Sponsored jointly by FAO and WHO


    The monographs

    Data and recommendations of the joint meeting
    of the FAO Panel of Experts on Pesticide Residues
    in Food and the Environment and the
    WHO Expert Group on Pesticide Residues
    Rome, 24 September - 3 October 1984

    Food and Agriculture Organization of the United Nations
    Rome 1985



         Maleic hydrazide was evaluated in 1976, 1977 and 1980 1/. A
    temporary ADI was estimated for the sodium and potassium salts in 1980
    and previously recorded Guideline Levels were converted to temporary

         An extensive review of the biological and economic features of
    maleic hydrazide in the USA was available which, together with other
    information, enabled the meeting to deal with questions raised by the
    1976 and 1980 JMPRs. (US Department of Agriculture, 1979).


    Purity of the Technical Product

         It was stated in the 1980 evaluation that the temporary ADI
    estimated at that meeting referred to maleic hydrazide containing up
    to 15 mg/kg hydrazine. This was a typographical error. This was the
    upper limit of the concentration of hydrazine present in maleic
    hydrazide used in animal feeding studies that did not give rise to the
    development of cancer in laboratory animals. The figure should have
    been 1.5 mg/kg. The figure was correctly recorded as 1.5 mg/kg in the
    report of the 1980 meeting.

         Liu et al. (1974) reported that commercial maleic hydrazide
    formulations may contain amounts of hydrazine varying from 0.14 to
    870 mg/kg. Bakker et al. (1983) reported the results of a survey
    of maleic hydrazide formulations including several stored at 50C for
    10 weeks. Fourteen commercial formulations with maleic hydrazide
    concentrations ranging from 180-360 g/l were investigated. The
    hydrazine content of the maleic hydrazide in these formulations ranged
    from less than 0.05 to 53 mg/kg. During the storage of two samples at
    50C for 10 weeks, the hydrazine contents increased from 2.2 to 120
    and 0.4 to 54 mg/l. Thirteen of the formulations were diethanolamine
    salts, the other a potassium salt. The hydrazine content of the
    potassium salt was less than 0.05 mg/kg. In later studies (personal
    communication) it was found that all samples of the potassium salt
    contained less than 1 mg/kg of hydrazine and that the potassium salt
    was stable to storage at 50C.


    1/  See Annex 2 for FAO and WHO documentation

         The withdrawal by the principal US manufacturer of the
    diethanolamine salt in 1980 has reduced the concern over the possible
    presence of hydrazine as an impurity in commercial formulations. The
    meeting was not convinced that all sources of the diethanolamine salt
    had yet disappeared from European markets but recommended that the FAO
    Specification for maleic hydrazide should be adopted and enforced
    world-wide. This incorporates a limit of 1 mg/kg for hydrazine as an
    impurity in maleic hydrazide.



         Two formulations of maleic hydrazide (MH), the diethanolamine
    (DEA-MH) and potassium (K-MH) salts, were marketed for many years but
    the DEA-MH formulation was withdrawn by the principal manufacturer in
    1980 when a means was found to increase the efficacy of the K-MH
    formulation. Since then the K-MH form has mainly been used. The rate
    of application is 3.3 kg/ha. The systemic action of MH makes it
    important for the control of tobacco suckers, and of sprouting in
    stored potatoes and onions.

         About 80% (1.45 million kg a.i.) of the total MH marketed in the
    USA is used to control tobacco suckers. Sprout inhibition in potatoes
    for storage accounts for 16.3% (300,000 kg), and about 1.5%
    (30,000 kg) is used to inhibit sprouting of stored onions. K-MH
    formulations are also registered for use on tobacco, potatoes and
    onions in Austria, Australia, Canada, Czechoslovakia, Italy, Kenya,
    The Netherlands and Taiwan among others. It is registered for use on
    tobacco only in Bulgaria, Colombia and Mexico. MH is used on other
    root crops in Canada.

    Use on potatoes

         Stored potatoes will sprout at temperatures above 5C unless
    sprouting is inhibited. At temperatures below 7C however the starch
    in potatoes is converted to sugar, a change which gives rise to an
    unacceptable dark brown colour in cooked potato products such as
    crisps and chips. The use of an inhibitor is therefore virtually

         The USA produces about 1.37 million tonnes of fall-crop potatoes
    annually, 1.2 million tonnes of which are stored for 1 month or
    longer. More than half of the total US production of potatoes is grown
    in three Pacific Northwest States, Idaho, Washington and Oregon, which
    are also the major processing States. Thirty-one percent of the total
    potato production used for food is processed into some type of frozen
    product, 13% is processed into potato chips, 7% is dehydrated, 2% is
    canned, and the remaining 47% is marketed fresh. Some kind of sprout
    inhibitor is used on about 60% of fall-produced potatoes.

         MH is applied in the field to the green vines of fall crop
    potatoes destined for storage. The treatment prevents sprouting and
    maintains the original high quality. The best figures available
    indicate that 220,000 tonnes of potatoes are treated each year with
    MH, using 272,000 to 320,000 kg of MH. This is applied to 80,000 to
    100,000 hectares of potatoes. Yields are not reduced if the
    recommended rate of one application at 3.3 kg ai/ha is applied to the
    vines 2 to 3 weeks after full bloom. Application of MH too early in
    the growing season, or application of more than double the recommended
    rate, can cause a cracking of the bud end, and in some cases an
    internal brown spot flecking on the apical end of the potato tuber. MH
    is absorbed through the leaves and translocated into the tubers, where
    it suppresses external and internal sprouting.

         Because of its internal mode of action, MH is the only chemical
    that will prevent tubers left in the field at harvest from growing
    into volunteer plants the next year. If allowed to grow, these
    volunteer plants serve as a source of leaf roll virus, which reduces
    yields and causes an internal discolouration in many varieties, called
    "net necrosis." This often renders the tubers unusable for table stock
    and many forms of processing. Inhibition of volunteer growth of
    potatoes reduces the potential source of leaf roll virus inoculum by
    as much as 98% (Sparks, 1978). Volunteers can also interfere with
    harvesting machinery and contaminate or compete with other vegetables.
    There is no alternative sprout inhibitor that will prevent volunteer
    potatoes, since all other inhibitors are applied after the tubers have
    been harvested and removed from the field. Other methods of control
    are only partially effective, and are more expensive.

         In addition to controlling sprouting of potato tubers, MH has
    been found by some workers to increase the food value and quality of
    tubers. Wittwer and Patterson (1951) state that MH-treated tubers held
    at low temperatures accumulate less sugar and produce lighter-coloured
    potato chips than the non-treated control. Patterson et al. (1952)
    indicated that treatment with MH appeared to result in lower contents
    of reducing sugars (see also the 1976 evaluation).

    Use on onions

         Maleic hydrazide is used to prevent sprouting in stored onions.
    It is applied to onions in the field at a rate of 2.2 kg/ha when the
    bulbs are mature. Five to seven green leaves are essential for
    adequate absorption of the compound. The crop is harvested after 2-4
    weeks and the tops are removed. The onions are then cured before

         In addition to its direct effects, an indirect benefit of MH
    Treatment is the elimination of volunteer onions, which often carry
    disease, in the following season.


    In plants

         MH is absorbed into plant leaves and readily translocated.
    Movement occurs from xylem to phloem and vice versa, and the chemical
    is distributed throughout plants after application to leaves (Crafts,
    1959, 1967; Crafts and Yamaguchi, 1958). In the early work on
    translocation, the translocated compound(s) containing the 14C from
    14C-MH were not identified, but generally assumed to be MH. More
    recent studies with tobacco show that 14C-labelled MH moves as the
    intact molecule from one leaf to another (Frear & Swanson, 1978).

         Burley tobacco was grown and treated with 14C-labelled MH under
    controlled conditions (Davis and Grunwald, 1974). More 14C-MH was
    absorbed at 100% than at 75% relative humidity. Light stimulated the
    uptake of MH in leaf disks, and the increased uptake was not due to an
    increase in transpiration. 14C-MH was translocated and most of the
    translocated radioactivity was recovered from the top leaves.

         In other work by Davis and Atkinson (1976), the concentration in
    the untreated lower leaves of tobacco reached about 100 mg/kg within
    24 hours after treating the upper leaves. Concentrations in the upper
    leaves declined rapidly from 300 mg/kg initially as MH was transported
    away from the treated area. In tests conducted in North Carolina,
    residues in mid-stalk flue-cured tobacco averaged 514 mg/kg
    immediately after application and decreased to 344 mg/kg over a 10-day
    harvest period of dry weather. The decrease was not statistically
    significant, but suggests that residues decline with time. Further, in
    a companion test at another location where 5.7 cm of rain fell on the
    third day after treatment, residues had decreased by 66% by the fourth
    day after treatment.

         Pendergrass (1969) applied 14C-MH to the green foliage of 22
    onion plants near maturity. The 14C-MH had a specific activity of
    0.3 MCi/m mole and was combined with 1.000 mg/l of Dupanol WAQ wetting
    agent. Volumes of 0.1 ml of this solution were injected into the
    internal cavity of four leaves per onion for a total dosage of 10 uCi
    per onion. In a separate treatment, the onions were sprayed with
    unlabelled MH at the normal rate of application for sprout control.
    Results indicated that distribution was fairly uniform, with 69% of
    the applied MH translocated to the bulb. At this point in onion
    growth, about 30 to 35% of the total weight of the plant is in the
    foliage. Of the total MH, 49% was found in the outer bulb leaves, 16%
    in the inner shoot leaves, and 4% in the root plate. The concentration
    in the root plate, however, was six times that found in the outer bulb
    leaves. In culinary preparation most of the root plate of an onion
    bulb is discarded as it is discoloured or appears soiled. It is not
    included in such preparations as, for example, frozen fried onion

         The results of Crafts (1959) showed that MH diffused readily into
    potato tuber tissue within 2 days after application to the cut
    surface. In tests conducted in Canada with MH as a 2500 mg/l spray
    applied to potato foliage 3 weeks after full bloom, sufficient was
    absorbed in 24 hours to inhibit sprouting (Franklin, 1959), and
    residues of 6 mg/kg were found in the tubers. A 48-hour absorption
    period provided complete inhibition of sprouting. MH residues in
    tubers increased to a maximum of 36 mg/kg 1 week after application.

         Although some early work on translocation suggested that MH was
    not readily held in storage tissues along the translocation path
    (Crafts, 1959), other research conducted during the same period
    (Towers et al., 1958) indicated that the -D-glucoside of MH is
    formed in Nicotiana rustica, N. sanderae, and representatives of
    three other plant species. About 15% of the MH in leaf tissues was
    present as the glucoside. Recent studies on the fate of MH in tobacco
    plants confirmed the formation of the -D-glucoside in N. tabacum
    (Frear & Swanson, 1978).

         Although MH residues decline with time, the growth regulator is
    only slowly degraded within plant tissues and residues may persist for
    extended periods. From 17 to 22% of the MH remained unchanged in the
    tobacco plant four weeks after application to leaves, and was
    extractable with methanol (Frear & Swanson, 1978). Methanol-soluble
    metabolites were present in amounts ranging from 14 to 18% of the
    applied 14C. In fresh tissue after short periods (Davis & Grunwald,
    1974), 99% of the 14C-label could be extracted with trichloracetic
    acide-acetone and perchloric acid. In cured tobacco, however, all the
    radioactivity was recovered from the RNA, DNA and proteins. Frear and
    Swanson (1978) also reported that 27 to 33% of the applied 14C was
    present as a methanol-insoluble fraction four weeks after leaf
    treatment. A large amount of this fraction was found in the roots, and
    30 to 40% of the foliar-applied MH was excreted into the growing
    medium unchanged. Davis and Grunwald (1974) showed that relative
    humidity affected the translocation of MH to the stem and to the root.
    Translocation, excretion and glucoside formation were similar in
    selected flue-cured and burley varieties of tobacco (Frear & Swanson,

         Morphological and physiological responses of plants to MH have
    been investigated by workers since its growth-regulating properties
    were first recognized. Summaries of the literature on MH (Zukel, 1957,
    1963) include many references and abstracts as well as a synopsis of
    the major observations in the abstracts cited. MH was discussed
    briefly in reviews by Crafts (1953), Woodford et al. (1958), and
    Shaw et al. (1960).

         Because MH has an exceedingly low vapour pressure (essentially
    zero at 50C), it was concluded that losses of MH from soil and plant
    surfaced by volatilization would be negligible. Observations on loss
    rates of MH from leaf surfaces support this conclusion (Smith
    et al ., 1959).

    In soil

         Published data on loss rates from soil show that MH disappears
    rapidly. In a study reported by Levi and Crafts (1952), oats planted 2
    months after treating the soil were not injured by 80 mg/kg of MH in
    any of 11 California soils. When oats were planted immediately after
    application, however, seedling plants were injured by 5 mg/kg of MH in
    the soil. In some soils, initial concentrations as high as 490 mg/kg
    were not toxic to oats planted 2 months after application. In later
    research with more refined detection methods and under laboratory
    conditions the level of MH in soil dropped from 100 to 5 mg/kg in 3
    weeks (Helweg-Anderson, 1971). Another experiment reported by the same
    author showed that about 90% of 4.5 and 9 kg/ha applications
    disappeared in 12 days. Only traces were present after 80 days. In
    work reported by Hoffman et al. (1962), an application rate of
    2.25 kg ai/ha resulted in 1 mg/kg of residue in the surface 15 cm of
    soil immediately after application. The disappearance of MH was very
    rapid from sand and muck and somewhat less rapid from clay soils.
    Following a farmer application of 15 kg ai/ha of MH on burley tobacco,
    it was not possible to detect any residue in the soil 12 months later.
    Also, in tobacco grown on soil where the crop had been treated with
    the recommended rate (170 mg/plant) of MH the previous year, leaf
    residues of MH were not detected (Davis & Massie, 1977).

         In later studies, Helweg (1975a, 1975b) showed that the
    decomposition of MH at concentrations of 20 mg/kg or less followed
    first-order kinetics, whereas at 120 mg/kg, zero-order kinetics more
    closely described the loss. The addition of activated carbon to soil
    retarded decomposition. With time in the soil, the capacity of carbon
    to retard degradation decreased.

         The rapidity with which phytotoxic effects of MH disappeared
    from soil in the studies by Levi and Crafts (1952) strongly suggested
    that micro-organisms were involved. Lembeck and Colmer (1957) reported
    the isolation of two soil bacteria (Alcaligenes faecalis and
    Flavobacterium diffusum) capable of using the diethanolamine salt of
    MH as an energy source. They also showed that the phytotoxic effects
    of MH were reduced by the action of the bacteria.

         The results of Helweg-Anderson (1971), showing that sterilizing
    the soil by autoclaving or gamma radiation prevented breakdown of MH,
    confirmed the earlier report of Lembeck and Colmer (1957). Kaufman and
    Kalayanova (1977) obtained similar results by the use of potassium
    azide as a soil sterilant. In another publication by Helweg (1975b) on
    the degradation of MH, 45% of the 14C added to soil in a 26 mg/kg
    treatment was liberated in 20 days and 56% was liberated in 255 days
    of incubation. However, micro-organisms capable of utilizing MH as a
    sole source of carbon were not isolated. It was concluded that the
    initial decomposition, if microbial, was by co-metabolism. Recent work
    by Kaufman and Kalayanova (1977) suggested that cleavage of the MH
    ring occurred by chemical mechanisms with subsequent degradation by
    soil micro-organisms. The degradation pathway of MH in soil was
    established in this work, with no evidence for the formation of

         Limited information is available on the movement of MH into soil.
    In a leaching experiment, Levi and Crafts (1952), through the use of a
    bioassay, showed that MH was displaced downward when relatively large
    quantities of water were added to the surface of soil columns. The
    fact that it is rapidly degraded generally eliminates movement below a
    few centimetres into the soil, except when rates of application are
    excessive and when rain occurs soon after application. At recommended
    rates, significant movement below the usual 15-cm plough layer would
    seldom, if ever, occur.



         The MRL for MH residues on potatoes is 50 mg/kg. The average
    residue is 15 to 25 mg/kg, and most residues would fall in a range of
    10 to 40 mg/kg (Sparks, 1978). Routine monitoring of MH on potatoes is
    not known to be carried out at any agency concerned with residue
    analysis, but the potatoes themselves serve as a useful bioassay for
    MH residues at or above the MRL because tuber injury occurs at the 45
    to 50 mg/kg level (Sparks, 1978). Potatoes damaged by MH account for
    an extremely small part of total production and are rejected for
    normal food use.

    On the basis of estimated consumption rates for potatoes and potato
    chips, the average daily exposure for a 60 kg person is about
    0.019 mg/kg per day.


         The MRL for MH in onion bulbs in 15 mg/kg. Experience has shown
    that the amount required for total sprout inhibition is 5 to 7 mg/kg.
    At the recommended application rate the amount of MH absorbed is
    usually within this range.

         Residues of 2 to 7 mg/kg occurred in New York in 1962 when MH was
    applied to onions at different stages of maturity (Isenberg, 1977). In
    the United Kingdom residues of 4.1 to 11.0 mg/kg were found in 1968
    and 2.8 to 3.1 mg/kg in 1969 (Whitewell, 1977).

         The principle manufacturer of MH undertook a survey throughout
    the USA and Canada (Uniroyal, 1984). 39 samples of potatoes from the
    major potato-growing areas were analysed for MH residues by the method
    of Lane et al. (1958) with the results shown in Table 1.

    Table 1.  Residues of maleic hydrazide in onion (survey data)

    Location                Residue, mg/kg
    New York                2, 6, 10
    Ohio                    5, 3, 4, 7
    Michigan                3, 2, 2
    Washington State        2, 2, 3, 3, 12, 5, 15, 7, 12, 5, 3, 3, 6
    Canada                  9, 10, 11, 8, 8, 10, 10, 6, 10, 9, 4, 10,
                            7, 10, 5, 6

    Mean of 39 results      6.5 mg/kg

         At recommended application rates of 2.2 kg/ha, residues are
    generally within the 5 to 7 mg/kg range. Per caput consumption of
    treated onions in 1976 was roughly 2 kg. On the basis of 2 kg and
    6 mg/kg residues, this equals about 0.00055 mg/kg per day for a 60-kg


         The 1976 meeting required that the GLC method should be further
    developed to make it suitable for regulatory purposes.

         The present meeting was advised that considerable time and effort
    had been given to this task but the outcome was still not
    satisfactory. The original method of Wood (1953) as modified by Lane
    et al. (1958) was later developed by the Naugatuck Chemical
    Division of the United States Rubber Company. This development was
    subjected to collaborative study (Lane, 1963, 1965), which showed that
    a wide range of concentrations of maleic hydrazide could be accurately
    determined in any likely substrate. The method recovers both free and
    conjugated maleic hydrazide.


         The meeting had available an extensive review of the biological,
    agricultural and economic implications of the use of maleic hydrazide
    in agriculture, which provided some of the information required by the
    1976 JMPR. Further information was received from the principal
    manufacturer. The diethanolamine salt is no longer available, the
    potassium salt being the only form now in use.

         Maleic hydrazide is essential to the production, storage and
    marketing of both potatoes and onions to prevent sprouting and
    subsequent deterioration of their acceptability and food value. It is
    applied to the growing crop and is distributed uniformally throughout
    the tuber or bulb by systemic transfer. It is therefore possible to
    obtain a long-lasting effect (9 - 12 months) on the whole crop with
    minimal rates of application. This avoids the need for grading and
    rejection of some of the crop prior to marketing.

         An additional advantage derived from spraying potato crops with
    maleic hydrazide is that it prevents the growth of volunteer potato
    plants from tubers left in the field, thus preventing the carry-over
    of potato virus diseases from one crop to the next.

         Valuable information on the fate of maleic hydrazide in various
    components of the environment was available. This confirmed that there
    was no risk of carry-over into subsequent rotational crops.

         Approved treatments of potatoes lead to residues in the tubers
    which, under some conditions, can be in the range 40-50 mg/kg.
    However, the average is between 15 and 25 mg/kg and most residues fall
    within the range 10-40 mg/kg. The potatoes themselves serve as a
    useful bioassay for maleic hydrazide residues at or above the MRL
    because tuber injury occurs at the 45-50 mg/kg level. Potatoes damaged
    in this way account for an extremely small proportion of the total
    production and are rejected for normal food use.

         The MRL for maleic hydrazide in onion bulbs is 15 mg/kg.
    Experience has shown that the amount required for total sprout
    inhibition is 5 to 7 mg/kg. Following the use of maleic hydrazide at
    the approved rate the amount absorbed is usually within this range.
    Monitoring studies indicated residues in the range 2-11 mg/kg in both
    potatoes and onions.

         The distribution of maleic hydrazide in onion bulbs has been
    studied with the labelled compound. Following approved treatments
    approximately 70% of the applied maleic hydrazide is transferred to
    the bulb, which represents about 65-70% of the total weight of the
    onion plant. Of the 70% in the bulb, 48% was found in the outer part,
    16% in the inner shoot leaves and 4% in the root plate. The root plate
    is discarded in culinary practice.

         It had been established that the diethanolamine salt was
    unstable, giving rise to hydrazine. Information available confirmed
    that the commercial formulation of the potassium salt contained less
    than 1 mg/kg of hydrazine. Hydrazine does not occur as a metabolite in
    plants, animals or soil because strong reducing conditions are needed
    to convert maleic hydrazide to hydrazine (e.g. sodium hydroxide and
    zinc dust).

         Maleic hydrazide residues occur as a mixture of the parent
    compound and various conjugates. The method of analysis, which
    involves heating at 160C in a 600g/l solution of sodium hydroxide,
    effectively converts these conjugates into free maleic hydrazide. The
    meeting agreed that the residue should therefore be defined as "sum of
    free and conjugated maleic hydrazide."

         Attempts to develop alternative methods of analysis based on GLC
    procedures have so far been only partially successful.

         Reliable information indicated that there were no significant
    uses on crops other than tobacco, potatoes and onions. The use on
    trees, grass and other ornamentals is small and does not lead to
    residues in food.

         There does not appear to be any possibility that maleic hydrazide
    residues on waste commodities such as potato skins could give rise to
    residues in foods of animal origin when such wastes are used as animal


         The meeting was satisfied that all the information and further
    work listed as required in 1976 had now been supplied and that there
    were no further questions concerned with residues evaluation to be
    pursued. It recommended that the maleic hydrazide used in agriculture
    be confined to the potassium salt which should be 99.9% pure and
    contain not more than 1 mg/kg of hydrazine.

         The meeting agreed that the TMRLs previously recommended were
    appropriate. As an ADI was estimated, the limits were converted to


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    1983      determination of hydrazine in maleic hydrazide formulations
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              genotypes. Final Prog. Rep., USDA Contract No.
              12-14-100-11042(34). pp. 1-31.

    Davis, D.L., & Massie, I. Univ. Kentucky. (Unpublished data).

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              treatment. Weed Res. 15:53-58.

    Helweg, A. Degradation of 14C-maleic hydrazide in soil as influenced by
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    See Also:
       Toxicological Abbreviations
       Maleic hydrazide (Pesticide residues in food: 1976 evaluations)
       Maleic hydrazide (Pesticide residues in food: 1977 evaluations)
       Maleic hydrazide (Pesticide residues in food: 1980 evaluations)
       Maleic hydrazide (Pesticide residues in food: 1984 evaluations)
       Maleic hydrazide (Pesticide residues in food: 1996 evaluations Part II Toxicological)
       Maleic Hydrazide (IARC Summary & Evaluation, Volume 4, 1974)