat dibromide
salt per gallon equivalent to 2 lbs diquat cation per gallon.  An
application rate of 0.5 lb ai/A is proposed with a limit of one
application per season.  A 7 day pre harvest interval (PHI) is required
with harvest no later than 10 days after application.  Ground or aerial
application equipment is permitted.  

The registrant is also proposing an HPLC analytical method as a new
enforcement method for the determination of residues of diquat in crops,
including oil seed crops (Analytical Method Number RAM 272/02, “The
Determination of Residues of Paraquat and Diquat in crops and Soil – A
High Performance Liquid Chromatographic Method”).   

Regulatory Recommendations and Residue Chemistry Deficiencies

HED has examined the residue chemistry database for diquat dibromide. 
There are no residue chemistry issues that would preclude establishing a
tolerance and granting a conditional registration for the requested use
of diquat dibromide   SEQ CHAPTER \h \r 1 or establishment of the
following tolerances for residues of the diquat cation dibromide:

	Canola, seed	2.0 ppm

	Canola, meal	6.0 ppm

The registration is conditional based on the submission the IR-4 data on
preharvest dessicant use of diquat dibromide on canola.  

Syngenta Crop Protection, Inc. has submitted field trial data from
studies conducted in W. Germany and the UK to support use of diquat
dibromide as a preharvest dessicant on canola.   No field trial data
from the United States or Canada are currently available.  However IR-4
is currently conducting studies for preharvest dessicant use of diquat
dibromide on canola.  

The HED ChemSAC determined that the EU data would support the
establishment of a tolerance of 1 ppm on canola seed and the conditional
registration of diquat dibromide on canola as a harvest aid/desiccant. 
The tolerance on canola seed has been increased from 1 ppm to 2 ppm to
harmonize with the Codex MRL, and a tolerance of 6 ppm is recommended
for canola meal, based on a maximum theoretical concentration factor of
3X.  HED is aware that IR-4 is currently conducting studies in the U.S.
to support the preharvest desiccant use of diquat dibromide on canola. 
Residues of diquat dibromide on canola grown in the U.S. are not
expected to differ significantly from residues reported in the EU
studies, since harvest aid/desiccant applications are made late in the
growing season with little time between application and harvest.  In
addition, since the recommended tolerances for canola seed and meal have
been increased by a factor of 2X to harmonize with Codex, there is
little chance residues in the U.S. trials will exceed these tolerances. 
Nevertheless, since the IR-4 field work has already been completed and
the study reports will be available in July, 2011, HED recommends that
these studies be required as a condition of registration to confirm the
recommended tolerance levels.

Syngenta provided insufficient information on storage
durations/conditions of canola samples in the European crop field trials
for use of diquat as a dessicant on canola.  Data on length of storage
of canola samples prior to analysis from the W. German and UK trials is
required.  Although the length of storage prior to analysis is unknown,
based on the structure of diquat, residues in frozen storage are
expected to be stable much longer than the 6 to 8 months for which
storage stability data are available.  Therefore, lack of data on length
of storage is not expected to impact the risk assessment.   

Syngenta has proposed a new enforcement method - Analytical Method
Number RAM 272/02 “The Determination of Residues of Paraquat and
Diquat in crops and Soil – A High Performance Liquid Chromatographic
Method.”  Samples are extracted by refluxing in dilute sulphuric acid.

The filtered digest is diluted and percolated through a column of cation
exchange resin which retains the diquat.  The column is washed with a
hydrochloric acid, ammonium chloride and deionized water solution. 
Diquat residues are eluted with saturated ammonium chloride solution. 
Final determination of diquat residues is made with ion-pair HPLC with
UV detection.  Reports on the method validation (RJ201B) and the ILV
(ZEN 0396) have been submitted to the Agency.  

Based on HED’s analysis, the proposed analytical method is classified
as scientifically acceptable.  

HED recently completed a risk assessment for a Section 18 Emergency
Exemption for use of diquat dibromide on canola grown in Oklahoma and
Kentucky (M. Doherty, D355407, 12/3/09).  The use requested for this
Section 3 registration is identical to that assessed for the Section 18
request. Therefore a new risk assessment is not needed.  

 

Background

Diquat dibromide is a non-selective contact herbicide, algicide,
dessicant, and defoliant.   As a herbicide/algicide, it is used to
control aquatic and terrestrial weeds.  It is used as a preharvest
dessicant/defoliant to facilitate the harvest of potatoes and various
crops grown for seed.  

Diquat dibromide works as a dessicant by reacting with molecular oxygen
to produce a superoxide anion in treated plants.  The oxidative
activity, which occurs subsequent to formation of the oxygen radicals,
rapidly destroys plant cell membranes.  The nomenclature and
physicochemical properties of diquat dibromide are shown in Tables 1 and
2.  

  

Common name	Diquat dibromide

Company experimental name	NA

IUPAC name	9,10-dihydro-8a,10a-diazoniaphenanthrene dibromide

CAS name	6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium dibromide

CAS #	85-00-7

End-use product (EP)	Reglone Dessicant herbicide (EPA Reg No. 100-1061



Table 2.	Physicochemical Properties of Diquat Dibromide

Parameter	Value	Reference

Mol Wt	344.1 (Cation – 184)	The Pesticide Manual 11th Edition

Melting point/range	300 °C	D277710

pH	Not Provided	NA

Density (20°C)	1.22-1.27	The Pesticide Manual 11th Edition

Water solubility (20°C)	700 g/L 	D277710.

Solvent solubility 	slightly soluble in alcohol and hydroxylic solvents,
and

practically insoluble in nonpolar organic solvents	D277710

Vapour pressure at 20°C	Negligible < 10-5 mbar   	The agrochemicals
handbook 1983

Dissociation constant (pKa)	Not Provided	NA

KOW Octanol/water partition coefficient as  Log POW  at 20°C	-4.60	The
Pesticide Manual 11th Edition

UV/visible absorption spectrum	Neutral and acidic conditions - one major
peak at 309.5 nanometers.  

Basic conditions - two major peaks at 309.0 and 217.0 nanometers.
D349998



Directions for Use

The petitioner submitted a draft label for label Reglone Desiccant (EPA
Reg. No. 100-1061).  An application rate of 0.5 lb ai/A is proposed with
a limit of one application per season.  A 7 day pre harvest interval
(PHI) is required with harvest no later than 10 days after application. 
Ground or aerial application equipment is permitted.  

Table 3.  Summary of Directions for Use of Diquat Dibromide on Canola

Applic. Timing, Type, and Equip.	Formulation

[EPA Reg. No.]	Applic. Rate 

(lb ai/A)	Max. No. Applic. per Season	Max. Seasonal Applic. Rate

(lb ai/A)	PHI

(days)	Use Directions and Limitations

Harvest Aid

Broadcast

Aerial or Ground 	2 lb (diquat cation)/gal SC

[100-1061]	0.5	1	0.5	7	Harvest no later than 10 days after application



Conclusions.  The label is adequate to allow evaluation of the residue
data relative to the proposed use.  

860.1300 Nature of the Residue - Plants

MARC Decision, DP# 277765, 11/8/2001, T. Morton

The reregistration requirements for plant metabolism are fulfilled.  The
qualitative nature of the residue in plants is adequately understood
based on an acceptable potato metabolism study and a rat bioavailability
study.  The HED Metabolism Assessment Review Committee (MARC) has
concluded the residue of concern for plants is the diquat cation (T.
Morton, 11/8/01, D277765).  The established tolerance expression for
residues of diquat dibromide in/on plant commodities is appropriate and
no changes are required.

The potato metabolism study indicated that no metabolism of diquat
occurred in potato tubers following preharvest application of
[14C]diquat as a desiccant to potato stalks and stems.  Soybean and
wheat metabolism studies had also been previously submitted but were
deemed marginal because of inadequate characterization and
identification of 14C-residues in the commodities of concern.  Attempts
to further characterize 14C-residues from these studies were
unsuccessful.  In lieu of the requirements for additional crop
metabolism studies, the Agency recommended several options to satisfy
reregistration requirements for this topic.  The registrant opted to
conduct a bioavailability study.  The results of the bioavailability
study showed that diquat plant residues are largely not bioavailable;
<5% of the 14C is absorbed as a result of feeding diquat field residues
in/on wheat chaff to rats.  The retention of diquat residues in tissues
was negligible ((0.004 ppm diquat equivalents) following dosing at  (25x
the maximum human dietary intake.

860.1300 Nature of the Residue - Livestock

Canola meal is a livestock feedstuff.  The qualitative nature of the
residue in plants is adequately understood and the terminal residue of
concern in plants is the diquat cation.  

860.1340 Residue Analytical Methods

DER 47910103

Enforcement Methods:  The Pesticide Analytical Manual (PAM) Vol. II.
lists a spectrophotometric method, designated as Method A (also
referenced as Chevron Chemical Company RM-8-7) as available for the
enforcement of tolerances for residues of diquat per se in/on plant and
in animal commodities.  In this method, residues are extracted with
sulfuric acid to free diquat from the bound state, absorbed on cation
exchange resin, and eluted with saturated ammonium chloride.  The diquat
in the eluate is reduced by sodium dithionate, forming an intense green
color and is measured spectrophotometrically at 337 µm.  The limit of
detection is 0.01 ppm.

The registrant is proposing analytical method number RAM 272/02 (“The
Determination of Residues of Paraquat and Diquat in crops and Soil – A
High Performance Liquid Chromatographic Method”) as an enforcement
method for the determination of residues of diquat in crops, including
oil seed crops.  RAM 272/02 has a limit of detection (LOD) in the range
of 0.01-0.05 mg/kg depending on the crop type and mass of sample
analyzed.  With this method, samples are extracted by refluxing in
dilute sulphuric acid.  The filtered digest is diluted and percolated
through a column of cation exchange resin which retains the diquat and
some of the natural crop constituents.  The column is washed with
hydrochloric acid, ammonium chloride solution to remove endogenous
materials.  Diquat residues are eluted with saturated ammonium chloride
solution.  A portion of the column eluent is passed through a
preconditioned C18 solid phase extraction cartridge to remove further
co-extractives.  Final determination of diquat residues is made using
ion-pair high performance liquid chromatography.  Samples are analyzed
against an appropriate concentration of diquat standard dissolved in
saturated ammonium chloride solution.  Based on HED’s analysis, the
proposed analytical method is classified as scientifically acceptable. 

Data Collection Methods:  Plant Protection Residue Analytical Method
No.5 was used for the measurement of diquat ion residues in canola seed.
 The samples were heated under reflux in 0.5 mol dm-3 sulfuric acid
solution.  The filtered digest was percolated through a column of cation
exchange resin which retained the diquat and some of the natural crop
constituents.  The column was sequentially washed with dilute
hydrochloric acid solution, 2.5% ammonium chloride solution and water. 
The diquat was then eluted with saturated ammonium chloride solution.  A
portion of the column eluent was treated with sodium dithionite in
alkali.  This reduced the diquat to a free radical, the light absorption
of which was measured using a spectrophotometer.  The estimated LOD of
the method was reported as 0.005 ppm in the UK study and 0.05 for the
West German study.  

Plant Protection Residue Analytical Method No 7A was used for the
measurement of diquat ion in canola oil samples for the West German
trials.  The oil samples were thoroughly mixed by rolling with cation
exchange resin for 2 hours.  The resin was then placed in a column and
was subjected to the same procedure as the seed samples.  The LOD of the
method was estimated to be 0.05 ppm.  

Conclusions:  The data collection method used in the submitted field
trials is similar to the existing enforcement method.  Adequate method
validation and concurrent recovery data were attained in the field trial
and processing studies.  Therefore, HED concludes that existing and
proposed enforcement methods are adequate for enforcement of the
existing tolerances.  

860.1360 Multiresidue Methods

The FDA's PESTDATA dated 11/6/90 (Pam Vol. I, Appendix) indicates that
recovery of diquat dibromide using Multiresidue Protocols is unlikely. 
The updated PESTDATA dated 08/93 does not have an entry for diquat
dibromide.

860.1380 Storage Stability

The requirements for storage stability data are fulfilled for purposes
of reregistration.  Adequate storage stability data on diquat dibromide
are available to support the storage conditions and intervals of samples
from magnitude of the residue studies in plants and animals.  Residues
of diquat per se are stable under frozen (-20(C) storage conditions for:
 (I) up to six months in/on bell pepper, carrot roots, clover (hay and
seed), lettuce, potato, rice (grain and straw), sorghum grain, soybean,
tomato and tomato processed fractions, and wheat (grain and straw); (ii)
up to 8 months in processed fractions of sorghum grain and soybean; and
(iii) up to 2 months in water and seafood samples. 

Syngenta provided very limited information on storage
durations/conditions of canola samples in the crop field trials for use
of diquat as a dessicant on canola.  For the West German trials,
representative samples of seed were received deep frozen at Jealott’s
Hill during September and October 1987; all samples were stored at -20
+/-5 ºC prior to analysis.  For the UK trials, samples were stored at
-20ºC prior to analysis.  Data on length of storage prior to analysis
was not provided.  Although the length of storage prior to analysis is
unknown, based on the structure of diquat, residues in frozen storage
are expected to be stable much longer than the 6 to 8 months for which
storage stability data are available.  Therefore, lack of data on length
of storage is not expected to impact the risk assessment.  However, data
on length of storage of canola samples prior to analysis should be
submitted.

860.1400 Water, Fish, and Irrigated Crops

There proposed label changes do not impact water, fish or irrigated
crops.  Therefore, data requirements for this guideline topic are not
relevant to the request for use on canola.  

860.1460 Food Handling

The requested label changes are not relevant to this guideline topic.

860.1480 Meat, Milk, Poultry, and Eggs

Canola meal is a livestock feedstuff.  Recalculated dietary burdens are
provided in Table 4.  Current tolerances are adequate.  Residues will
not exceed those used in the dietary exposure analysis conducted for the
Section 18 Emergency Exemption.

Table 4.  Recalculated Dietary Burden

Feed Commodity	% Dry Matter a	% Diet a	Est Residues (ppm)	Dietary
Contribution (ppm) b

Beef Cattle

    Canola Meal	88	5	6.0	0.34

    Soybean hulls	90	15	0.6 d	0.10

    Potato processed waste	15	80	0.5 f	2.66

  TOTAL BURDEN



3.11

Dairy Cattle





   Canola Meal	88	10	6.0	0.68

   Soybean hulls	90	20	0.6 d	0.13

   Potato processed waste	15	40	0.5 f	1.33

   Sorghum grain	86	30	2.0 e	0.70

  TOTAL BURDEN



2.84

Poultry

   Soybean Seed	NA	10	0.2 c	0.04

   Canola Meal	NA	15	6.0 d	0.09

   Sorghum grain	NA	75	2.0 e	1.50

  TOTAL BURDEN



2.44

Swine

   Canola Meal	NA	15	6.0	0.09

   Sorghum grain	NA	85	2.0 e	1.78

  TOTAL BURDEN



2.60

a Table 1 (OPPTS Guideline 860.1000, August 1996).

b Contribution = [tolerance / % DM (if cattle)] X  % diet).

c Tolerance level for soybean seed.

d Tolerance level for soybean hulls.

e Tolerance level for sorghum grain.

f  Tolerance level for potato processed waste (wet peel residue levels
used).	

In a cattle feeding study, residues of diquat were non-detectable
(<0.003 ppm) in milk and tissues following feeding of diquat-treated
ryegrass silage to cattle at 3.6 ppm (1.2x and 1.3x the maximum dietary
intake for dairy and beef cattle, respectively) for 30 days.  In another
study, residues of diquat were nondetectable (<0.01 ppm) in milk
following feeding of diquat-treated clover hay to cattle at 11 ppm
(3.9x) for 34 days.  The established 0.02 ppm tolerance level for diquat
residues in milk is adequate.  The established tolerances of 0.02 ppm
for diquat residues in the fat, meat, and meat byproducts of cattle,
goats, hogs, horses, and sheep may be raised to 0.05 ppm to achieve
compatibility with the Codex maximum residue limit (MRL).

The maximum dietary burden for poultry is 2.44 ppm; the calculation of
the poultry burden is presented above.  In a poultry feeding study,
residues of diquat were mostly non-detectable (<0.005 ppm) in samples of
eggs, fat, muscle, liver, and skin of chickens fed diquat at 1, 4.3, and
8.2 ppm diquat cation (0.4x, 1.8x, and 3.4x, respectively, the estimated
dietary burden) in the diet for 28 days.  A single skin sample from the
day-21, 8.2 ppm treatment group bore residues of 0.006 ppm.  Residues in
gizzard ranged from non-detectable to 0.022 ppm.  The established
0.02-ppm tolerance level for diquat residues in poultry fat, meat, meat
byproducts and eggs may be raised to 0.05 ppm to achieve compatibility
with the Codex maximum residue limit (MRL).

860.1500 Crop Field Trials

DER References: 47910101.der.doc 

Syngenta Crop Protection, Inc. has submitted field trial data for use of
diquat dibromide as a preharvest dessicant on canola preharvest
dessicant on canola. Field trials were conducted in West Germany and the
United Kingdom.  No field trial data from the United States or Canada
are currently available.  However IR-4 is currently conducting studies
for preharvest dessicant use of diquat dibromide on canola.  Data from
the IR-4 studies is expected to be available by July 2011.  The results
of the field trials conducted in Europe are summarized in Table 5.

TABLE 5.  Summary of Residue Data from Crop Field Trials with Diquat

Commodity	Total Applic. Rate

 (lb a.i./A)

 (kg a.i./ha)	PHI (days)	Residue Levels

 (ppm)



	n	Min.	Max.	HAFT*	Median

(STMdR)	Mean

(STMR)	Std. Dev.

Canola Seed	0.54/0.6	5-8	10	<0.05	0.48	0.48	0.155	0.232	0.13

Canola Seed	0.54/0.6	10-13	2	0.03	0.21	0.38	0.12	0.12	0.13

Canola Seed	0.54/0.6	20	2	0.18	0.38	0.38	0.28	0.28	0.14

Canola Oil	0.54/0.6	5-8	8	<0.05	<0.05	<0.05	<0.05	<0.05	0



Conclusion. Based on ChemSAC’s recommendation that the diquat team
consider requiring data from trials conducted in the U.S. to supplement
the available European field trial data, submission of the IR-4 data
should be required as a condition of the canola registration.  

860.1520 Processed Food and Feed

 

A maximum theoretical concentration factor of 3X has been applied for
the processed fraction canola meal.  Residue data from field trials
conducted in Germany demonstrated that diquat residues were not found in
any of the oil samples.  

  SEQ CHAPTER \h \r 1 860.1650 Submittal of Analytical Reference
Standards

An analytical reference standard for diquat dibromide monohydrate is
available in the Pesticide Standards Repository (Personal Communication,
Theresa Cole 3/25/10). 

860.1850 & 1900 Confined and Field Accumulation in Rotational Crops 

There are no rotational crops associated with the proposed label
changes.  Therefore, data requirements pertaining to confined and field
accumulation in rotational crops are not relevant to this tolerance
petition.

860.1550 Proposed Tolerances

The residues of concern in plant and animal commodities is the diquat
cation.  The tolerances proposed by Syngenta and those recommended by
HED are listed in Table 5.  Codex has an established maximum residue
limit (MRL) at 2 ppm Canada has an established tolerance of 1 ppm.  The
residue definitions are identical.  Therefore to harmonize with Codex
HED recommends a tolerance of 2 ppm.

Tolerances are established in 40CFR 180.226 for residues of the plant
growth regulator and herbicide diquat, (6,7-dihydrodipyrido
(1,2-a:2'1'-c)pyrazinediium) derived from application of the dibromide
salt and calculated as the cation.  The current tolerance expression is
adequate.

Table 5. Tolerance Summary for Diquat Dibromide

Commodity	Established/

Proposed Tolerance

(ppm)	Recommended

Tolerance

(ppm)	Comments; 

Correct Commodity Definition

Proposed tolerances under §180.226

Canola, seed	1.0	2.0

	Canola, meal	3.0	6.0

	

References

DP#:	277710

Subject:	Tolerance Reassessment of Diquat dibromide: Product and Residue
Chemistry Considerations.

From:	T. Morton

To:	R. Daiss 

Dated:	12/13/2001

MRID:	NA

Template Version September 2005



:2′,1′-c]pyrazinediium dibromide	Common Name:  Diquat

	(Proposed tolerance

( Reevaluated tolerance

( Other	Date:  4/1/10

Codex Status (Maximum Residue Limits)	U. S. Tolerances

( No Codex proposal step 6 or above

( No Codex proposal step 6 or above for the crops requested	Petition
Number 9F639

DP Barcode:  375811

Other Identifier:

Residue definition (step 8/CXL): Diquat cation.

Generally available as dibromide.

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9,10-dihydro-8a,10a-diazoniaphenanthrene ion

	Residue definition: Mexico uses US tolerances and/or Codex MRLs for its
export purposes.

Crop(s)	MRL (mg/kg)	Crop(s)	MRL (mg/kg)



Rapeseed	

1.0	

	





Notes/Special Instructions:  S. Funk, 04/01/2010



Diquat Dibromide	                    Summary of Analytical Chemistry and
Residue Data                        DP#: 375811

Page   PAGE  1  of   NUMPAGES  11 

