DIQUAT JMPR 1972
Diquat was evaluated at the Joint Meeting in 1970. Since the previous
evaluation (FAO/WHO, 1971), some new experimental studies have been
presented to fulfill the request made at that meeting for a further
three-generation reproduction study, investigations on the mechanism
of cataractogenesis in animals and clinical studies on factory workers
exposed to diquat to assess cataractogenic risk.
EVALUATION FOR ACCEPTABLE DAILY INTAKE
Levels of diquat in lung, muscle, kidney and liver have been measured
at 1, 3, 5, 7 and 10 days after administration of 20 mg/kg
intravenously to rats. Levels in lung and muscle were constantly
lower, and in liver constantly higher, than those observed with
paraquat. Up to five days, kidney levels of diquat and paraquat were
comparable, after which diquat levels exceeded those of paraquat
(Sharp et al., 1972).
Special studies on cataractogenicity
Cataractogenicity has been studied by Pirie and Rees (1970) and Pirie
et al. (1970). These workers consider that such prolonged
administration of diquat is required that cataract is unlikely to
occur in man. Diquat cataracts differ from others in the rat. They
develop through a posterior opacity, which may be rich in
ribonucleoprotein, to a dense nuclear cataract, with cortical opacity,
developing only at a late stage as the lens shrinks. In this respect,
it resembles cataract due to irradiation, but is unlike it as no
damage to cell nuclei of lens epithelium occurs. An important
biochemical change is a fall in concentration of ascorbic acid in the
lens and intraocular fluids. A major difference between cataracts
following diquat administration and others (e.g. those developing from
X-irradiation, galactose, diabetes, naphthalene feeding or senility)
is that lens GSH remains high in diquat treated rats, even in the
presence of severe cataracts. The concentration of diquat in the lens
is below that of serum when diquat is administered intraperitoneally.
Fresh extracts of bovine lens form free radicals of diquat in the dark
by reduction with glutathione reductase and NADPH. This reaction is
reversible. Sunlight catalyses the formation of free diquat radical in
the presence of amino acids, aqueous humour or lens diffusate and also
the oxidation of ascorbic acid in aqueous humour by diquat. At present
it is possible only to speculate on the role of these biochemical
reactions on the mechanism of cataract formation by diquat.
Special studies on reproduction
In a three-generation reproduction study, 3 groups of 12 male and 24
female rats were fed on diet containing 0, 125 and 250 ppm diquat ion
from the age of 35 days; thereafter, they and their progeny remained
on these diets. After 100 days, two females were mated with one male
to produce the F1a litter. The male was replaced if conception did
not occur within three weeks. Second matings took place ten days after
weaning. The F1b litter was used to provide the second generation
parents. Subsequent generations were similarly produced. Animals
receiving 500 ppm failed to maintain a normal rate of growth and their
food intake was reduced. At the 125 ppm level, rats of the F2
generation had body-weights significantly below that of controls at
weaning and just prior to mating but not in the intervening period.
Neither treatment level affected reproduction as shown by fertility,
period between mating and litter production, mean litter size, number
of stillborn and sex distribution. The mean body weight of the young
at weaning was reduced at the 500 ppm level in F1a, F1b, F2a and F2b
litters and at the 125 ppm level in F1b and F2a litters. Examination
of all animals for changes in behaviour, congenital abnormalities and
gross and microscopic pathology showed no differences between test and
control groups, except for the occurrence of lens opacities. These
were seen in animals receiving 500 ppm diquat and first appeared at
91, 106 and 124 days in the parent, F1b and F2b generation adults.
After about 280 days these generations showed an incidence of the
lesion of 55%, 70% and 47%. The time of onset and incidence were thus
no different in groups which had been exposed during intrauterine and
neonatal periods. No opacities were found in the young or in adults of
the 125 ppm groups (Fletcher et al., 1972).
OBSERVATIONS IN MAN
Observations have been made on 22 workers engaged in manufacturing
and/or packing diquat dibromide for periods varying from 6 months to
11´ years (average 3´ years). Nine are still employed by the company.
Review of the medical records of the 22 workers over this period
showed that there were 20 medical incidents which could be attributed
to contact with diquat and that exposure was principally by splashing
while drums were being filled. All were of a minor nature and due to
contact effects on the eyes, nails and skin. No case of lens opacity
has been observed in these or other workers exposed to diquat (Shaw,
A three-generation reproduction study has confirmed that 500 ppm does
not affect reproduction or produce teratogenic effects. This study has
confirmed the cataractogenicity of this compound and has demonstrated
that exposure of animals in utero and neonatally to diquat does not
decrease the exposure time needed to develop cataracts. Human data are
limited to cases of intermittent skin and inhalation exposure. Human
subjects who have been exposed over several years to diquat and in
some cases suffered injuries from direct eye, skin or nail contact,
have not shown evidence of cataract formation. This parallels previous
findings in animals. It seems unlikely that cataract formation will
occur in humans exposed to low levels of diquat such as occur as
pesticide residues in food. The acceptable daily intake was assessed
from two-year studies in rats and dogs.
Level causing no toxicological effect
Rat: 10 ppm in the diet, equivalent to 0.5 mg/kg body-weight/day
(corresponds to 0.36 mg diquat ion/kg body-weight/day)
Dog: 68 ppm in diet, equivalent to 1.7 mg/kg body-weight/day
(corresponds to 1.22 mg diquat ion/kg body-weight/day)
ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN
0 - 0.005 mg/kg body-weight (equivalent to
0 - 0.0036 mg diquat ion/kg body-weight.
RESIDUES IN FOOD AND THEIR EVALUATION
The uses and chemical characteristics of diquat together with its mode
of action and toxicity have been reviewed by Wheeler (1971) and
RESIDUES RESULTING FROM SUPERVISED TRIALS
Table 1 depicts residue data on various crops following desiccant use
of diquat (Plant Protection Ltd., 1972).
Residues of diquat and its degradation products in rapeseed and oil
have been determined by Leahey and Allard (1971). Rape plants were
desiccated with 14C-diquat at various rates of application and the
oil and meal analysed. No detectable residues of diquat or its
breakdown products were found in rapeseed oil when the seeds were
harvested 7 days after desiccation, and residues of less than 0.02 ppm
were found when seeds were harvested 14 days after desiccation. Values
obtained from the analysis of the meal following varying application
rates are given in Table 2. It was confirmed that a large part of the
radioactive extract consisted of unchanged diquat.
TABLE 1 Residues of diquat in various crops
Rate of Interval Diquat found
Crop application harvest Samples range mean
(g diquat/ha) (days) (no.) (ppm)
Wheat (grain) 600 - 1000 4 - 7 43 ND1 - 1.58 0.61
(flour) 600 - 1000 4 - 7 8 ND - 0.67 0.22
Rice (with husk) 200 - 400 3 - 5 35 ND - 6.40 0.89
(polished) 200 - 400 3 - 5 26 ND - 0.16 0.07
Sorghum (grain) 400 - 600 4 - 10 25 ND - 5.90 0.81
Rape seed 400 - 600 4 - 10 77 ND - 1.50 0.37
Sunflower seed 600 4 -15 16 ND - 0.20 0.07
Cotton seed 400 - 1000 10 14 ND - 0.98 0.37
Potatoes 600 - 1000 4 - 10 36 ND - 0.25 0.03
Peas 300 - 1000 4 - 10 20 ND - 0.07 0.05
Beans 300 - 1000 4 - 10 15 ND - 0.57 0.10
Poppy 800 6 - 10 9 0.56 - 4.90 2.84
sugar beet (root) 300 - 800 2 - 7 2 0.09 - 0.11 0.10
(juice) 300 - 800 2 - 7 4 - <0.01
1 ND = not detected (usually <0.05 ppm)
TABLE 2 Residues of diquat in rape seed meal
Diquat ion Radioactivity
Rate of Time between equivalent extracted
Plant application spraying and to the with
no. (kg a.i./ha) harvest radioactivity 1.0 M HBr
(days) detected (%)
1 0.3 7 0.37 83
2 6.3 7 0.18 80
3 0.6 7 0.96 85
4 0.6 7 0.94 85
5 1.1 7 0.48 67
6 1.1 7 0.66 75
7 0.3 14 0.52 86
8 0.3 14 10.02 76
9 0.6 14 1.49 75
10 0.6 14 1.51 71
11 1.1 14 3.22 85
12 1.1 14 1.87 89
FATE OF RESIDUES
Diquat is a contact herbicide that kills or severely scorches all
green herbage with which it comes into contact. It is not readily
translocated. Furthermore, it is quickly rendered biologically
inactive by sorption into clay minerals in the soil and is thus not
mobile in soil or available for root uptake. These properties make it
highly successful as a pre-planting or pre-crop-emergence total
weedkiller or as a directed, inter-row spray between rows of emerged
crops. Contamination may occasionally arise when spray is misdirected
or drifts onto growing crops or when young seedlings emerge through
dense swards of sprayed herbage containing diquat residues. In such
cases severe contamination will kill or severely scorch the plants,
and small residues (below 0.5 ppm) have been detected in the foliage
of some crops (e.g., oats and maize) seven to eight weeks after
application. However, the great majority of crops treated in this way
show no detectable residues (<0.05 ppm) in edible parts when
harvested from one to four months later (Plant Protection Ltd., 1972).
No residues of diquat were found in milk, urine or faeces of cows fed
daily with 5 kg of ground sunflower seed, containing 1 mg of residual
diquat, for periods of 185 and 257 days (Lembinski et al., 1971). No
residues of diquat were found in the liver and kidneys of a calf born
to a cow fed for 257 days with the ground seed or in the liver and
kidneys of wethers fed daily for 141 days with 0.5 kg of ground seed
containing 0.1 mg of diquat. The limits of detection were 0.01 ppm in
milk and urine and 0.03 ppm in liver, kidneys and faeces.
Brian (1970) showed that the behaviour of diquat in plants was
complex. Studies using tomato plants demonstrated that the activity
was influenced by light before and after treatment, and that the
apparent loss at intervals was not due to exudation from leaves or
downward movement into the roots. When diquat was used as a silaging
agent it was found that the protein content in the hay was increased
and the glycidic content reduced (Jambrich, 1970). The moisture
content of maize was not reduced significantly when diquat was used as
a desiccant, but the application appeared to reduce the crop yield
(Wilkins and Tetlow, 1970). The changes in plants occurring after
application of diquat have been discussed by Dodge (1971).
The cationic exchange properties of diquat in soil clays, vermiculite
and smectite has been investigated. Exchangeable diquat ion was
replaced by potassium ion, and values are given for various samples. A
direct relationship between the exchange and layer charge density was
obtained (Dixon et al., 1970).
Diquat was used at a rate of 4 lb/acre (surface) on two New York
lakes; residues were less than 0.005 ppm in 4-8 days and a similar
shoreline treatment showed no residue after 1 day (Sewell, 1970).
METHODS OF RESIDUE ANALYSIS
Herbicides of the diquat type have been determined by a colorimetric
method (Zhemchuzhin and Akimova, 1970) based on chloramine-T and
chlorophenol red. A semi-quantitative thin-layer chromatographic
method for the determination of diquat has been used for residues in
bees (Mueller and Worseck, 1970). The polarographic response of diquat
in five supporting electrolytes has been reported (Hance, 1970). The
method of Calderbank and Yuen (1966) was used for the determination of
diquat in sunflower seeds by Lembinski et al. (1971); the limit of
detection was 0.03 ppm. The method was also used, with the
modifications of Black et al. (1966), for residues in milk, urine,
faeces, kidneys and liver after feeding animals with the sunflower
seed. These procedures should be suitable for regulatory purposes.
Since the evaluation of diquat in 1970 (FAO/WHO, 1971) further
residues data have become available. These indicated a need to
increase the recommended tolerance levels for residues of diquat in
polished rice and potatoes; some new tolerances are also proposed.
Tolerances on barley and wheat cover occasional required desiccant
uses; the bulk of any cereals so treated should be used for animal
feed or seed purposes only.
The following tolerances are recommended to replace those listed in
Annex 1 of FAO/WHO (1972).
Barley, poppy seed, rice (in husk) 5
Rapeseed, sorghum, wheat 2
Beans, sunflower seed 0.5
Rice (polished), potatoes, wheat flour 0.2
Onions, maize, sugar beet, peas 0.1
Sesame, sunflower, rape, cotton seed oils 0.1
Other vegetable crops 0.05*
Milk ) from the feeding 0.01*
Meat and meat products ) of treated forage 0.05*
* at or about the limit of determination
FURTHER WORK OR INFORMATION
1. Further study on the mechanism of cataractogenic activity.
2. Identification of the toxicologically active substance (i.e.,
parent compound or metabolite).
3. If treated cereals are to be used for human consumption, further
data would be required on residues occurring in barley, wheat,
rye and oats and their products (flour, bread, beer, etc.)
To FAO/WHO (1971), p. 549 and FAO/WHO (1972), p. 31: diquat
(cation), column 4, after Peas, beans, sunflower seed for 0.1
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Metzner, Ed.), Tübingen III, 1737.
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Residues in herbage and silage and feeding experiments following the
use of diquat as a desiccant. J. Sci. Fd. Agric., 17: 506-509.
Brian, R.C. (1970) Effect of light environment on the activity and
behaviour of diquat and paraquat in plants. Pestic. Sci., 1: 38-41.
Calderbank, A. and Yuen, S.H. (1966) An improved method for
determining residues of diquat. Analyst, 91: 625-629.
Dixon, J.B., Moore, D.E., Agnihotri, N.P. and Lewis, D.E. (1970)
Exchange of diquat in soil clays, vermiculite and smectite. Soil Sci.
Soc. Amer. Proc., 34: 805-808.
Dodge, A.D. (1971) Mode of action of the bipyridylium herbicides,
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Hance, R.J. (1970) Polarography of herbicides - a preliminary survey.
Pestic. Sci., 1: 112-113.
Jambrich, J. (1970) Desiccants as silaging agents. Agrochemia, 10:
Leahey, J.P. and Allard, J. (1971) Bipyridylium herbicides: residues
in rape seed and oil following desiccation with diquat. Report TMJ
674A Plant Protection Ltd. (unpublished)
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determination by thin-layer chromatography of diquat (Reglone) and
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establishment of permanent tolerances for diquat in human food crops.
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