DIQUAT JMPR 1978
Explanation
Diquat was evaluated by the 1970, 1972 and 1976 Joint Meetings
(FAO/WHO, 1971b, 1973b, 1977b); brief mention is also made in the
evaluations of the 1977 Meeting (FAO/WHO, 1978b). The 1972 Meeting
indicated that further data on residues occurring in barley, wheat,
rye and oats and their products would be required if diquat-treated
cereals were to be used for human consumption. The limits
recommended at that Meeting for cereals were set on the
understanding that the bulk of the treated crops would be used for
animal feed or seed purposes only. Similar questions were raised at
the 1977 Meeting of the Codex Committee on Pesticides Residues
(CCPR, 1978). Data concerning these matters have now been made
available and are reviewed together with certain other information,
in this monograph addendum.
RESIDUES IN FOOD AND THEIR EVALUATION
USE PATTERN
Cereals are desiccated with diquat to prevent the loss of laid
crops. The most important potential source of human intake of
diquat residues is wheat, to which this review in primarily
directed. Uses on other cereals do not contribute significantly to
the potential human Intake of diquat residues. Diquat in
recommended for the desiccation of laid barley and laid oats
intended for animal feed use only. Moreover, laid barley and laid
oats considered to be of generally inferior quality and are not
normally used in brewing or in porridge manufacture. Diquat is not
currently recommended for use on laid rye but there is some use on
rice.
Lodging of wheat is not a widespread problem every year. In
North-Western Europe, lodging in generally a problem only one year
in three. This is because the most important of the several factors
which can affect lodging is the weather - heavy wind and rain can
result in considerable yield losses, particularly if damage occurs
about two months before harvest. Daring the last twenty years, bad
weather and heavy lodging occurred in the UK in 1958, 1962, 1965,
1968, 1974 and, to a lesser extent, 1977. 1971 was a year in which
wind and rain were insufficient to cause wheat to lodge, but
continuing mild, cloudy weather with high humidity resulted in
considerable loss owing to grain sprouting in the ears of standing
crops.
Little data have been published on the extent to which laid
cereals are a problem in a bad year. However, on a national basis
only a small proportion of the crop in likely to be affected. In
the UK in 1965, when laid wheat was widespread, approximately 15%
of the crop was estimated to be severely lodged, with moderate
lodging in a further 10-20% (Barrett et al., 1967). In France, it
is estimated that in a normal year about 1-2% of the cereal acreage
can be lodged and that in a wet year, or year following a dry year,
up to 10% of the total acreage can be lodged. A similar pattern
appears in Germany were estimates indicate that between 0.5% and
5% of the total cereal acreage can be affected, depending upon the
year.
Although only a small proportion of the crop is likely to be
affected nationally, considerable regional variations can occur and
lodging can be a matter of agronomic and economic importance in
localized areas or to individual growers. In 1965, when laid wheat
was a particular problem in the UK, in those regions where lodging
occurred up to 70% of the acreage was affected (Barrett et al.,
1967). In fields where lodging occurs, up to 70% of the potential
yield can be lost (Calidcott, 1966).
Although the use of chlormequat and/or short-strawed varieties
of wheat is becoming more widespread, they are not used exclusively
and it is anticipated that laid wheat will continue to be a matter
of economic importance to individual growers in some years. Where
serious lodging occurs, diquat can be used to minimize potential
yield losses by removing weed growth and secondary tillers. It is
effective on wheat, barley and oats at a rate of 0.6-1.2 kg ai per
ha, applied at least four days before harvest, under conditions
which generally prevail in North-Western Europe. Lower rates, of
the order of 0.3-0.4 kg ai per ha, can be effective under
conditions of high light intensity, eg in Australia, New Zealand
and Canada.
RESIDUES RESULTING FROM SUPERVISED TRIALS
Wheat
Data on diquat residues in desiccated wheat are available from
trials in Australia, New Zealand, Canada, Germany, Switzerland and
the UK, in the period 1963/75. In the range of effective use rates,
a mean residue of 0.5 mg/kg was found (Table 1). More than 200
analytical results are available, only four of which exceeded the
maximum residue limit of 2 mg/kg recommended by the 1972 Meeting
(FAO/WHO, 1973a,b) and none exceeded 3 mg/kg (Calderbank and Yuen,
1963, Calderbank and McKenna, 1964; Edwards et al., 1976; Gardiner,
1977; Hayward, 1974, 1975; McKenna, 1966, 1967; Reeve, 1972; Ward
1973; Willis, 1968).
Barley
Unlike wheat, barley does not lose its hull and hence the
associated spray-deposited residue of diquat, during threshing.
Therefore diquat residues in barley grain are likely to be higher
than those in desiccated wheat grains. Trials in Canada, France,
Germany, Sweden and the UK in the period 1962-75 showed a mean
diquat residue of 1.7 mg/kg in barley grain desiccated at effective
use rates (Table 2). Three of the 43 samples analysed contained
residues in excess of the 5 mg/kg maximum residue limit recommended
by the 1972 Meeting (FA0/WHO, 1973a,b). (Calderbank and Yuen, 1963;
Calderbank and McKenna, 1964; Gardiner, 1977; McKenna, 1966;
Edwards et al., 1976).
TABLE 1. Diquat residues in desiccated wheat
Application Residues (mg/kg) of diquat
Country Year rate ion
(kg diquat
ion/ha) Range Mean
Australia 1967 0.28 - 0.55 <0.05 <0.05 (6)
New Zealand 1966 0.4 0.32 - 1.3 0.63 (23)
0.8 0.91 - 2.3 1.5 (20)
1972-74 0.28 - 0.3 (<0.05 - 0.25 0.10 (26)
0.55 - 0.6 0.05 - 0.8 0.25 (28)
1.2 0.14 - 1.6 0.61 (12)
Canada 1973-75 0.28 < 0.02 - 0.08 0.05 (3)
0.56 < 0.02 - 0.12 0.07 (2)
Germany 1974-75 0.6 < 0.05 - 0.21 0.11 (4)
1.2 < 0.05 - 0.44 0.19 (4)
Switzerland 1965 0.8 - 2.1 (1)
UK 1962-65 0.4 - 0.75 < 0.05 - 3.0 0.44 (41)
1.2 0.15 - 1.5 1.3 (5)
1973 0.6 <0.1 - 0.3 0.14 (15)
1.2 <0.1 - 0.8 0.26 (15)
Overall mean 0.49 (205)
Figures in brackets are the numbers of results upon which each mean
is based.
In calculating the mean of finite and `non-detectable' results, a
value equal to the limit of determination was applied to the
`non-detectable' results.
TABLE 2. Diquat residues in desiccated barley
Application Residues (mg/kg) of diquat
Country Year rate ion
(kg diquat
ion/ha)
Range Mean
Canada 1965 0.25 0.41 - 0.44 0.43 (2)
0.5 1.2 - 1.5 1.3 (2)
France 1963 0.5 - 0.8 1.5 - 1.7 1.6(2)
Germany 1974-75 0.6 0.28 - 0.81 0.58 (4)
1.2 1.2 - 2.0 1.6 (4)
Sweden 1962 0.5 - 1.1 (1)
UK 1962-65 0.4 - 0.75 0.40 - 6.2 1.9 (27)
1.2 1.1 - 7.4 3.2 (5)
Overall mean 1.7 (47)
Figures in brackets are the numbers of results upon which each mean is
based.
TABLE 3. Diquat residues in desiccated oats
Application Residues (mg/kg) of diquat
Country Year rate ion
(kg/diquat
ion/ha)
Range Mean
Canada 1975 0.28 0.14 - 0.51 0.33 (2)
0.56 0.14 - 1.4 0.80 (2)
UK 1962-63 0.4 - 0.6 0.24 - 1.7 0.93 (5)
1.2 2.4 - 3.8 3.1 (3)
Overall mean 1.3 (12)
Figures in brackets are the numbers of results upon which each mean is
based.
Oats
Like barley, oats do not lose their hull, and hence the
associated spray-deposited residue of diquat, during threshing. In UK
and Canadian trials, residues up to 3.8 mg/kg diquat were found in oat
grains which had been desiccated with diquat at effective use rates.
The mean residue of 1.3 mg/kg was similar to that found in barley
(Tables 2 and 3) (Calderbank and Yuen, 1962; Calderbank and McKenna,
1963; Gardiner, 1977).
Rice
No new data on diquat residues in rice were available to the
Meeting. A review of earlier data (FAO/WHO, 1971b. 1973b) confirmed
that the bulk of the residue is associated with the husk, residues
were seldom detected in de-husked (brown) or polished rice and when
observed were below 0.16 mg/kg with a mean level of 0.07 mg/kg.
FATE OF RESIDUES
In water and on plants
Diquat undergoes rapid and extensive photochemical degradation on
plants and in water. Photochemical degradation products of diquat are
listed in Figure 1 and a proposed scheme for the photochemical
degradation of diquat in water is displayed in Figure 2.
In water maintained in sunlight, 70% degradation of diquat occurs
in 1-3 weeks (Cavell et al., 1978a; Smith and Grove, 1969). The major
photochemical degradation product is 1,2,3,4-tetrahydro-1-oxopyrido-
2H-[1,2-a]-5-pyrazinium ion (TOPPS) (II) (Slade and Smith 1967;
Smith and Grove, 1969). A four-carbon or smaller moiety must also be
formed. On further irradiation, TOPPS is cleaved to yield picolinamide
(III) and picolinic acid (IV) (Smith and Grove, 1969).
6-hydroxypicolinic acid (V) is formed from picolinic acid by
ultra-violet light (Kurokawa et al.# 1973) and is further degraded by
sunlight (Redemann and Youngson, 1968). It is quite likely that the
photochemical degradation of 6-hydroxypicolinic acid parallels the
known bacterial oxidation of picolinamide, picolinic acid and
6-hydroxypicolinic acid studies by Orpin et al., (1972). During
bacterial oxidation, 6-hydroxypicolinic acid is degraded to
2,5-dihydroxypyridine, to maleamic acid and thence to maleic acid
which isomerises to fumaric acid (Orpin et al., 1972). Photochemical
cleavage of TOPPS also yields glyoxal (VI) which is then oxidised
further to oxalic acid (VII), formic acid (VIII) and carbon dioxide
(Cavell at al., 1978a).
FIGURE 1a;V078PR07.BMP
The photochemical degradation of diquat on plants is more complex
than that found in water. On diquat-desiccated wheat and barley more
than 90% of the residue has been accounted for. Diquat itself normally
constitutes the most important single compound, eg 10-30% of the
residue. TOPPS constitutes the most important single photochemical
degradation product. No other well-defined major degradation product
is formed. Oxalic acid and diquat monopyridone (IX) are minor
degradation products, normally constituting approximately 2% of the
total residue in each case. A large proportion (eg 50-70%) of the
14C-radiolabel applied as diquat is found as an ill-defined
complex mixture of compounds which streak on thin-layer
chromatoplates. A spectrum of 14C-products is formed, with molecular
weights ranging from less than 700 to greater than 10,000. No
individual 14C-labelled component predominates and each generally
constitutes less than 5% of the total residue (Cavell et al., 1978a,b;
Leahey et al., 1973). At least in part, the higher molecular weight
compounds are likely to have been formed by the reaction or
incorporation of small molecular weight compounds, for example
glyoxal, formic acid and carbon dioxide, with/into natural plant
constituents, to yield water soluble polysaccharides and proteins.
TOPPS is of low mammalian toxicity (Figure 1). All other
degradation products are individually minor. Diquat itself constitutes
the single major residue of potential toxicological concern and
therefore it is this residue which has been the subject of detailed
scrutiny in the other parts of this review.
In animals
The available data on the fate of diquat and its photoproducts in
ruminants and poultry were reviewed at the 1976 Meeting (FA0/WHO,
1977b).
Following oral administration, diquat and its photoproducts are
rapidly eliminated by ruminants, primarily (90% or more) in the
faeces. Diquat undergoes little metabolism in ruminants. Only very
small residues are secreted in milk - eg less than 0.02% of a
radiolabelled dose of diquat. The radioactivity secreted in milk is
due mainly (to 90%) to incorporation into natural milk constituents
such as lactose, fats and proteins. Diquat, TOPPS (II) and the
monopyridone (IX) have been identified as components of the residue in
milk.
When diquat-desiccated alfalfa, clover, or sunflower seeds, or
silage made from diquat desiccated grass, were fed to cows and sheep
for approximately 1-4 months, no ill effects were observed. Diquat
residues in meat and milk were consistently within the maximum re
residue limits proposed by the 1972 Meeting (0.05 mg/kg in meat, 0.01
mg/kg in milk).
Approximately 95% or more of an oral dose of diquat is excreted
by hens in 3-5 days, principally (75-80%) as unchanged diquat.
Approximately 0.06% of the dose is transferred to the eggs, 35-40% as
diquat and 55-60% as the monopyridone (IX).
Cereals can form up to 50% of the diet of lactating ruminants,
70% of the diet of poultry and 100% of the diet of animals grown for
meat production. The dietary levels used in these studies on residues
of diquat in products of animal origin are sufficient to cover the
residue levels of diquat which are likely to appear in animal
feed-stuffs as a result of laid cereals being desiccated. The maximum
residue limits recommended by the 1972 and 1976 Meetings of 0.01 mg/kg
in milk and eggs and 0.05 mg/kg in meat are sufficient to cater for
the use of diquat as a desiccant of laid cereals (FAO/WHO, 1973b,
1977b).
In processing
Wheat
Extensive data are available from trials conducted in the UK,
Germany and New Zealand in the period 1963-75 (Calderbank and McKenna,
1974; Calderbank and Springett, 1971; Edwards et al., 1976; Reeve,
1972).
The spray-deposited residue of diquat in desiccated wheat is
located primarily in the outer layers of the grain. Overall, residue
levels in bran are approximately twice those in whole grains. The
highest residue found in bran following desiccation at effective use
rates was 2.7 mg/kg (Table 4). Residues in fine offal are comparable
to those in whole grains (Table 4).
Residues in white flour are generally 20-25% of those in whole
grains. At effective use rates all of the 50 samples of white flour
analysed contained residues within the maximum residue limit of 0.2 mg
recommended by the 1972 Joint Meeting. The highest value recorded was
0.19 mg/kg (Table 4).
The baking process does not materially affect diquat residue
levels, and diquat residues in white flour and in white bread are
essentially the same. Levels in wholemeal bread are slightly smaller
than those in whole grains - this is accounted for by the higher
moisture content of bread than grain (Tables 4 and 5).
Barley
As stated previously diquat is recommended for the desiccation of
laid barely intended for animal use only. In any case, laid barley is
generally considered to be of an inferior quality and not normally
used in brewing. Nevertheless to cater for the possibility that a
small quantity of treated barley might occasionally be used in beer
production, a limited amount of data has been produced on diquat
TABLE 4. Diquat residues in processing fractions of wheat
Application Residues (mg/kg) of diquat ion
Country rate
and year (kg diquat
ion/ha) Whole grain Bran Fine offal White flour White bread
New Zealand 0.28 <0.05 - 0.25 - <0.05 - 0.33 <0.05 -
1972
(Means) 0.10 (4) - 0.19 (4) <0.05 (4) -
UK 0.6 <0.1 - 0.3 <0.1 - 1.5 <0.1 - 0.6 <0.05 - 0.08 <0.05
1973
1965 0.75 0.95 - 1.0 1.2 - 2.4 0.57 - 0.58 0.10 - 0.12 0.09 - 0.14
Germany 0.6 <0.05 - 0.21 <0.05 - 0.48 - <0.05 - 0.13 <0.05 - 0.08
1974-75
New Zealand 0.55 - 0.25 - 0.84 - <0.05 - 0.09 -
1971 (means) 0.27 (14) 0.55 (24) 0.28 (10) 0.07 (28) 0.07 (8)
UK 1.2 0.71 - 1.5 0.59 - 2.3 0.66 - 1.9 <0.05 -
1963
1973 <0.1 - 0.8 <0.1 - 2.7 0.1 - 0.84 <0.05 - 0.08 <0.05
Germany 1.2 <0.05 - 0.44 0.16 - 1.1 - <0.05 - 0.19 <0.05 - 0.07
1974-75
(means) 0.39 (16) 0.78 (16) 0.43 (16) 0.07 (19) <0.05 (6)
Figures in brackets are the numbers of results upon which each mean is based.
TABLE 5. Diquat residues in wheat grain and wholemeal bread
Application Residues (mg/kg) of diquat
Country Year rate ion in
(kg diquat
ion/ha)
Whole grain Wholemeal
bread
UK 1973 0.6 <0.1 - 0.2 <0.05 - 0.1
1965 0.75 0.95 - 1.0 0.41 - 0.51
Germany 1974-75 0.6 <0.05 - 0.21 0.09 - 0.33
(Means) 0.29 (10) 0.20 (10)
UK 1973 1.2 <0.1 - 0.3 <0.05 - 0.6
(Means) 0.16 (5) 0.19 (5)
Overall mean 0.25 (15) 0.20 (15)
Figures in brackets are the numbers of results upon which each mean in based.
TABLE 6. Diquat residues in barley grain and in beer
Commodity Residues (mg/kg) of diquat ion
analysed
whole grain 4.2 2.4 0.94
Beer 0.15, 0.12, 0.14 0.07, 0.06, 0.03, 0.03,
0.03 0.02
0.10, 0.13, 01.3 0.05, 0.06, 0.03, 0.01,
0.04 0.03
0.12 0.05
(Means) 0.13 0.05 0.025
Ratio of
Mean residue in beer
Residue in grain 3.1% 2.1% 2.7%
(expressed as %)
residues in melting and brewing (Calderbank and Springett, 1971). The
figures indicate that residues in beer would be approximately 2-3% of
those found in whole grain (Table 6). With mean residues in barley
grain of 1.7 mg/kg at effective use rates (Table 2), it can be
anticipated that diquat residues in beer would be of the order of 0.05
mg/kg in the event that treated barley was used for melting and was
not diluted with untreated grain first.
NATIONAL MAXIMUM RESIDUE LIMITS
The following additional national MRLs have been reported to the
Meeting.
COUNTRY COMMODITY MRL, mg/kg
Australia Barley, poppyseed, rice in husk 5
Rapeseed, sorghum, wheat 2
Cottonseed, beans, sunflower seed, rice (polished) 1
Potatoes, wheat flour 0.2
Onions, maize, sugarbeet, peas 0.1
Cottonseed, rape, sesame, sunflower oils,
other vegetables crops 0.05
Meat and meat products 0.05
Milk (whole) 0.01
Czechoslovakia Barley for feeding 3
Clover, alfalfa 3
Rye for feeding 2
Silage 1:1 (desilated to non-desilated) 1.5
Rapeseed 1
Peas 0.1
Potatoes 0.05
Milk (smallest detectable quantity of
their method) 0.01
COUNTRY COMMODITY MRL, mg/kg
Federal Republic Potatoes 0.1
of Germany
Rapeseed 0.7
All other crops 0.05
Hungary Animal feed 2
Oily crops 0.8
Other crops 0.5
Netherlands Fruit, vegetables (incl. potatoes)
spices 0.05
Sweden Fruit, vegetables, potatoes 0.1
Cereals, beans 0.5
Milled cereal products 0.2
Switzerland Fruit, vegetables 0.02
Appraisal
In response to requests from earlier Meetings, further data
concerning residues of diquat in some cereals and their products were
submitted for evaluation. These data covered the potential use of
diquat as a desiccant to rescue lodged wheat for possible human
consumption; barley and oats so treated are normally only regarded as
suitable for animal feed or seed purposes. Beer made from treated
barley contained only 2-3% of the diquat present in the grain. Further
information on the degradation of diquat in water, on plants, in
animals and during the processing of cereals was also reviewed. The
data provided generally confirmed the maximum residue limits
recommended by the 1972 and 1976 Meetings with the exceptions of the
additions and amendments listed below.
RECOMMENDATIONS
Commodity Limit, mg/kg diquat ion
Wheat bran 5
Wheat flour (wholemeal) 2
Wheat flour (white) rice (hulled
and/or polished) 0.2
REFERENCES
Barrett, D.W.A., Meens. B.E. and Mees, G.C. The control of lodging
in cereals by (2-chloroethyl)-trimethyl ammonium
chloride (CCC). J. Agric. Sci., Camb., 68:39-46.
Calderbank, A. and Yuen, S.H. Bipyridylium herbicides: residues of
(1963) diquat and paraquat in food crops. ICI Plant
Protection Ltd., Report No. PP/E/231
(unpublished).
Calderbank, A. and McKenna, R.H. Bipyridylium herbicides: residues of
(1964) diquat and paraquat in food crops from 1963
trials. ICI Plant Protection Ltd., Report No.
PP/E/292 (unpublished).
Calderbank, A. and Springett, R.H. Diquat residues in cereal grain
(1971) and processed parts (eg flour and malt) following
use of `Reglone' as a pre-harvest desiccant. ICI
Plant Protection Ltd., Report No. TMJ644A
(unpublished).
Caldicott, J.J.B. The interaction of Cycocel (2-choroethyl)-trimethyl
(1966) ammonium chloride and nitrogen top dressing on the
growth lodging and yield of wheat. Proc. 8th Dr.
Weed Control Conf., 1:229-239.
Cavell, D.B., Francis, P.D., Goddard, C and McIntosh, S.
(1978a) Photochemical degradation of diquat in water and
on plants. Status report. ICI Plant Protection
Division Report No. RJ0038A (unpublished).
Cavell, B.D., Francis, P.D. and Goddard, C. Fractionation of the
(1978b) photoproducts formed from 14C-diquat on barley
ICI Plant Protection Division Report No. RJ0039A
(unpublished).
CCPR Report of the 1977 Meeting of the Codex Committee on Pesticide
(1978) Residues, ALINORM 78/249 para. 91-93.
Edwards, M.J., Hayward, G.J. and Ward, R.J. Diquat: residues in
(1976) grain, flour and bread - UK and German trials,
1973-5. ICI Plant Protection Division Report No.
AR2682A (unpublished).
FAO/WHO 1970 evaluations of some pesticide residues in food.
(1971) FAO/AGP:1970/M/12/1, WHO Food Add./71.42.
FAO/WHO 1972 evaluations of some pesticide residues in food.
(1973) FA0/AGP:1972/M/9/1, WHO Pest. Res. Series No, 2.
FAO/WHO 1976 evaluations of come pesticide residues in food.
(1977) FAO/AGP:1976/M/14.
FAO/WHO 1977 evaluations of some pesticide residues in food. (in
(1978) press).
Gardiner R.P. Summary of residues data from 1975 small plot trials.
(1977) Chipman Chemicals Ltd. Canada data (unpublished).
Hayward, G.J. Diquat residues data on wheat grain from New Zealand
(1974) trials 1974. ICI Plant Protection Division data
(unpublished).
Hayward G.J. Diquat residues data on wheat grain from Canadian
(1975) trial 1973. ICI Plant Protection Division data
unpublished).
Kurokawa, H., Furihata, T., Takeuchi, F. and Sugimori, A.
Photoreactions of heteroaromatic compounds. III
Photohydroxylation and alkoxylation of 2-pyridine-
carboxylic acid in acidic aqueous and alcohol
solutions. Tetrahedron Letters, 28:2623.
Leahey, J.P., Griggs, R.E. and Allard, G.B. Diquat: residues of
(1973) diquat and its photoproducts on barley and oats
after desiccation with 14C-diquat. ICI Plant
Protection Ltd. Report No. AR2478B (unpublished).
McKenna, R.H. Bipyridylium herbicides: residues of diquat in
(1966) food crops from 1964 and 1965 field trials. ICI
Agricultural Division Report No. A 126, 493
(unpublished).
McKenna, R.H. Diquat residues data on wheat grain from New Zealand
(1967) trials, 1966. ICI Plant Protection Ltd, data
(unpublished).
Orpin, C.G., Knight, M. and Evans, W.C. The bacterial oxidation
(1972) of picolinamide, a photolytic product of diquat.
Biochem. J., 127:819.
Redemenn, C.T. and Youngson, C.R. The partial photolysis of
(1968) 6-chloropicolinic acid in aqueous solution. Bull.
Env. Cont. and Tox., 3:97.
Reeve, M. Diquat residues data on wheat grain, white flour, offal
(1972) and bran from New Zealand trials, 1971-72. ICI
Plant Protection Ltd. data (unpublished).
Slade, P. and Smith, A.E. Photochemical degradation of diquat.
(1967) Nature, 213, No. 5079:919.
Smith, A.E. and Grove, J. Photochemical degradation of diquat in
(1969) dilute aqueous solution and on silica gel. J.
Agric. Food Chem. 17, No. 3:609.
Ward, R.J. Diquat residues data on wheat grain from New Zealand
trials, 1973. ICI Plant Protection Division data
(unpublished).
Willis G.A. Diquat residues data on wheat grain from Australian
(1968) trials 1967. ICI Plant Protection Ltd. data
(unpublished).