DIQUAT         JMPR 1978


         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.



         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

         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

         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 

         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.



         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).


         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                                   
                                                      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

    TABLE 3. Diquat residues in desiccated oats

                                    Application       Residues (mg/kg) of diquat
    Country           Year          rate                         ion
                                                      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


         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).


         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.


    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 1

    FIGURE 1a;V078PR07.BMP

    FIGURE 2

         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,

         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

         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,

    In processing


         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,

         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).


         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               -
                   (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
    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

    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             -
    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
                   (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                                       
                                                 Whole grain      Wholemeal

    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

    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.


         The following additional national MRLs have been reported to the

    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

         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.


    Commodity                               Limit, mg/kg diquat ion

    Wheat bran                              5
    Wheat flour (wholemeal)                 2
    Wheat flour (white) rice (hulled
    and/or polished)                        0.2


    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

    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

    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

    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

    Hayward G.J. Diquat residues data on wheat grain from Canadian
    (1975)              trial 1973. ICI Plant Protection Division data

    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

    McKenna, R.H. Diquat residues data on wheat grain from New Zealand
    (1967)              trials, 1966. ICI Plant Protection Ltd, data

    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

    Willis G.A. Diquat residues data on wheat grain from Australian
    (1968)              trials 1967. ICI Plant Protection Ltd. data

    See Also:
       Toxicological Abbreviations
       Diquat (HSG 52, 1991)
       Diquat (PIM 580F, French)
       Diquat (AGP:1970/M/12/1)
       Diquat (WHO Pesticide Residues Series 2)
       Diquat (Pesticide residues in food: 1976 evaluations)
       Diquat (Pesticide residues in food: 1977 evaluations)
       Diquat (Pesticide residues in food: 1993 evaluations Part II Toxicology)