DIQUAT JMPR 1976
Since diquat was last evaluated by the Joint Meeting in 1972
(FAO/WHO, 1973), additional studies have been undertaken on
residues occurring in meat, milk and eggs. The available data on
studies with ruminants and hens is reviewed below.
These studies show that diquat, and its photochemical
degradation products which are formed on plant surfaces, are poorly
absorbed from the gut of animals and do not accumulate in tissues,
milk and eggs.
FATE OF RESIDUES
Diquat undergoes rapid photochemical degradation on plant
surfaces. When barley and oats were desiccated with
14C-ring-labelled and 14C-bridge-labelled diquat and kept in
daylight for one or two weeks, unchanged diquat accounted for only
about 30% of the radioactivity present on the plants. Compound II
(Figure 1) was a minor degradation product, normally constituting
approximately 10% of the residue and traces (2% each) of compounds
III and VI were formed. The majority of the residue on the plants,
usually 50-70%, was present as a complex mixture of photochemical
degradation products which appear to be strongly associated with
natural plant materials (Leahey et al., 1973).
Animal transfer studies have therefore been carried out with
diquat alone and with diquat plus a mixture of photochemical
degradation products. This was necessary as in some instances
diquat is the sole or major residue in animal feed (e.g. in meal
from oil seed rape), whereas in others, e.g. in small grain cereals
and alfalfa, the photoproducts may constitute up to 70% of the
residue in the feed. In reviewing the feeding studies conducted on
ruminants and hens, the studies carried out on diquat alone are
considered separately from those carried out with diquat plus its
photoproducts. The structures of diquat and its derivatives
referred to in this evaluation are shown in Figure 1.
Studies with ruminants using diquat alone
Diquat is poorly absorbed from the gastro-intestinal tract of
ruminants and the small amount absorbed is rapidly excreted in the
urine. Residues of diquat and its metabolites in milk and tissues
are extremely low.
Thus, when a single dose of 14C-bridge-labelled diquat was
administered orally to a goat, 96.3% of the radioactivity was
excreted within 7 days, 94.2% in the faeces and 2.2% in the urine.
Diquat was the major radioactive compound present in the faeces,
with only 1-4% of compounds II and III. Diquat accounted for 20% of
the radioactivity present in urine collected on the first day after
dosing, compound II for 6% and compound III for 44%.
Only very small amounts of radioactivity (0.02%) were
transferred to the milk. The administered dose was equivalent to
145 mg/kg of diquat in the daily diet whereas the maximum
radioactive residue in the milk was only 0.014 mg/kg diquat ion
equivalent. Diquat accounted for 22% of this total, compound II 13%
and compound III 7%. Incorporation of radioactivity into lactose,
fat and protein accounted for a further 25% (Griggs and Davis,
In other experiments, when cows were fed a single dose of up
to 20 mg/kg of either 14C-bridge-or 14C-ring-labelled diquat
dibromide, only 0.001-0.015% of the radioactive dose was excreted
in the milk (Stevens and Walley, 1966).
When a cow was dosed for seven consecutive days with
14C-ring-labelled diquat, at a rate equivalent to 30 mg/kg in its
diet, the radioactivity was again eliminated rapidly, mainly in the
faeces. 91% of the dose was recovered in the faeces, principally as
unchanged diquat, and 0.4% in the urine.
After three days the total radioactive residue in milk reached
a maximum of 0.004 mg/kg diquat ion equivalents. 13% of this
residue was unchanged diquat, 7% compound II, 15% compound III and
38% due to incorporation of radioactivity into lactose, fat and
protein. Within four hours of the final radioactive dose the cow
was slaughtered. The radioactive residues found in the tissues were
all very low - less than 0.01 mg/kg diquat ion equivalents except
in liver (0.05 mg/kg) and kidney (0.08 mg/kg) (Leahey et al.,
In a longer term study, non-radiolabelled diquat was
incorporated into the diet of cows to give a residue of 0.5 mg/kg
in the total diet. The diet was fed for 31 days. No residues of
diquat were found in milk, sampled daily (limit of detection 0.005
mg/kg), or in various tissues (limit of detection 0.01-0.035 mg/kg)
Studies with ruminants using diquat plus its photoproducts
Since a mixture of diquat and its photoproducts will be the
residue consumed by animals fed crops such as desiccated barley and
alfalfa, several studies have been carried out to investigate the
safety of this residue mixture.
In metabolic studies, a single dose of barley straw desiccated
with 14C bridge- and ring-labelled diquat was fed to goats at
levels of 2 and 7% (approx.) of the daily intake. Virtually all of
the administered radioactivity, which contained 59-62% unidentified
photoproducts, was eliminated within 10 days, mainly in the faeces,
with about 5% in the urine. A small radioactive residue (0.0028
mg/kg diquat ion equivalent) was detected in the milk. This
radioactivity was shown to be mainly due to incorporation of 14C
into the natural milk constituents. Residues of diquat and compound
II were less than 0.0003 mg/kg (Hemingway et al., 1973).
In an extension of this study a cow was given a single oral
dose of barley straw containing radioactive diquat and its
photoproducts. Total residues were equivalent to about 10% of a
daily intake for a cow maintained wholly on diquat-desiccated
Virtually all of the radioactivity was eliminated from the cow
within 10 days, mainly in the faeces. Approximately 0.4% of the
dose was excreted in the urine. Only small radioactive residues
(maximum 0.0014 mg/kg diquat ion equivalent) were detected in the
milk. The radioactivity in milk was shown to be mainly (77-90%)
incorporated into the natural milk constituents, i.e. the lactose,
fats and proteins. A small percentage of the radioactivity in the
milk was identified as diquat (0.3-2.0%) and compounds II, III, V
and VI (each 0.3-1.0%) (Hemingway et al., 1974).
A study, in which a goat was dosed for five successive days
with diquat-desiccated barley, showed that if a ruminant was
maintained on a diet consisting solely of diquat desiccated fodder,
the residue in the milk would be unlikely to exceed 0.04 mg/kg
diquat ion equivalent. This residue was again due mainly to
incorporation of 14C into natural milk constituents. Residues in
the meat and fat are unlikely to exceed 0.01 mg/kg diquat ion
equivalent (Leahey, 1974).
In feeding trials with non-radiolabelled materials,
diquat-desiccated alfalfa or clover, or silage made from
diquat-desiccated grass, was fed to cows and sheep for
approximately 1-4 months without ill effects. No diquat was
detected in meat, fat or milk (the limit of detection of the
analytical method was normally 0.01 mg/kg) (Black et al., 1966;
Calderbank et al., 1966; and Cardinali et al., 1967).
When diquat-desiccated sunflower seed was fed to cows for
approximately nine months and to sheep for 4 1/2 months, again no
ill effects were noted. No residues of diquat were detected in meat
or milk. Furthermore, no residues of diquat were detected in the
liver and kidneys of a calf born to one of the cows at the end of
the study (limit of detection of the analytical method: 0.03 mg/kg
in meat, 0.01 mg/kg in milk) (Lembinski et al., 1971).
More recently cows were fed for 30 days on diets containing
20, 50 and 100 mg/kg diquat ion (on a dry weight basis) together
with diquat photoproducts which were present as field weathered
residues in grass nuts prepared from grass sprayed with 4 kg diquat
ion/ha. No residues of diquat were detected in milk samples (limit
of detection 0.01 mg/kg) taken throughout the trial. Similarly
residues of diquat in samples of liver, kidney, fat and muscle taken
from animals at the end of the trial were all less than 0.02 mg/kg
diquat (Edwards et al., 1976).
Studies in hens using diquat
When hens were fed on a diet containing residues of diquat,
there was little transfer of residue to the eggs or tissues.
In a series of radiochemical experiments, bridge-labelled
14C-diquat was given to three hens.
i) A single oral dose equivalent to 4-5 mg. diquat ion/kg in the
diet was recovered quantitatively within three days. 98.5% of
the dose appeared in the faeces, 90% of it in the first 24
ii) Following five such daily doses, 94.5% of the radioactivity
was again recovered in the faeces within five days of final
dosing; diquat accounted for 75-80% of this radioactivity.
Approximately 0.06% of the administered radioactivity was
present in eggs: 35-40% as diquat and 55-60% as compound III.
Total radioactive residues in the eggs did not exceed
approximately 0.02 mg/kg diquat ion equivalents. Tissue levels
in this hen seven days after the final dose did not exceed
0.004 mg/kg diquat ion equivalent.
iii) The eggs of a hen given 14 daily doses of 14C-diquat at a rate
equivalent to 0.4-0.5 mg diquat/kg in the daily diet,
contained a maximum residue of less than 0.03 mg/kg diquat ion
equivalent. Tissue levels four hours after the final dose did
not exceed 0.0005 mg/kg diquat ion equivalent (Leahey &
In a longer term experiment, hens were fed a diet containing
up to 10 mg/kg of diquat ion for 6 weeks. Eggs collected throughout
the trial contained no residues of diquat (<0.05 mg/kg) in white
or yolk. Tissues from hens slaughtered after 16, 28 and 45 days
feeding also contained no detectable residues of diquat (<0.05
mg/kg in meat and liver; <0.2 mg/kg in kidney). Food consumption,
egg production and hatchability were not affected by diquat at
these levels. (Edwards & Smith, 1975).
Studies in hens with diquat and its photoproducts
Mature barley plants were sprayed with 14C-ring-labelled
diquat and left in sunlight for 4 days before harvesting. Residues
of diquat and its photoproducts on the grain were measured and the
sample was then fed to three hens. The first was given a single
oral dose, 96% of which was recovered in the faeces within 5 days.
The other two hens were dosed for 11 consecutive days at rates
equivalent to 1.0-1.5 mg/kg diquat and photoproducts in the total
diet. There was a very small transfer of radioactivity into the
eggs, the maxima in the albumen and yolk being 0.0006 and 0.0039 mg
diquat ion equivalent/kg. Radioactive residues in the tissues of one
of the hens sacrificed 4 hours after its final dose did not exceed
0.005 mg/kg diquat ion equivalents, except in the kidney where 0.014
mg/kg was detected.
These results show that, as with other animals, residues of
diquat and its photoproducts are rapidly excreted by hens and there
is only an extremely small transfer of residue into eggs and
tissues (Hughes & Leahey, 1975).
Safety to animals consuming treated fodder
In the above-mentioned studies, ingestion of diquat and its
photoproducts, at levels higher than would be found in practice,
did not produce ill effects in ruminants or hens.
The following national tolerances have been reported to the
TABLE 1. National tolerances reported to the Meeting
Country Commodity Tolerance,
West Germany Potatoes 0.1
Oil seed rape 0.7
All other crops 0.05
Holland Apples )
Green vegetables ) 0.05
Hungary All crops 0.5
USA Sugarcane 0.05
Since diquat was evaluated in 1972 additional information has
been obtained on the fate of the compound in goats, cows, sheep and
Using radio-labelled diquat and its photoproducts it has been
shown in the above ruminants that about 90-96% of these compounds
are excreted in the faeces; small amounts were found in the urine.
Longer term experiments with cows using unlabelled diquat again
showed that over 90% was excreted in the faeces and only a very
small amount (ca 0.01%) in the milk. In another long-term
experiment where cows were fed for 30 days on diets containing both
diquat and its photoproducts, residues in milk were undetectable
throughout the trial (limit of detection 0.01 mg/kg).
When hens were fed on a diet containing residues of
radio-labelled diquat there was little transfer to the eggs or
tissue, 94-98% was excreted in the faeces and only 0.06% was found
in the eggs. Food consumption, egg production and hatchability were
not affected during these experiments.
These further studies support the recommendations made in 1972
for limits in milk, meat and meat products at or about the limit of
determination. A further residue limit in eggs is recommended.
In addition to the previous recommendations the following
maximum residue limit is recommended.
Commodity Limit, mg/kg
* at or about the limit of determination.
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