4.21 Terbufos (167)

RESIDUE AND ANALYTICAL ASPECTS

Terbufos, a systemic nematicide and soil insecticide, was evaluated for the first time by JMPR in 1989. A further residue review was undertaken in 1990. At the 36^th Session of the CCPR the compound was scheduled for a residue evaluation within the periodic review programme for 2005. The toxicological review was conducted in 2003, which established an ADI of 0.0006 mg/kg bw/day and an ARfD of 0.002 mg/kg bw/day.

The Meeting received information on identity; metabolism and environmental fate; analytical methods; relevant storage stability studies; use pattern; residues resulting from supervised trials on a number of crops including bananas, coffee beans, sugar beets, maize, sorghum, and sweet corn; residues in food in commerce and at consumption and national maximum residue limits.

List of terbufos and related metabolites:

Animal metabolism

The Meeting received information on the fate of [methylene-¹⁴C]terbufos in rats, lactating goats and laying hens dosed orally.

Studies on metabolism in rats were evaluated by the WHO Expert Group of the 2003 JMPR, which concluded that absorption of single doses of ¹⁴C-labelled terbufos was rapid and fairly complete. Most of the radiolabel was excreted within 24-48

h. Excretion was primarily by the urinary route (about 70-80% of the administered dose). Terbufos was extensively metabolized and little radioactivity was found in the tissues. Sulfoxidation and desulfuration of terbufos is followed by hydrolysis of the thiolophosphorus bond, enzymatic S-methylation and then additional S-oxidation. On the basis of a 14-day study of repeated doses, terbufos showed little potential for accumulation.

[Methyllene-¹⁴C]terbufos at doses equivalent to 0.281 and 2.53 mg/kg body weight, were administered via capsule to two lactating goats separately, i.e., one dose regime per goat. Each goat was dosed once daily for seven consecutive days. The major route of excretion was via the urine, which accounted for 96.0 and 86.9% of the administered radioactivity respectively. The main metabolic pathway in lactating goats and rats is qualitatively similar, thus suggesting a common metabolic pathway. Neither terbufos nor any of the phosphorylated oxidative metabolites - sulfoxide, sulfone, oxygen analog and its sulfoxide and sulfone - were observed in milk. None of the phosphorylated oxidative metabolites were detected in tissues. However, terbufos (parent) was observed at low concentrations in liver (< 0.01 mg/kg eq) and in kidney (< 0.01 mg/kg eq).

The total radioactive residue (TRR) in daily milk samples were < 0.01 mg/kg eq (low dose, 0.28 mg/kg eq in diet, day 7) and 0.02-0.03 mg/kg eq (high dose, 2.53 mg/kg eq in diet, day 7). Residues in the liver, kidney, muscle and fat of the low dose animal were all < 0.01 mg/kg eq. In the high dose animal, residues were 0.08, 0.04, < 0.01 and < 0.01 mg/kg eq, respectively.

Two groups of laying hens were dosed via capsules with [methyllene-¹⁴C]terbufos for five consecutive days with the feed equivalent of 0.35 ppm for one group (Group B) and an exaggerated level of 1.05 ppm equivalent for the second group (Group C). Recovery of [¹⁴C] residues in excreta over the 5-day treatment period averaged 91.4% of the total administered dose for the 1^st group, and 88.9% for the 2^nd group. For both dose levels, residues in eggs (days 1 through 5, both white and yolk), skin with adhering fat, muscle, liver or kidney tissues were all less than the LOQ of the radioassay (< 0.05 mg/kg eq).

The results of the hen study showed that terbufos when orally ingested at highly-exaggerated levels does not give rise to residues in the eggs or edible tissues of the laying hen.

Plant metabolism

The Meeting received information on the metabolic fate of [¹⁴C]terbufos in soybeans, sugar beet, sweet corn, cabbage and rape seed.

Soybean plants were grown under field conditions from seed treated in the furrow at a rate of 1.1 kg ai/ha with [methylene-¹⁴C]terbufos. The TRR levels found in the plant, expressed as terbufos equivalent, were 13.3 and 1.5 mg/kg in plants at one and two months after treatment, respectively. At harvest, residue levels were 1.8 mg/kg in fodder, 1.6 mg/kg in hulls and 1.3 mg/kg in the seed.

At the one-month sampling, 43% of the total extractable residue was identified as the phosphorylated metabolites: sulfoxide, sulfone, oxygen analog sulfone, and oxygen analog sulfoxide. The non-phosphorylated metabolites accounted for 11% of the residue. The remaining residue was comprised of five unknown metabolites (4%) and origin-bound compounds (17%). At harvest only non-phosphorylated metabolites were identifiable at low (< 10%) levels in all three commodities, i.e., hulls, fodder and seed. The remaining residue was shown to be very polar extractable materials or to have the ¹⁴C incorporated into the cellulose and lignin of the hulls, fodder and protein and oil of the seed.

In conclusion, soybean seedlings can readily take up terbufos applied to the soil. The absorbed compound is then translocated and metabolized by oxidation, hydrolysis, methylation and subsequent oxidation to eventually yield principally non-phosphorylated, non-toxic metabolites.

In sugar beet metabolism studies, plants were grown from seed in soil treated with [methyllene-¹⁴C]terbufos at a rate of 6.8 kg ai/ha. The levels of radioactivity in both foliage and roots were determined at 4.5, 8, l6, and 32 weeks after treatment. The TRR levels found in the various samples declined with time from 6.27 to 1.07 mg/kg eq in foliage and from 7.44 to 0.284 mg/kg eq in roots. The levels of ¹⁴C recovered in all plants represented a total of only 2.3% of the applied dose. The data showed that metabolism of terbufos occurred at a faster rate in the roots. Chromatographic data obtained at different stages of plant growth indicated that terbufos is degraded mainly by way of oxidation, hydrolysis and methylation followed by subsequent oxidation to yield principally non-phosphorylated, non-toxic metabolites.

There is also evidence of incorporation of terbufos-derived radioactivity into the sucrose fraction of sugar beets.

In sweet corn metabolism studies, corn was grown in metal cylinders contained in greenhouses and treated with [methylene-¹⁴C]terbufos at 1.1 kg ai/ha. Sweet corn contained 0.34, 2.64, 4.70 and 6.85% of the applied dose at 2, 4, 7 and 10 weeks of growth. The identified phosphate esters found as metabolites in sweet corn accounted for about 89% of the radioactivity. Levels of ¹⁴C extracted from plants were separated into at least 19 radioactive metabolites using thin layer chromatography (TLC). The expected oxidation products of terbufos, i.e., the sulfoxide, the sulfone, the oxygen analog of sulfoxide and sulfone, were confirmed to be present as residues in the corn plants. In the corn plants sampled at 10 weeks the phosphorylated metabolites, terbufos sulfoxide (8.1 mg/kg eq), terbufos sulfone (2.8 mg/kg eq), terbufoxon (0.3 mg/kg eq), terbufoxon sulfoxide (16.9 mg/kg eq) and terbufoxon sulfone (5.6 mg/kg eq) accounted for 34% of the chloroform-soluble extractable radioactivity. A significant amount of the total hydrophilic radioactivity could be in the form of natural products.

In the cabbage metabolism study, plants were grown in a greenhouse and externally from seed in soil treated with [methyllene-¹⁴C]terbufos at a rate of 2.2 kg ai/ha, using both a 15-G granular formulation and a liquid concentrate. The levels of radioactivity found in the cabbage plants, expressed as mg/kg equivalent of terbufos, declined with time (4 to 16 weeks) from 3.93 to 0.09 mg/kg eq for external granular treatment, from 1.48 to 0.04 mg/kg eq for the external liquid treatment and from 1.71 to 0.07 mg/kg eq for the greenhouse liquid treatment. The absolute amounts of radioactivity (in µCi) recovered in plants did not vary much with time. The recovered radioactivity represents a maximum of 1.5% of the total applied dose. At the end of 12 weeks, 92% (0.07 to 0.22 mg/kg eq) of the total radioactivity consisted of unidentified water-soluble metabolites and the total amount of phosphate compounds were less than 0.01 mg/kg eq. There was no apparent metabolic difference between granular (15-G) or liquid-treated soil in developing cabbage. The metabolism of terbufos in cabbage is similar to that reported for sugar beet.

In a rape metabolism study, rape seed was grown in soil treated with [methylene¹⁴C]terbufos in the furrow at 0.28 kg ai/ha. The total residual radioactivity in rape plants expressed as parent was 0.63 and 0.68 mg/kg eq for 1 and 2 month post-treatment samples respectively. Residues were 0.42 mg/kg eq in the 2 month hulls sample. At harvest (3-months post treatment), the residue levels in fodder, hull and seed were 3.21, 3.63 and 1.11 mg/kg eq, respectively. The extractable radioactivity from the 1-month old rape plant was 90%, of which 48% was organosoluble and 42% was aqueous soluble. By two-dimensional TLC analysis, about 16.3% of the radioactive organosolubles migrated away from the plate origin and the remaining 31.7% of the radioactivity stayed at the origin. Among the migrating radiocomponents, non-phosphorylated compounds predominated with 4.9%, terbufoxon sulfoxide accounted for 4.0% and non-phosphorylated compounds and terbufos sulfoxide contributed to 1.7 and 1.3% of the resolved organoextractables respectively. The remaining 4.4% of the migrating radioactivity was made up of at least 6 minor components.

Rape plants can readily take up terbufos and closely related metabolites from the soil. The absorbed compounds are then initially metabolized in plant tissues by way of oxidation to phosphorylated metabolites such as terbufos sulfoxide and terbufoxon sulfoxide. These oxidized products degrade further through hydrolysis, methylation and subsequent oxidation thus leading to the formation of certain non-phosphorylated metabolites. In rape seeds, the hexane fraction comprised of 22% of the radioactivity which was probably associated with fatty acids or lipid-type compounds. The acetonitrile fraction, accounting for about 12%, mainly consisted of oil-related compounds and a non-phosphorylated compound along with trace amounts of several other minor components., The hydrolysis study indicated that incorporation of [¹⁴C]formaldehyde or ¹⁴CO_2, derived from [¹⁴C]terbufos, into natural products of various rape tissues accounts for a very large fraction of the radioactivity present in the plants or seeds.

In conclusion, the metabolic pathway for the formation of observed metabolites arises from sulfoxidation and desulfuration of terbufos, hydrolysis of the thiol-phosphorous bond (S=P), enzymatic S-methylation and finally S-oxidation. The studies evaluated show that the same oxidative phosphorylated metabolites of terbufos occur in plants and in animals. In addition, terbufos has been shown to be taken up by the roots, with the residues and metabolites translocated to all parts of the plants examined.

Environmental fate

The Meeting received information on aerobic degradation in soil, hydrolysis rates and products and a confined rotational crop study.

Degradation in soil (aerobic)

The metabolic fate of terbufos in soil was investigated in silt loam soil under aerobic conditions using [methylene-¹⁴C]terbufos. The half-life of terbufos was approximately 5 days and of the total terbufos related residues was approximately 100 days. Major degradation products were carbon dioxide and the oxidative metabolites terbufos sulfoxide and terbufos sulfone. The concentration of terbufos sulfoxide in soil increased rapidly to a maximum of 2.6 mg/kg eq (52% of the applied dose) after 30 days and then declined to 0.3 mg/kg eq (6% of dose) after one year. Terbufos sulfone residues increased slowly to a maximum level of 1.0 mg/kg eq (20% of applied dose) at 60-days and then decreased to 0.1 mg/kg eq (2.3% of dose) after one year.

Hydrolysis rate and products

Terbufos hydrolyses rapidly under abiotic conditions at environmentally relevant temperatures and would not be expected to persist in aquatic systems. Hydrolysis of terbufos sulfoxide and terbufos sulfone occurs more slowly, but the des-ethyl derivatives that formed are not expected to be of toxicological concern.

Confined rotational crop study

Residues of terbufos and related compounds were determined in soil and rotational crops (cabbage, red beets, and wheat) from a treated corn field. In the study in Wisconsin, corn was planted in a silt loam soil and treated at planting with 2.24 kg ai/ha. Residues of terbufos and related compounds were less than the LOQ of the method (0.05 mg/kg) in all cabbage, red beet and wheat grain samples. Wheat straw contained residues of 0.1 mg/kg. The soil half-life of terbufos and related compounds was calculated to be 30 days.

In another study conducted in Nebraska, corn planted in silt loam soil was treated at planting by soil incorporation with terbufos at the rate of 2.24 kg ai/ha. Residues of terbufos and related compounds were less than the LOQ of the method (0.05 mg/kg) in all cabbage, sugar beet and wheat grain samples. Spring wheat forage contained residues of 0.15 mg/kg. No residues were detected in winter wheat straw and grain. The soil half-life of terbufos and related compounds was calculated to be 17 days in beet plots, 16 days in cabbage plots, and 10 days in wheat plots.

Methods of analysis

The Meeting received information on validated methods of analysis of terbufos in plant matrices, animal matrices and environmental samples that were used in supervised trials, rotational crops studies and storage stability studies. Enforcement methods and multiresidue methods of analysis were also submitted to the Meeting.

Several analytical methods have been developed for the determination of terbufos in plant commodities and animal tissues, suitable for data collection and enforcement. All analytical methods for terbufos residues are designed to extract parent terbufos and its oxygenated metabolites: terbufos sulfoxide, terbufos sulfone, terbufoxon and terbufoxon sulfoxide. Terbufos and its metabolites are oxidized to the common moiety terbufoxon sulfone using m-chlorobenzoic acid, which is then analysed by gas chromatograph equipped with a phosphorus-selective detector. The methods vary slightly, usually in the extraction solvent used.

In plant samples, the LOQ for most of the reported trials was 0.05 mg/kg, but limits for some methods/substrates were 0.01 or 0.005 mg/kg. Recoveries of terbufos and its related metabolites were tested over the concentration range of 0.01-1.0 mg/kg on samples from all plant commodities reported in the trials.

In animal tissue samples, the LOQ for the milk is 0.005 or 0.01 mg/kg, for the tissue, 0.05 mg/kg, and for eggs, 0.01 mg/kg. Recoveries of terbufos and its related metabolites were tested on the samples over the concentration range of 0.005-1.0 mg/kg.

An adequate method is available for enforcement of terbufos MRLs in or on plant commodities. The GC method for determining terbufos and its phosphorylated metabolites is described in the Pesticide Analytical Manual (PAM), Vol.II as Method I modified by Method M-1754 substituting acetone for benzene and dichloromethane for chloroform.

Terbufos and its metabolites were taken through the US FDA Multiresidue Method with limited success.

Stability of pesticide residues in stored analytical samples

The stability of terbufos residues has been determined in freezer storage stability studies (from < 0 to -10ºC or -17ºC) in the representative plant commodities of corn (grain, plants and straw); sugar beet (tops and roots); and banana (unpeeled and pulp). Terbufos residues fortified in representative crop samples (root, grain, watery and oily commodities) were shown to be stable in frozen storage for approximately 18 months.

The stability of terbufos residues in milk (1.7-3.3ºC) has been determined and 79% of the residues were recovered after 14 days.

No stability studies were submitted to the Meeting on other animal matrices.

Definition of the residue

Metabolic studies on animals and plants have demonstrated that terbufos is metabolized in much the same way in all the biological systems studied. The decrease in the parent compound is accompanied by a short-term build-up of the sulfoxide and sulfone metabolites. The corresponding oxygen analogues are also formed, but at a much slower rate. Cleavage of the P=S bond yields, after methylation of the resulting thiol, a series of methylated metabolites differing in the oxidation state of the sulfur atoms.

Terbufos and all oxidation products are considered potent anticholinesterase agents.

Terbufos is readily metabolized in both plant and animal tissues by way of oxidation, hydrolysis and methylation which is then followed by further oxidation to principally non-toxic metabolites.

All analytical methods used to determine terbufos residues are designed to extract parent terbufos, and its oxygenated metabolites terbufos sulfoxide, terbufos sulfone, terbufoxon, and terbufoxon sulfoxide.

The Meeting confirmed the previous (JMPR 1989) residue definition for terbufos, both for enforcement and for risk assessment and for both animal and plant commodities as follows:

The sum of terbufos, its oxygen analogue and their sulfoxides and sulfones expressed as terbufos.

Although terbufos has a log k_ow of 4.71 based on the parent terbufos, the total residue of terbufos and related metabolites are not considered fat soluble.

Results of supervised trials on crops

Supervised residue trials were available for bananas, sugar beets, sweet corn, cereal grains (maize and sorghum); coffee beans, fodder and forage of cereal grains (maize and sorghum); and miscellaneous forage and fodder crops (sugar beet tops). A large number of trials were submitted from the 1970s based on analytical methods with an LOQ of 0.05 mg/kg. More recent trials were provided which had an improved LOQ of 0.01 and were used in estimating residues and establishing MRLs. In cases of finite residues, then relevant data from trials with an LOQ of 0.05 were considered acceptable to include in the data set. Supervised trials on the remaining commodities that currently have CXLs were not provided. The Meeting decided to withdraw the current recommendations for broccoli, cabbages (head), mustard seed, onion (bulb), peanut, peanut fodder, peanut forage (green), popcorn, rape seed, rapeseed oil (crude), soy beans (dry); straw and fodder of cereal grains, sugar beet fodder and wheat.

In situations where residues from supervised trials from GAP show nil residues, the MRL was chosen to reflect a level of sensitivity that is compatible with enforcement activities. Where analytical methods applied had different LOQs, the lowest value was chosen only if the nil residue could be expected. In this case, the High Residue value would be recommended at the highest LOQ used in the study unless a majority of the observations were derived from the more sensitive LOQ.

In situations where supervised trials from GAP showed nil residues, even at exaggerated rates, the MRL was chosen to reflect an LOQ that is compatible with enforcement activities. However, both the STMR and high residue values were recommended at zero.

Banana

Thirty six field trials were submitted to the Meeting from banana producing areas of the world including Australia, Costa Rica, Ecuador, Honduras, Panama, Philippines and Mexico. In the trials 100 g ai/kg (10G) or 150 g ai/kg (15G) granule (G) terbufos was applied to the soil at the base of daughter banana plants at 1-9 g ai/plant/application. Application rates varied with a maximum rate of application per plant per year at of 41 g ai. GAP application rates ranged from 2-4 g ai/plant with a maximum of 12 g ai/year in Australia and Central America, 2 g ai/plant with a maximum of 8 g ai/year in Philippines and 3 g ai/plant to the maximum of 9 g ai/year in Mexico. No PHI was specified in the various national GAPs.

Residue levels ranged from <LOQ (< 0.01 or < 0.002) to 0.03 mg/kg for those trials where substantially exaggerated rates (2-3 times GAP) were applied. However, the majority of the trials did not conform to GAP. The residues from trials that were conducted according to GAP were < 0.01(6) and 0.02(2) mg/kg.

The Meeting estimated a maximum residue level for bananas of 0.05 mg/kg, and STMR of 0.01 mg/kg and a HR of 0.02 mg/kg.

Sugar beets (roots)

Field trials involving at-planting and post-emergence treatments with terbufos were made available to the Meeting from the USA. The trials were conducted during the 1986, 1989 and 1994 growing seasons. In 1986, terbufos (15G) was applied at planting (banded, knifed-in, or in-furrow) at 2.2 kg ai/ha. In the trials conducted in 1989, terbufos (15G) was knifed in as a band at planting at 4.9 kg ai/ha, in excess of the current USA GAP. Residues reflecting GAP were < 0.01(6) and 0.01(2) mg/kg where the PHI was considered equivalent to GAP, i.e., from 91-141 days.

In more recent field trials (1994), terbufos (15G) was applied as a band over the row to sugar beets at 2.2 to 2.4 or 4.4-4.9 kg ai/ha. The lower rate reflects the maximum GAP rate. Again, residues reflecting GAP were < 0.01(5) mg/kg. The PHI was considered equivalent to GAP at 90 days.

For knifed-in applications data was available at only 2 times the GAP rate where low finite residues could be found in some cases (< 0.01, 0.01, 0.02 and 0.03). Another knifed-in application trial had residues at < 0.01. The PHI for these trials ranged from 139-180 days (GAP is 150 days).

For all trials conducted according to GAP, total terbufos-related residues were: < 0.01(11) and 0.01(2) mg/kg.

The Meeting withdrew its previous recommendation of 0.1 mg/kg and estimated a maximum residue level for sugar beets of 0.02 mg/kg, an STMR of 0.01 mg/kg and a highest residue of 0. 01 mg/kg.

Sweet corn kernels and corn-on-the-cob

Field trials involving at-planting and post-emergence treatments with terbufos were made available to the Meeting from the USA. In trials from 1972-1974, terbufos granules were applied in the furrow or in a band at the time of planting at rates of 1.1 to 9.0 kg ai/ha. In 1986 terbufos granules were applied to the soil at planting (in furrow or in a band), at post-emergence or at cultivation at a combined rate of about 6.0 kg ai/ha. GAP in the USA for 15G or 20G (200 g ai/kg) terbufos formulations is at a maximum rate of 1.5 kg ai/ha applied once at planting, post-emergent, or at cultivation. For post-emergent applications, the PHI is 30 days for forage, and 60 days for corn-on-the cob.

For post-emergent use, samples were analysed only where the PHI was less than that for GAP. Residue values from the majority of trials (7) were lower than the LOQ (0.01 mg/kg). For two trials, where the equivalent of three times the GAP rate was applied in two applications, residues found were 0.01 mg/kg (2).

The Meeting withdrew its previous recommendation of 0.01 (*) mg/kg and estimated a maximum residue level for sweet corn of 0.01(*) mg/kg, an STMR of 0.01 mg/kg and a HR of 0.01 mg/kg.

Cereal grains

Maize grain

Field trials involving at-planting and post-emergence treatments with terbufos were made available to the Meeting from the USA. GAP in the USA for terbufos 15G or 20G formulations is at the maximum rate of 1.5 kg ai/ha applied once at planting, post-emergent, or at cultivation. A PHI of 30 days is required for forage if applied post-emergent. In trials conducted from 1981 to 1986, terbufos granules were applied to the soil at planting, either in furrow or as a band, at the rate of 1.1 to 1.8 kg ai/ha. In some trials, additional plots were treated with terbufos at rates up to five times the recommended label rates. In trials conducted from 1990 to 1996 terbufos granules were applied post-emergent at the recommended rate of 1.5 kg ai/ha as well as at higher rates up to five times the recommended application rates.

In all the trials conducted on maize grain according to GAP, total terbufos-related residues were below the LOQ of the analytical method: < 0.01 mg/kg (13). In trials where higher rates of application or more than one application was made, the residue levels were also below the LOQ. Since there were finite residues found in the trials for sweet corn at exaggerated rates, the use pattern for maize grain is not considered a nil residue situation and relevant values for STMR and HR have been proposed.

The Meeting confirmed its previous recommendation for a maximum residue level of 0.01(*) mg/kg and estimated an STMR of 0.01 mg/kg and a highest residue of 0.01 mg/kg for maize.

Sorghum grain

Field trials involving at-planting and post-emergence treatments with terbufos were made available to the Meeting from the USA. GAP in the USA for terbufos 15G or 20G formulation consists of a maximum rate of 2.0 kg ai/ha applied once with a PHI of 50 days for forage, and 100 days for grain and fodder.

Results of all trials conducted according to the GAP for sorghum grain, including post emergent applications, showed total terbufos-related residues below the LOQ: < 0.01 mg/kg (5). Residues were at non-detectable levels even in trials where higher rates of application or shorter PHI 58-76 days (6 trials) were used.

The Meeting estimated a maximum residue level for sorghum grain of 0.01(*) mg/kg, an STMR of 0.

Coffee beans

Residue trials were conducted during 1982-1988 in Costa Rica, Guatemala, and El Salvador.

In field trials in Costa Rica conducted in 1982-1983, a 10G granular formulation of terbufos was applied to the soil at the base of established coffee plants at the rate of 0.75-7.5 g ai/plant. Berries were collected from treated plants at various intervals, field dried according to common practice, and the outer shell removed from the dried beans.

In the trials in El Salvador and Guatemala (1988), terbufos (10G) was band applied to plants after flowering but before bean formation, at the rate of 1 or 5 g ai/plant. From treated plants field dried berries, with outer shell removed, were collected at 38-56 days in El Salvador and at 163-197 days in Guatemala.

GAP in coffee bean plantations permits the application of terbufos at a maximum rate of 1.1g ai/plant for up to 2 applications with a PHI of 60 days. No trials were conducted at the maximum GAP. However, residue levels were below the LOQ (< 0.05 mg/kg) in all coffee bean samples (10) collected 58-120 days after treatment with terbufos at 0.75-7.75 g ai/plant rate. At one site, where coffee beans had been treated with 3.75 and 7.5 g ai/plant and shorter than GAP PHI of 60 days (47 or 35 days after treatment), maximum residues of 0.12 and 0.17 mg/kg respectively, were found. Residues declined to < 0.05 mg/kg at the next sampling interval, 124 or 53 days post-treatment.

The Meeting confirmed its previous recommendation for a maximum residue level of 0.05 (*) mg/kg and estimated an STMR of 0.05 mg/kg for coffee beans.

Animal feed commodities

Fodder and forage of cereal grains

As maize forage, sorghum forage and sugar beet tops are not moving in international trade the Meeting made no recommendations regarding maximum residue levels for these commodities.

Maize forage and fodder

The GAP for terbufos 15G or 20G formulation in the USA allows for a maximum application rate of 1.5 kg ai/ha applied once either at-planting, early post-emergence, or at cultivation. A PHI of 30 days is required for forage when applied post-emergent. The same GAP applies to both maize and sweet corn. Trials on maize and sweet corn for residues in fodder and forage were conducted in the USA during 1972-1990. Terbufos granules were applied to the soil either in-furrow or in a band during planting at the rate of 1.1-5.8 kg ai/ha. In a few trials, tests were performed where two applications were made to maize one at planting and a second treatment 5-6 weeks after planting.

The residues deriving from trials conducted in sweet corn and maize were found to represent similar populations which could be combined (Mann-Whitney U-test). Residues, on a fresh weight basis, from trials conducted according to GAP were, with median underlined, < 0.05 (11), 0.07(2), 0.14, 0.16, 0.17, 0.23, 0.32 and 0.96 mg/kg. The highest residue value (HR) was 0.96 mg/kg from trials in Colorado, USA from forage samples taken 90 days after treatment at planting at a rate of 1.5 kg ai/ha. Applying the default percent dry matter content (average between %DM of sweet corn forage and field corn forage, as listed in the FAO Manual (FAO, 2002) for maize forage (44%)), the highest residue on dry weight basis is estimated as 2.2 mg/kg.

The Meeting withdrew its previous recommendation of 1 mg/kg and estimated an STMR of 0.10 mg/kg and a highest residue of 2.2 mg/kg for maize forage.

Residue levels from trials according to GAP for maize fodder were: < LOQ i.e., < 0.05(38) and 0.08 mg/kg (from one trial in Colorado, USA, sampled at harvest after treatment at the rate of 1.5 k ai/ha at planting). Applying the default percent dry matter value of 83% for corn fodder, as listed in the FAO Manual (FAO, 2002), the highest residue on dry weight basis was calculated as 0.10 mg/kg.

The Meeting withdrew its previous recommendation of 0.1 mg/kg and estimated, on a dry weight basis, a maximum residue level of 0.2 mg/kg, an STMR of 0.06 mg/kg and a highest residue of 0.10 mg/kg for maize fodder.

Sorghum forage and fodder

Supervised trials on sorghum were conducted during 1978-1996. In the 1996 trials, terbufos granules were applied post-emergent, at the rate of 2.1 or 2.2 kg ai/ha. Forage samples were harvested 48 to 72 days after treatment while fodder samples were taken at normal grain harvest time, 88 to 90 days after treatment. In the rest of the trials (1978-1991), terbufos granules were applied at planting, at the GAP rate (2 kg ai/ha) and at twice that rate (4-4.3 kg ai/ka).

All trials according to the GAP resulted in residues below the LOQ for sorghum forage (< 0.05 mg/kg), except one trial (Louisiana, USA) where a level of 0.07 mg/kg was recorded. This highest residue value was from forage samples taken 50 days after treatment with terbufos at a rate of 2.0 kg ai/ha at the vegetative stage. The moisture content of samples was only determined from some trials with the results showing wide variations. The Meeting therefore decided to use the default percent dry matter for sorghum forage (35%), as listed in the FAO Manual (FAO, 2002) to estimate the highest residue value.

The Meeting estimated an STMR for sorghum forage, on a dry weight basis, of 0.14 mg/kg and a highest residue of 0.20 mg/kg.

Residue levels in sorghum fodder ranged from < 0.05 to 0.19 mg/kg. Residues from trials conducted according to GAP were < 0.05 (12), 0.12 and 0.19 mg/kg. The highest residue value was 0.19 mg/kg from trials where fodder samples were taken 88 days after a post-emergent treatment at a rate of 2.2 kg ai/ha. Applying the default percent dry matter for sorghum fodder/stover of 88%, as listed in the FAO Manual (FAO, 2002), the highest residue on dry weight basis was estimated as 0.22 mg/kg.

The Meeting estimated, on a dry weight basis, a maximum residue level of 0.3 mg/kg, an STMR of 0.057mg/kg and a highest residue of 0.22 mg/kg for sorghum fodder.

Sugar beet tops

Field trials were conducted in the USA and Canada during 1971-1975 in which terbufos (15G) was either applied in-furrow or banded at 1.0 to 2.5 kg ai/ha or at exaggerated rates of 4.0-12.3 kg ai/ha. Several trials were also conducted which consisted of sequential at-planting and post emergence banded applications, typically utilizing exaggerated rates.

Several trials were also conducted in the USA during the 1989 growing season in which terbufos (15G) was knifed in at-planting at 4.9 kg ai/ha. Samples were harvested by hand at maturity, 150-180 days after treatment. Residues found in all control samples of tops were < 0.05 mg/kg.

In US field trials in 1994, terbufos (15G) was applied as a band over the row to sugar beets at the maximum GAP rate of 2.2 to 2.4 kg ai/ha and at 2 × GAP rates of 4.4 to 4.9 kg ai/ha. Residue levels ranged from < LOQ (0.01 or < 0.05) to 0.82 mg/kg for sugar beet tops samples. Residues found from trials conducted according to GAP were < 0.01(3), 0.01, 0.04, < 0.05 (18), 0.12, 0.15 and 0.82 mg/kg. The highest residue value (HR) found was 0.82 mg/kg, from samples taken 91 days following an at-planting treatment of 1.8 kg ai/ha Applying the default percent dry matter for sugar beet tops (23%), as listed in the FAO Manual (FAO, 2002), the highest residue on dry weight basis was estimated as 3.57 mg/kg.

The Meeting withdrew its previous recommendation for a maximum residue level of 1 mg/kg for fodder beet leaves or tops and estimated, on a dry weight basis, an STMR of 0.22 mg/kg for sugar beet tops and a highest residue of 3.6 mg/kg.

Dietary burden in farm animals

The Meeting estimated the dietary burden of terbufos residues in farm animals on the basis of the diets listed in Appendix IX of the FAO Manual (FAO, 2002). One feed commodity from each Codex Commodity Group was used. Calculation from the HR values provides the concentrations in feed suitable for estimating MRLs for animal commodities, while calculation based on STMR values for feed is suitable for estimating the STMR values for animal commodities.

Estimated maximum dietary burden of farm animals

Estimated median dietary burden of farm animals

The highest residues or STMR values for feed commodities were used in calculating the worst-case dietary burden for dairy cows, beef cattle and poultry while the STMR values were used in the estimation of the median dietary burdens. The respective dietary burdens were then compared with the results of the feeding studies at various dose levels (mg/kg in diet) to estimate the maximum residue levels and STMR in animal commodities.

The dietary burdens of terbufos for estimates of STMR and highest residue level values in animal commodities (residue levels in animal feeds expressed as dry weight) are 0.088 mg/kg and 1.60 mg/kg for beef cattle, 0.076 mg/kg and 1.47 mg/kg for dairy cows and 0.009 mg/kg and 0.009 mg/kg for poultry.

Farm animal feeding studies

Feeding studies indicated that at a dose (2ppm for 21 days) approximately equivalent to the calculated animal diets, no residues (< 0.05 mg/kg) of terbufos or its metabolites were detectable in cattle tissues and milk. In another study, done at an exaggerated rate (50 ppm), only one milk sample had a finite residue (0.011 mg/kg) while one sample had residue at the LOQ (0.005 mg/kg) and the rest were below the LOQ.

The Meeting received a feeding study in poultry. Hens were fed at 2 ppm terbufos for 21 days and residues were determined in poultry tissues and eggs. The LOQ was 0.05 and 0.01 mg/kg for tissues and eggs, respectively. All tissues and eggs samples contained residues below the LOQ value.

Maximum residue levels

The estimated maximum dietary burdens for beef cattle (1.60 mg/kg) and for dairy cows (1.47 mg/kg) matched the feeding level from the respective cattle feeding studies (2 mg/kg). As a result the Meeting decided to use the residue levels from the feeding studies as estimates of the maximum residue levels for cattle tissues and milk. Residues in cattle tissues and milk in the feeding studies were all below the LOQ (< 0.05 mg/kg for cattle fat, muscle, liver, and kidney, and < 0.01 mg/kg for milk). The calculated median dietary burdens were lower than the actual feeding level in both transfer studies, 0.088 mg/kg for beef cattle and 0.076 mg/kg in dairy cattle therefore the calculated median residues would also be expected to be lower.

The actual feeding level of laying hens was (2 ppm for 21 days), the calculated maximum and median dietary burdens (0.009 ppm) were lower than the residue levels in both tissue and eggs. Consequently, no detectable residues are expected in both tissues and eggs. Therefore, residues are expected to be well below the LOQ for the method used (< 0.05 mg/kg for poultry tissues and < 0.01 mg/kg for eggs).

The calculations confirmed the findings of the animal metabolism studies as well as the results of the feeding studies, that showed no residues of terbufos or its metabolites were detectable in cattle tissues, poultry tissues, milk, and eggs. The MRL and STMR for residues of terbufos in animal commodities are proposed at the limit of quantification of the analytical method.

The Meeting withdrew its previous recommendation of 0.05 (*) mg/kg for cattle meat, cattle edible offal, chicken meat and chicken edible offal and 0.01 (*) mg/kg for cattle milk. The Meeting confirmed its previous recommendation of 0.01 (*) mg/kg for eggs and estimated a maximum residue level of 0.05 (*) mg/kg for meat from mammals other than marine mammals and mammalian edible offal, and 0.01(*) mg/kg for milks. The Meeting recommended an STMR of 0.05 mg/kg for mammalian meat and edible offal and poultry tissues and 0.01 mg/kg for milk and eggs. The estimated high residues are 0.05 mg/kg for mammalian meat, edible mammalian offal, chicken meat and edible chicken offal and 0.01 mg/kg for milks and eggs.

DIETARY RISK ASSESSMENT

Long-term intake

The International Estimated Dietary Intakes (IEDIs) were calculated for the five GEMS/Food regional diets using the STMR for banana, coffee beans, edible offal (mammalian), eggs, maize (fresh, flour), meat from mammals other than marine mammals, milks, poultry meat, poultry edible offal, sorghum, sugar beet and sweet corn (corn on the cob) estimated by the current Meeting (Annex 3). The ADI is 0-0.0006 mg/kg and the calculated IEDIs were 9-40% of the maximum ADI. The Meeting concluded that the intake of residues of terbufos resulting from the uses considered by the current JMPR were unlikely to present a public health concern.

Short-term intake

The International Estimated Short-Term Intakes (IESTIs) of terbufos by the general population and by children were calculated for commodities by the current Meeting. This was based on HRs estimated by the Meeting from available information on consumption. The ARfD is 0.002mg/kg and the calculated IESTIs for children up to 6 years range from 0-60% and those for general population from 0-30% of the ARfD. The Meeting concluded that the short-term intake of residues of terbufos resulting from the uses considered by the current Meeting were unlikely to present a public health concern.