29 January 2008
ISBN: 978-0-662-47704-4 (978-0-662-47705-1)
Cat. No.: H113-27/2008-6E (H113-27/2008-6E-PDF)
The text provided on this page reflects only the body of the report. To obtain an electronic copy of the complete document, including appendices (PRVD2008-06, Proposed Re-evaluation Decision: Rimsulfuron), please contact our publications office.
Should you require further information please contact the Pest Management Information Service.
After a re-evaluation of the herbicide rimsulfuron, Health Canada's Pest Management Regulatory Agency (PMRA), under the authority of the Pest Control Products Act and Regulations, is proposing continued registration for the sale and use of products containing rimsulfuron in Canada.
An evaluation of available scientific information found that products containing rimsulfuron have value in the food and crop industry and do not present unacceptable risks to human health or the environment when used according to label directions. As a condition of the continued registration of rimsulfuron use on field corn, potatoes, tomatoes and lowbush blueberries, new risk reduction measures are proposed for inclusion on the labels of all rimsulfuron products. No additional data are being requested at this time.
The PMRA's pesticide re-evaluation program considers potential risks as well as the value of pesticide products to ensure they meet modern standards established to protect human health and the environment. Regulatory Directive DIR2001-03, PMRA Re-evaluation Program, presents the details of the re-evaluation activities and program structure. Re-evaluation draws on data from registrants, published scientific reports, information from other regulatory agencies and any other relevant information available.
This proposal affects all end-use products containing rimsulfuron registered in Canada. Once the final re-evaluation decision is made, registrants will be instructed on how to address the new risk-reduction measures.
This Proposed Re-evaluation Decision is a consultation document1 that summarizes the science evaluation for rimsulfuron and presents the reasons for the proposed re-evaluation decision. It also proposes additional risk-reduction measures to further protect human health and the environment.
The information is presented in two parts. The Overview describes the regulatory process and key points of the evaluation, while the Science Evaluation provides detailed technical information on the human health, environmental and value assessment of rimsulfuron.
The key objective of the Pest Control Products Act is to prevent unacceptable risks to people and the environment from the use of pest control products. Health or environmental risk is considered acceptable if there is reasonable certainty that no harm to human health, future generations or the environment will result from use or exposure to the product under its conditions or proposed conditions of registration2. The Act also requires that products have value3 when used according to label directions. Conditions of registration may include special precautionary measures on the product label to further reduce risk.
To reach its decisions, the PMRA applies rigorous, modern hazard and risk assessment methods and policies. These methods consider the unique characteristics of sensitive subpopulations in both humans (e.g. children) and organisms in the environment (e.g. those most sensitive to environmental contaminants). These methods and policies also consider the nature of the effects observed and the uncertainties present when predicting the impact of pesticides. For more information on how the PMRA regulates pesticides, the assessment process and risk-reduction programs, please visit the PMRA's website at www.pmra-arla.gc.ca.
Before making a re-evaluation decision on rimsulfuron, the PMRA will consider all comments received from the public in response to this consultation document4. The PMRA will then publish a Re-evaluation Decision document5 on rimsulfuron, which will include the decision, the reasons for it, a summary of comments received on the proposed registration decision and the PMRA's response to these comments.
For more details on the information presented in this overview, please refer to the Science Evaluation section of this consultation document.
Rimsulfuron is a selective systemic sulfonylurea herbicide. It is registered for use on several terrestrial food and feed crops including pre-emergent use on field corn (Eastern Canada only) as well as postemergent use on field corn (Eastern Canada and Manitoba), potatoes (Eastern Canada, irrigated potatoes in Western Canada and seed potatoes), tomatoes (transplanted processing and fresh market field) and lowbush blueberries (Eastern Canada only). Rimsulfuron may be used alone or in co-formulation with nicosulfuron to control quackgrass and annual grassy weeds as well as black bulrush, redroot pigweed (including triazine resistant) and lamb's-quarters (suppression only). One application per year is made by ground equipment only. Typical maximum application rates for rimsulfuron range from 12.5-15.0 g a.i./ha.
People could be exposed to rimsulfuron by consuming food and water, working as a mixer/loader/applicator or by entering treated sites. The PMRA considers two key factors when assessing health risks: the dose levels where no health effects occur and the dose levels to which people may be exposed. The dose levels used to assess risks are established to protect the most sensitive human population (e.g. children and nursing mothers). Only those uses where exposure is well below levels that cause no effects in animal testing are considered acceptable for continued registration.
Toxicology studies in laboratory animals describe potential health effects from varying levels of exposure to a chemical and identify the dose where no effects are observed. The health effects noted in animals occur at doses more than 100-times higher (and often much higher) than levels to which humans are normally exposed when using rimsulfuron products according to label directions.
An acute overexposure to rimsulfuron can produce a variety of symptoms in animals and humans. Symptoms may include weight loss, nasal secretions, ruffled fur and hair loss. Local effects of an acute dermal exposure may include slight erythema. Contact with the eye may cause corneal opacity, iritis, conjunctival redness and chemosis.
Additional toxic effects on the liver, kidney and testes, as well as effects noted in pregnant females (increased abortions and mortality), were observed in animals at very high doses only; therefore, they would not occur when rimsulfuron products are used according to label directions. Based on the weight of evidence, rimsulfuron is considered non-carcinogenic. A cancer risk assessment was not required.
Although there are no risks of concern based on current uses of rimsulfuron, some additional protective measures (personal protective equipment and improved work practices) have been proposed to update the product labels to the current standard and to further reduce the level of human exposure to rimsulfuron.
Reference doses define levels to which an individual can be exposed over a single day (acute) or lifetime (chronic) and expect no adverse health effects. Generally, dietary exposure from food and water is acceptable if it is less than 100% of the acute reference dose or chronic reference dose (acceptable daily intake). An acceptable daily intake is an estimate of the level of daily exposure to a pesticide residue that, over a lifetime, is believed to have no significant harmful effects.
Human exposure to rimsulfuron was estimated from residues in treated crops and drinking water, including the most highly exposed subpopulation (e.g. children 1 to 6 years old). This aggregate exposure (i.e. to rimsulfuron from food and drinking water) represents less than 6% of the chronic reference dose.
The Food and Drugs Act prohibits the sale of adulterated food, that is, food containing a pesticide residue that exceeds the established maximum residue limit (MRL). Pesticide MRLs are established for the Food and Drugs Act purposes through the evaluation of scientific data under the Pest Control Products Act. Each MRL value defines the maximum concentration in parts per million (ppm) of a pesticide allowed in/on certain foods. Food containing a pesticide residue that does not exceed the established MRL does not pose an unacceptable health risk.
Rimsulfuron is currently registered in Canada for use on lowbush blueberries, field corn, potatoes and tomatoes. MRLs for rimsulfuron are currently specified for blueberry and tomatoes or processed foods derived from these foods. No specific MRLs have been established for corn and potatoes. Where no specific MRL has been established, a default MRL of 0.1 ppm applies, which means that pesticide residues in a food commodity must not exceed 0.1 ppm. The proposed amendments to the default MRL for rimsulfuron can be found in the Science Evaluation Section 8.1.2 of this consultation document.
Rimsulfuron is not registered for use in any residential areas. Therefore, no residential or other non-occupational risks are expected.
Based on both the precautions and directions for use on the original product labels reviewed for this re-evaluation, and also considering the use of appropriate protective equipment, the risk estimates associated with mixing, loading and applying activities meet current standards for all the scenarios and are not of concern. The proposed personal protection equipment include long pants, long-sleeved shirt and chemical-resistant gloves.
Occupational postapplication risk assessments consider exposures to workers re-entering treated sites in agriculture. Based on the precautions and directions for use on the original product labels reviewed for this re-evaluation, and also considering proposed protective measures (revised re-entry interval), postapplication non-cancer risk estimates to re-entry workers performing high exposure activities (scouting) meet current standards and are not of concern.
When rimsulfuron is applied for control of weeds in crops, some of it finds its way into soil and water. However, the chemical is not expected to persist as it is rapidly broken down by soil microbes and by chemical reaction in water. The breakdown chemical products that are formed persist for a longer period of time than rimsulfuron. Both rimsulfuron and its breakdown products are mobile and hence can move freely in soil. Water runoff on the soil surface can move the residues into nearby bodies of water, such as ponds and rivers. Water monitoring of these bodies of water have generally revealed residues at concentrations below levels of concern. To reduce the potential for leaching and runoff, it is important that additional risk reduction measures are observed.
When rimsulfuron is used for weed control in crops, there is a potential that sensitive plant species on land and in water may be exposed to the chemical as a result of the spray drifting or runoff. Some of these species are sensitive to the chemical and would be adversely affected. To minimize the potential exposure, strips of land (buffer zones) between the agricultural field and the non-target terrestrial or aquatic areas will be left unsprayed. The width of these buffer zones will be specified on the product label. Rimsulfuron poses negligible risk to wild birds and mammals, bees and other arthropods as well as to aquatic organisms like fish, amphibians and invertebrates because concentrations in the environment are not expected to be harmful. Rimsulfuron's major transformation products pose a negligible risk to earthworms and terrestrial plants, as well as to aquatic organisms such as fish, aquatic invertebrates, amphibians and aquatic plants.
Rimsulfuron reduces a portion of the economic losses caused by weeds estimated at $94.4 million in the early 1990s for field corn, potatoes, tomatoes and blueberries. Rimsulfuron (co-formulated with nicosulfuron) is the only herbicide registered for the control of black bulrush in lowbush blueberries. Compared to alternative non-sulfonylurea postemergent herbicides, rimsulfuron has greater flexibility in application timing when used on field corn, has no or fewer restrictions on crop variety and soil type for use on tomatoes and potatoes. Although rimsulfuron plays a role in mitigating resistance development in weeds to other herbicide groups and may be used to control triazine-resistant redroot pigweed, consideration has to be given to resistance management as more weed species are reported to be resistant to herbicides that inhibit acetolactate synthase (such as rimsulfuron) than to herbicides having other modes of action.
Labels of registered pesticide product labels include specific instructions for use. Directions include risk-reduction measures to protect human and environmental health. These directions must be followed by law. As a result of the re-evaluation of rimsulfuron, the PMRA is proposing further risk-reduction measures for product labels.
Before making a re-evaluation decision on rimsulfuron, the PMRA will consider all comments received from the public in response to this consultation document. The PMRA will then publish a Re-evaluation Decision document, which will include the decision, the reasons for it, a summary of comments received on the proposed decision and the PMRA's response to these comments.
When the re-evaluation decision is made, the PMRA will publish an Evaluation Report on rimsulfuron in the context of this re-evaluation decision (based on the Science Evaluation section of this consultation document). In addition, the test data on which the decision is based will be available for public inspection, upon application, in the PMRA's Reading Room (located in Ottawa).
Rimsulfuron is a selective systemic herbicide. It belongs to the sulfonylurea chemical family and is classified as a Group 2 herbicide. The herbicidal activity of rimsulfuron is due to the inhibition of the plant enzyme acetolactate synthase (ALS), also called acetohydroxyacid synthase (AHAS).
Following the re-evaluation announcement for rimsulfuron, E.I. du Pont Canada Company, the registrant of the technical grade active ingredient (TGAI) and primary data provider in Canada, indicated it would continue to support all uses included on the label of Commercial Class end-use products. There are no Domestic Class products containing rimsulfuron registered in Canada.
Based on the manufacturing process, the product is not expected to contain impurities of human health or environmental concern as identified in Regulatory Directive DIR98-04, Chemistry Requirements for the Registration of a Technical Grade of Active Ingredient or an Integrated System Product, Section 2.13.4 or Toxic Substances Management Policy (TSMP) Track 1 substances as identified in Regulatory Directive DIR99-03, The Pest Management Regulatory Agency's Strategy for Implementing the Toxic Substances Management Policy, Appendix II.
Appendix I lists all rimsulfuron products registered under the authority of the Pest Control Products Act, including one technical grade active ingredient, two manufacturing concentrates, and six Commercial Class end-use products. Four of the Commercial Class end-use products contain rimsulfuron alone and the remaining two rimsulfuron products are co-formulated with nicosulfuron.
Appendix II lists all the uses for which rimsulfuron is presently registered. All uses were supported by the registrant at the time of initiation of re-evaluation and were, therefore, considered in the health and environmental risk assessments. Also presented is whether any of the uses were added through the PMRA minor use program. While currently supported by the registrant, the data supporting these minor uses were originally generated by a user group.
Uses of rimsulfuron belong to the following use site categories: terrestrial food crops and terrestrial feed crops. The crops specifically include field corn, lowbush blueberries, potatoes and tomatoes.
Toxicology studies in laboratory animals describe potential health effects resulting from various levels of exposure to a chemical and identify dose levels where no effects are observed. Unless there is evidence to the contrary, it is assumed that effects observed in animals are relevant to humans and that humans are more sensitive to effects of a chemical than the most sensitive animal species. The health effects noted here were observed in animals at dose levels at least 100-fold (often much higher) above levels to which humans are normally exposed through use of products containing this chemical.
The toxicology database supporting rimsulfuron is primarily based on studies from the technical registrant. In laboratory animals, rimsulfuron is of low acute toxicity by the oral, dermal, and inhalation routes of exposure, non-irritating to rabbit skin, moderately irritating to rabbit eyes, and a non-sensitizer in Guinea pigs. Signs of acute toxicity induced by very high doses of rimsulfuron are weight loss, ruffled fur, nasal discharge and hair loss.
After oral exposure, rimsulfuron was rapidly absorbed from the gastrointestinal tract, more than 80% of the administered dose was excreted within the first 24 hours and more than 90% within 72 hours. Accumulation within tissues was minimal, with 58-72% of the administered activity recovered from the urine, and 21-35% recovered from the feces. The main component was the parent compound. The major metabolic pathway was a sulfonylurea bridge contraction followed by oxidation and hydroxylation reactions. A number of minor metabolites were found in the urine and in the feces.
In short- and long-term studies, the major effects were a reduction in body weight or body-weight gain and changes in the liver. In subchronic mouse, rat and dog studies reduced body-weight gains occurred with an increase in liver weight. An increase in the incidence of hepatocellular hypertrophy was also noted in the rat. Additionally, in the 90-day mouse study there was an increase in several hematological parameters (RBC, Hgb, HCT, eosinophils) and in the dog studies (90-day and 1-year) there was an increase in testicular atrophy/degeneration of the seminiferous tubules, an increase in kidney weight, and an increase in levels of serum alkaline phosphatase, and serum cholesterol.
In the mouse oncogenicity study, in addition to reduced body weight and body-weight gain and increased relative liver weight, there was an increase in the relative weights of the kidney, testes and heart. At the highest dose tested, there were testicular effects (deformation, degeneration of the testicular artery) and increased incidences of liver masses and hepatocellular adenomas. In the combined chronic/oncogenicity rat study, in addition to reduced body weight and body-weight gain, and increased relative and absolute liver weights, other effects included diffuse and focal fatty changes in the liver, a slightly increased incidence of centrilobular hepatocellular hypertrophy, and an increased relative kidney weight. The incidence of lymphosarcomas and histiocytic sarcomas was increased at interim sacrifice, but overall the increased incidences at the high dose were not statistically significant and were within normal ranges of in-house historical data for the latter part of the study. All genotoxicity studies were negative. Rimsulfuron is considered non-carcinogenic.
In a rat reproduction study and a rabbit developmental study, reductions in body weight/body-weight gain and food consumption were noted. In rabbits, maternally toxic effects including increased mortality and increased abortions (late) were noted at the highest dose (1500 mg/kg bw/day). Reproductive and offspring sensitivities were not observed.
Reference doses have been set based on the NOAEL for the most sensitive indicator of toxicity, namely decreases in body weight and body-weight gain, increases in organ (liver, kidney) weight and testicular effects in dogs. Reference doses incorporate uncertainty factors to account for extrapolating between laboratory animals and humans.
The toxicology profile of rimsulfuron and endpoints used in the risk assessment of rimsulfuron are summarized in Appendix III and IV.
Occupational and non-occupational risk is estimated by comparing potential exposures with the most relevant endpoint from toxicology studies to calculate a margin of exposure (MOE). This is compared to a target MOE incorporating safety factors protective of the most sensitive subpopulation. If the calculated MOE is less than the target MOE, it does not necessarily mean that exposure will result in adverse effects. However, MOEs less than the target MOE require mitigation measures to reduce risk.
To estimate the risk from short-, intermediate- and long-term oral, dermal and inhalation exposure, an oral NOAEL of 9.6 mg/kg bw/day from the combined results of a 90-day and a 1-year dog study was used. The NOAEL was based on decreased body weight and body-weight gain, increased absolute and relative liver and kidney weights, increased incidence of seminiferous tubule degeneration and increased number of spermatid giant cells in the epididymides, at a LOAEL of 81.8 or 193 mg/kg bw/day. A target MOE of 100 is based on the standard uncertainty factors of 10 for interspecies extrapolation and 10 for intraspecies variability.
The PMRA does not consider rimsulfuron to be a possible human carcinogen based on a weight-of-evidence approach.
All dermal exposure scenarios included in this assessment were considered to be of short- or intermediate-term in duration. As the toxicological endpoint for these scenarios is based on an oral NOAEL, and no dermal absorption study was available, the PMRA used a dermal absorption factor of 100% for a tier one assessment.
Workers can be exposed to rimsulfuron through mixing, loading or applying the herbicide during normal use, and when re-entering a treated site to conduct activities such as scouting and/or handling treated crops.
There are potential exposures to mixers, loaders and applicators. The following supported uses were assessed:
Occupational handlers of rimsulfuron include farmers and custom agricultural applicators who mix, load and apply the herbicide. As only one application is permitted per season, the duration of exposure for farmers is short-term (1 to 30 days). In the case of custom applicators applying rimsulfuron to several fields per season, the duration of exposure may be considered intermediate-term (1 to 6 months). Exposure for all crop scenarios is predominantly dermal, with inhalation accounting for a minor component of overall exposure.
The PMRA estimated handler exposure based on the following level of personal protective equipment (PPE):
Baseline PPE: Long pants, long-sleeved shirt and chemical-resistant gloves (unless specified otherwise, i.e. for groundboom application, chemical-resistant gloves are not included in the scenario).
Mixer/loader/applicator exposure estimates are based on the best available data at this time. The assessment might be refined with product-specific exposure data, biological monitoring data, or dermal absorption data.
No chemical specific exposure studies were available for use in the re-evaluation of rimsulfuron. Thus, appropriate dermal and inhalation exposures were estimated using the Pesticide Handlers Exposure Database (PHED), Version 1.1. PHED is a compilation of generic mixer/loader and applicator passive dosimetry data with associated software which facilitates the generation of scenario-specific exposure estimates based on formulation type, application equipment, mix/load systems and level of personal protective equipment. In some cases, PHED did not contain appropriate data sets to estimate exposure to workers wearing additional PPE and/or using engineering controls. Surrogate data for wettable granules in WSP were not available in PHED, therefore closed mixing and loading data for wettable granules were used.
For some scenarios (e.g. hand-held equipment), estimating exposure from mixing, loading and applying wettable granules in WSP was not possible using PHED. In these situations, exposure units from mix/load/apply with open pour liquid for low pressure handwand and backpack were used.
PHED unit exposures coupled with information on the amount of rimsulfuron handled per day was used to estimate handler exposure. The amount handled per day is based upon the maximum label application rate and default assumptions on the crop area which can reasonably be treated in one day.
Calculated MOEs exceed the target MOE for all exposure scenarios and are summarized in Table 1 of Appendix V.
The postapplication occupational risk assessment considered exposures to workers entering treated agricultural sites to conduct activities involving foliar contact. Based on the rimsulfuron use pattern, there is potential for short-term (1 to 30 days) postapplication exposure to rimsulfuron residues for workers.
Chemical-specific dislodgeable foliar residue (DFR) data and activity specific transfer coefficients (TC) were used to estimate postapplication exposure resulting from contact with treated foliage at various times after application. DFR data include the amount of residue that can be dislodged or transferred from a surface, such as the leaves of a plant. A TC is a factor that relates worker exposure to dislodgeable residues. TCs are specific to a given crop and activity combination (e.g. hand harvesting apples, scouting late season cotton) and reflect standard agricultural work clothing worn by adult workers. Postapplication exposure activities include scouting, thinning, hand pruning, hand harvesting, hand weeding, staking, tying, training, irrigation and detasseling.
For workers entering a treated site, restricted-entry intervals (REIs) are calculated to determine the minimum length of time required before people can safely re-enter. An REI is the duration of time that must elapse before residues decline to a level where performance of a specific activity results in exposures above the target MOE (i.e. >100 for short-term exposure scenarios).
Although rimsulfuron is applied to agricultural areas including terrestrial feed and food crops, no relevant DFR studies were available. As a result, a conservative default value of 20% of the application rate with a 10% dissipation per day was used.
The postapplication risk estimates include a number of conservative inputs, such as the assumption that workers are exposed to residues following the maximum application rate.
Calculated MOEs exceed the target MOE on day 0, and proposed REIs are 12 hours. Postapplication exposure calculations for each use site are summarized in Table 2 of Appendix V.
There are no domestic class products registered for rimsulfuron in Canada; therefore, a residential risk assessment was not required.
In a dietary exposure assessment, the PMRA determines how much of a pesticide residue, including residues in milk and meat, may be ingested with the daily diet. Exposure to rimsulfuron from potentially treated imports is also included in the assessment. These dietary assessments are age specific and incorporate the different eating habits of the population at various stages of life. For example, the assessments take into account differences in children's eating patterns, such as food preferences and the greater consumption of food relative to their body weight when compared to adults. Dietary risk is then determined by the combination of the exposure and the toxicity assessments. High toxicity may not indicate high risk if the exposure is low. Similarly, there may be risk from a pesticide with low toxicity if the exposure is high.
The PMRA considers limiting use of a pesticide when risk exceeds 100% of the reference dose. The PMRA's Science Policy Note SPN2003-03, Assessing Exposure from Pesticides, A User's Guide, presents detailed acute and chronic risk assessments procedures.
Residue estimates used in the dietary risk assessment (DRA) may be conservatively based on the maximum residue limits (MRL) or the field trial data representing the residues that may remain on food after treatment at the maximum label rate. Surveillance data representative of the national food supply may also be used to derive a more accurate estimate of residues that may remain on food when it is purchased. These include the Canadian Food Inspection Agency's National Chemical Residue Monitoring Program and the United States Department of Agriculture Pesticide Data Program (PDP).
Chronic dietary risk assessments were conducted using the Dietary Exposure Evaluation Model (DEEM-FCID™, Version 2.03), which uses updated food consumption data from the United States Department of Agriculture's Continuing Surveys of Food Intakes by Individuals, 1994-1996 and 1998.
For more information on dietary risk estimates or residue chemistry information used in the dietary assessment, see Appendix VI and VII.
An acute (1 day) reference dose (ARfD) was not calculated since there was not an acute endpoint of concern.
The acceptable daily intake (ADI) is the dose that an individual can be exposed to over the course of a lifetime and expect no adverse health effects. The ADI selected for rimsulfuron was based on a NOAEL of 9.6 mg/kg bw/day from the combined results of a 90-day and a 1-year study in dogs. The endpoint selected was based on decreased body weight and body-weight gain, increased absolute and relative liver and kidney weights, and an increased incidence of seminiferous tubule degeneration and an increased number of spermatid giant cells in the epididymides at a LOAEL of 81.8 or 193 mg/kg bw/day. An overall uncertainty factor of 100 was required to account for interspecies extrapolation (10-fold) and intraspecies variability (10-fold), resulting in an ADI of 0.096 mg/kg bw/day (9.6 mg/kg bw/day ÷ 100). This value was considered protective of all populations, including infants and children, and females of childbearing age (13-49 years).
The chronic dietary risk was calculated by using the average consumption of different foods and water considering average residue values. This expected intake of residues from food and water was then compared to the ADI.
The chronic potential daily intake for both food and water accounted for less than 6% of the ADI for all subpopulations and is, therefore, not of concern.
A cancer risk assessment was not conducted since the rimsulfuron database did not suggest any carcinogenic potential in mice or rats.
A cancer risk assessment was not conducted since the rimsulfuron database did not suggest any carcinogenic potential in mice or rats.
Rimsulfuron residues in potential drinking water sources were estimated using computer simulation models. The chronic estimated drinking water concentration value used in the exposure model is 0.04 parts per billion (ppb).
Drinking water estimates were incorporated directly in the exposure model. For more information, refer to Section 3.5, Aggregate Risk Assessment.
Aggregate exposure is the total exposure to a single pesticide that may occur from food, drinking water, residential and other non-occupational sources as well as from all known or plausible exposure routes (oral, dermal and inhalation).
As residential use of rimsulfuron is not permitted, aggregate exposure is from dietary and drinking water exposures only (see Sections 3.3 and 3.4). Chronic aggregate (food and water) exposures were less than the respective reference doses. Therefore, aggregate exposure from all relevant sources is not considered a health concern.
Based on its physical-chemical properties (Section 2.2), rimsulfuron is soluble in water, is not likely to volatilize from moist soil or water surfaces under field conditions, and is not likely to bioaccumulate in organisms. Environmental fate data for rimsulfuron are summarized in Table 1 of Appendix X. Rimsulfuron is relatively labile and dissipates from soil and aquatic systems by hydrolysis and biotransformation. At cooler temperatures the dissipation of rimsulfuron is slower in all environmental media. Phototransformation is not an important route of transformation for rimsulfuron, however, it may assist the breakdown of the major transformation products associated with rimsulfuron. The major transformation products IN-70941, IN-70942, and IN-E9260 are generally more persistent than rimsulfuron. IN-70942 and IN-E9260 are stable to hydrolysis, although IN-E9260 was not detected as a major transformation product in the water or sediment phases of aquatic test systems. Water/sediment studies demonstrated that the majority of the applied radioactivity (-21 to - 93%) is preferentially associated with the sediment phase by study termination (89-100 days), and that much of that applied radioactivity is bound (up to 42%) suggesting that rimsulfuron and its transformation products are less available to aquatic organisms.
Laboratory studies on adsorption/desorption, thin layer chromatography, and soil column leaching indicate that rimsulfuron and its transformation products, IN-70941, IN-70942, IN-E9260, have a potential to be mobile in a variety of soils. Laboratory studies indicate that rimsulfuron and its transformation products, IN-70941, IN-70942 and IN-E9260 have the potential to leach. Rimsulfuron and its transformation products met all criteria identifying them as leachers. Suitable terrestrial field studies conducted in Canada, or in Canadian equivalent ecoregions, were not available to assess the leaching potential of rimsulfuron and its transformation products in the field. Terrestrial field studies conducted in the US and Europe indicate varying degrees of leaching. The extent of rimsulfuron leaching is related to the time of application, amount of rainfall and soil characteristics. The potential for leaching is especially prominent in non-acidic soils because of increased solubility and decreased adsorption. Leaching of rimsulfuron and its transformation products in terrestrial field studies ranged from a soil depth of 7.6 cm to a depth of 30 cm. Evidence of leaching may be hampered by the fact that soils were only sampled to depths of 7.6 cm or 30 cm depending on the study. Canadian water monitoring data have shown detections in surface water at concentrations of less than 1.0 µg a.i./L, while at present, there are no monitoring data for groundwater (Appendix VIII).
The proposed transformation pathway is the bridge-contraction reaction to form IN-70941 and IN-70942, followed by the cleavage of the sulfonylurea linkage to form IN-E9260, IN-T5831 and IN-J290 (Appendix IX, Diagram 1).
The environmental risk assessment determines the potential for adverse ecological effects in each environmental compartment by comparing the ratio of the estimated environmental exposure to the ecotoxicological effect. The estimated environmental exposure concentration (EEC) is the initial or cumulative concentration of pesticide in the various sources of food, water and soil to which the organism is exposed. EECs are clculated by different methods for each media (food, water or soil). If multiple applications of pesticide are used, cumulative EECs are determined by using the time taken to decline to 50% of the original application (DT50) using the minimum time interval between applications for each environmental media.
The risk assessment is initially conducted using a screening-level scenario which assumes maximum exposure (EEC) and the most sensitive toxicological endpoint for the organism of interest. This assumes direct application or overspray to the environmental media (food, water, soil) to which the organism is exposed. This is the most conservative scenario and generally does not reflect the exposure to which an organism would be subject to when the pesticide is applied according to the label instructions. Risk to the environment is calculated as a risk quotient (RQ) which is the ratio between the environmental exposure and the toxicological endpoint for the organism (i.e. RQ = EEC/toxicological endpoint). The threshold or level of concern for potentially harmful effects to an organism is an RQ value of 1 where the exposure exactly equals the toxicological endpoint. RQ values greater than or equal to 1 are considered to equal or exceed the level of concern which may result in potentially harmful effects to the organism. RQ values less than 1 indicate negligible risk to the organism because they are below the threshold for harmful effects. In the latter case, no further assessment is carried out. If the RQ is greater than or equal to 1, the level of concern, then a refinement of the risk assessment is carried out to assess the level of concern using scenarios which are a better approximation of exposure or toxicological effects and less conservative. Refinements can include (i) exposure from the fraction of pesticide which drifts onto non-target habitats, instead of assuming 100% overspray, and (ii) exposure from the amount of pesticide predicted in runoff, instead of assuming direct overspray to water (i.e. 100% exposure). The refinements may also consider different toxicity endpoints or a percentile of a species sensitivity distribution rather than the most sensitive endpoint. They may also consider the results of a mesocosm study using several species rather than the toxicity from a single species. Further refinements to the risk assessment may consider the use of monitoring data collected in the field rather than EECs generated by a model.
A risk assessment of rimsulfuron to terrestrial organisms was based upon an evaluation of toxicity data on rimsulfuron to earthworms (acute contact), bees (acute oral and chronic), two species of birds (acute oral, dietary, and chronic), two species of mammals (acute oral, dietary, and chronic), and a variety of terrestrial plants (emergence). Toxicity data on formulated products were also available for earthworms (acute contact), bees (acute oral and contact), beneficial invertebrates (acute contact), birds (acute oral and dietary), and terrestrial plants (vegetative vigor). As well, toxicity data for the transformation products IN-70941, IN-70942, and IN-E9260 were available for earthworm (reproduction) and terrestrial plants (emergence and vegetative vigour). A summary of terrestrial toxicity data for rimsulfuron, as technical active or in formulation, and its transformation products are presented in Table 2 of Appendix X. For the assessment of risk, toxicity endpoints chosen from the most sensitive species were used as surrogates for the wide range of species that can be potentially exposed following treatment with rimsulfuron (Table 3a, 3b, and 3c of Appendix X).
Rimsulfuron and its transformation products demonstrated no adverse toxicological effects on terrestrial invertebrates. This was confirmed in a field study in which no treatment related effects were observed in macro-fauna collected from a rimsulfuron treated field (40 g a.i./ha) using pitfall traps and soil cores over a 17-month period. As well, rimsulfuron demonstrated no treatment related adverse toxicological effects to birds or mammals on an acute oral, dietary and reproductive basis. Plant emergence and vegetative vigor studies conducted with ten plant species (four monocot and six dicot) indicated that, although the seeds of most plant species emerged successfully, plants did not follow normal growth patterns. Studies conducted with rimsulfuron transformation products indicated that IN-70941, IN-70942, and IN-E9260 did not affect emergence or growth of any of the seventeen plant species tested (11 monocot and six dicot).
The risk assessment was conducted using data for the earthworm, honeybee, bobwhite quail, mallard duck, and rat. In the case of terrestrial plants, the most sensitive monocot species was sorghum (Sorghum bicolor), while the most sensitive dicot was canola (Brassica napus).
The screening level risk assessment indicated that exposure to rimsulfuron and its transformation products do not pose a risk to terrestrial invertebrates, wild birds and mammals at the current registered label rates. As well, risk to terrestrial plants from exposure to the transformation products IN-70941, IN-70942, and IN-E9260 is not expected as the no observed effects concentration (NOEC) for each product was 400 g/ha, well above the currently registered label rates for rimsulfuron. A risk assessment of the end-use products was not conducted as the end-use products used in the terrestrial toxicology studies are not currently registered for use in Canada and, with the information provided, it was not possible to determine if the products used are similar to those products registered for use in Canada. Table 3a and 3b of Appendix X summarizes the risk assessment from rimsulfuron and its transformation products to terrestrial organisms.
The screening level risk assessment concluded that rimsulfuron poses a potential risk to non-target terrestrial plants. The level of concern (LOC) was exceeded by as much as 187 times. A refinement of the risk assessment was conducted taking into consideration the concentrations of rimsulfuron that could be present in terrestrial habitat directly adjacent to the application field through drift of spray. Spray drift data for a medium American Society of Agricultural Engineers (ASAE) droplet size, as are generally used in ground boom applications of herbicides, indicate that the maximum amount of spray that will drift one metre down wind from the point of application during spraying is 6%. Using this percent drift, the off-site EECs for rimsulfuron were calculated. Based on this method of refinement, rimsulfuron poses a reduced risk to non-target terrestrial plants directly adjacent to the application field. Exceedance of the LOC was reduced to 11 times from 187 times. Incident reports documenting adverse effects to cropped species (corn, peas, and soybean), as well as an ash tree, support the conclusions of the risk assessment. Buffer zones will be required to reduce the risk of rimsulfuron to non-target terrestrial plants. The transformation products IN-70941, IN-70942, and IN-E9260 are not expected to be in the environment at concentrations that would result in a risk to terrestrial non-target plants. Table 4 of Appendix X summarizes the refined risk assessment from rimsulfuron to non-target terrestrial plants.
Risk to aquatic organisms, acute and chronic, is based on an evaluation of toxicity data on rimsulfuron for nine freshwater species (two invertebrates, three fish, three algae, and one vascular plant) and four estuarine/marine species (two invertebrates, one fish, and one algae). Acute and chronic toxicity data on formulated products for freshwater invertebrates, fish, algae, and vascular plants were also available. As well, acute toxicity data on the transformation products IN-70941, IN-70942, and IN-E9260 were available for freshwater invertebrates, fish, algae, and vascular plants. A summary of aquatic toxicity data for rimsulfuron is presented in Table 2 of Appendix X. For the assessment of risk, toxicity endpoints chosen from the most sensitive species were used as surrogates for the wide range of species that can be potentially exposed following treatment with rimsulfuron (Table 5a and 5b of Appendix X).
Rimsulfuron and its transformation products are not toxic to freshwater or estuarine/marine invertebrates or fish on an acute bases, or to freshwater fish and invertebrates on a chronic bases. Rimsulfuron significantly affected biomass and cell density of freshwater algae and freshwater and marine/estuarine diatoms. However, data indicate that there is a potential for algae to recover from acute exposure to rimsulfuron. Rimsulfuron significantly affected frond density and biomass of duckweed, however, in contrast to the algae data, duckweed may not be able to recover after acute exposure to rimsulfuron. The transformation products IN-70941, IN-70942, and IN-E9260 did not adversely affect algae or duckweed at the maximum concentrations tested.
According to acute toxicity, the most sensitive freshwater invertebrate, fish, algae and vascular plant species were Daphnia carinata, common carp, green algae and duckweed, respectively. Acute toxicity data were available for only one species of estuarine/marine invertebrate (mysid shrimp), fish (sheepshead minnow), and aquatic plant (diatom). Chronic toxicity data were available for only one species of freshwater invertebrate (Daphnia magna) and fish (rainbow trout), while no chronic toxicity data were available for estuarine/marine species.
The screening level risk assessment indicated that rimsulfuron and its transformation products do not pose a risk to freshwater or estuarine/marine invertebrates, fish, algae and amphibians (based on surrogate data from fish studies). A risk assessment of the end-use products was not conducted as the end-use products used in the aquatic toxicology studies are not currently registered for use in Canada and, with the information provided, it was not possible to determine if the products used are similar to those products registered for use in Canada. Table 5a and 5b of Appendix X summarizes the risk assessment from rimsulfuron and its transformation products to aquatic organisms.
The screening level risk assessment concluded that rimsulfuron poses a potential risk to non-target aquatic vascular plants. The LOC was exceeded by 3.7 times. A refinement of the risk assessment was conducted taking into consideration the concentrations of rimsulfuron that could be present in non-target water bodies directly adjacent to the application field through drift of spray (using the same methods as for the refined terrestrial risk assessment) and through runoff (as modelled by PRZM/EXAMS). The refined risk assessment reduced the exceedance of the LOC to less then one from spray drift and to 1.5 times from runoff. To mitigate risk to aquatic vascular plants from rimsulfuron, a buffer zone and precautionary label statements will be required on the label. Table 6 of Appendix X summarizes the refined risk assessment to non-target aquatic plants from rimsulfuron spray drift and runoff.
Although surface water monitoring data are available for rimsulfuron (Appendix VIII), these data are limited and were not used in the aquatic risk assessment because of the following source of uncertainties: 1) sampling was conducted at regular time intervals and may not have coincided with runoff events in the watershed; 2) rimsulfuron use information from the areas surrounding where the samples were collected were not available.
All rimsulfuron uses are supported by the registrant. There are no risk concerns for any of the registered uses. Consequently, no further information regarding alternatives to the use of rimsulfuron is considered.
There are no Domestic Class products containing rimsulfuron registered in Canada.
Quackgrass is one of the most troublesome and difficult to control weeds. A recent survey estimated that about 56% of Canadian farmland has quackgrass present. It can reduce corn yields by as much as 25 to 85%. Rimsulfuron is one of few herbicides which effectively control quackgrass in field corn.
In Canada, the estimated average annual losses caused by weeds in the early 1990s were $54.4 million in field corn, $32.5 million in potatoes, $4.7 million in tomatoes, and $2.8 million in blueberry. Rimsulfuron was reported to be widely used in field corn, potatoes and tomatoes to prevent and reduce a portion of the economic losses caused by weeds. In addition, rimsulfuron (co-formulated with nicosulfuron) is the only herbicide registered for the control of black bulrush in lowbush blueberries.
Rimsulfuron has no, or fewer, restrictions than the alternative postemergent grass and broadleaf herbicides for use on tomato and potato. Rimsulfuron may be used on both fresh market field tomatoes and transplanted tomatoes grown for processing while the registered alternatives may only be used on fresh market field tomatoes. When used on potatoes, rimsulfuron has no variety restrictions while its alternative has several variety restrictions (e.g. it cannot be used on red skinned varieties, Shepody or Atlantic potatoes, or potatoes grown for the early market). Similarly, soil restrictions associated with the use of rimsulfuron are fewer than the alternatives.
When used in field corn, the application window of rimsulfuron is wider (1-8-leaf stage for corn) than that of alternative non-sulfonylurea postemergent grass herbicides (spike to 2- or 3-leaf stage for corn).
The development of herbicide resistance in weeds associated with the use of rimsulfuron must be considered. Due to the widespread and frequent use of acetolactate synthase inhibiting (Group 2) herbicides such as rimsulfuron, and lack of rotation with herbicides with other modes of action, more resistant weed biotypes have been documented in Canada with Group 2 herbicides than with any other herbicide group, including 12 broadleaf weeds (cleavers, chickweed, hemp-nettle, kochia, lamb's-quarters, giant ragweed, ball mustard, wild mustard, redroot pigweed, Russian thistle, spiny annual sowthistle, and stinkweed) and two grassy weeds (green foxtail and wild oats). Nevertheless, rimsulfuron still plays a role in managing and/or mitigating resistance development in weeds to other herbicide groups. For instance, rimsulfuron is registered for the control of triazine resistant redroot pigweed.
The management of toxic substances is guided by the federal government's Toxic Substances Management Policy (TSMP), which puts forward a preventive and precautionary approach to deal with substances that enter the environment and could harm the environment or human health. The policy provides decision makers with direction and sets out a science-based management framework to ensure that federal programs are consistent with its objectives. One of the key management objectives is virtual elimination from the environment of toxic substances that result predominantly from human activity and that are persistent and bioaccumulative. These substances are referred to in the policy as Track 1 substances.
During the review process, rimsulfuron was assessed in accordance with the PMRA Regulatory Directive DIR99-03, The Pest Management Regulatory Agency's Strategy for Implementing the Toxic Substances Management Policy. Substances associated with the use of rimsulfuron were also considered, including major transformation products formed in the environment, microcontaminants in the technical product and formulants in the end-use products. The PMRA has reached the following conclusions:
Therefore, the use of rimsulfuron is not expected to result in the entry of Track 1 substances into the environment.
Formulant issues are being addressed through the PMRA formulant initiatives and Regulatory Directive DIR2006-02, Formulant Policy and Implementation Guidance Document published on 31 May 2006.
The occupational application and postapplication risks are acceptable for the exposure scenarios involving the use of rimsulfuron. The calculated margins of exposure for application and postapplication occupational exposures are all above the Agency's target assuming that workers wear baseline personal protective equipment.
Chronic dietary risk assessments demonstrate that there were no dietary concerns for any population subgroup in Canada, including infants, children, teenagers, adults and seniors. In addition, no dietary concerns were evident for nursing or pregnant females or based on gender in general.
The potential for the contamination of drinking water with rimsulfuron is expected to be negligible. There are no drinking water concerns for any population subgroup in Canada, including infants, children, teenagers, adults and seniors.
Rimsulfuron is not registered for use in any residential areas so a residential risk assessment was not required.
Aggregate exposure and risk are from dietary and drinking water exposures only. This aggregate exposure (i.e. to rimsulfuron from food and drinking water) represents less than 6% of the chronic reference dose and was not considered a health risk.
Rimsulfuron is non-persistent in most soils and water systems, although its transformation products are more persistent than the parent compound. There is a potential that under appropriate environmental conditions, rimsulfuron and its transformation products may leach to groundwater and/or runoff to surface water. The risk assessment of rimsulfuron indicate that adverse effects on non-target terrestrial and aquatic plants are expected.
Rimsulfuron controls quackgrass and many annual grassy weeds in several important crops at a low application rate.
Rimsulfuron reduces a portion of the economic losses caused by weeds (estimated at $94.4 million in the early 1990s) for field corn, potatoes, tomatoes and blueberries. Rimsulfuron (co-formulated with nicosulfuron) is the only herbicide registered for the control of black bulrush in lowbush blueberries. Compared to alternative non-sulfonylurea postemergent herbicides, rimsulfuron has greater flexibility in application timing when used on field corn, has no or fewer restrictions on crop variety and soil type for use on tomatoes and potatoes. Although rimsulfuron plays a role in mitigating resistance development in weeds to other herbicide groups and may be used to control triazine resistant redroot pigweed, consideration has to be given to resistance management as more weed species are reported to be resistant to herbicides that inhibit acetolactate synthase (such as rimsulfuron) than to herbicides having other modes of action.
The PMRA is proposing that rimsulfuron is acceptable for continued registration with the implementation of the proposed risk-reduction measures. These measures are required to further protect human health and environment. As a condition of the continued registration of rimsulfuron uses, new risk reduction measures must be included on the labels of all products. No additional data are being requested at this time.
The PMRA has determined that the dietary and drinking water risks, worker risks during mixing, loading and application are acceptable for all uses provided that the mitigation measures listed in Appendix XI are implemented.
Division 15, Table II, of the Food and Drug Regulations currently identifies rimsulfuron (N-[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]-3-(ethylsulfonyl) as the residue for risk assessment and enforcement. This residue definition is consistent with that of the USEPA.
In general, when the re-evaluation of a pesticide has been completed, the PMRA intends to update Canadian maximum residue limits and to remove MRLs that are no longer supported.
As shown in Table 8.1.2, the Food and Drug Regulations specify MRLs for rimsulfuron residues in blueberries and tomatoes. Residues in all other agricultural commodities, including corn and potatoes approved for treatment in Canada but without a specified MRL, must not exceed the general MRL of 0.1 ppm.
Where no specific MRL for a pest control product has been established in the Food and Drug Regulations, subsection B.15.002(1) applies. This requires that residues do not exceed 0.1 ppm and has been considered a general MRL for enforcement purposes. Currently, residues of rimsulfuron in all agricultural commodities including corn and potatoes approved for treatment in Canada are regulated by subsection B.15.002(1). However, changes to this general MRL may be implemented in the future, as indicated in Discussion Document DIS2006-01, Revocation of 0.1 ppm as a General Maximum Residue Limit for Food Pesticide Residues [Regulation B.15.002(1)].
Extrapolation of available residue data for corn and potatoes following good agricultural practices (GAP), as described by the current product labels, indicated that residues would not exceed the limit of quantitation of the analytical method of 0.05 ppm. If and when the general MRL is revoked, a transition strategy will be established to allow permanent MRLs to be set for corn and potatoes.
Table 8.1.2 Rimsulfuron MRLs for Commodities Approved in Canada
Commodity MRL (ppm)
* By virtue of subsection B.15.002(1) of the Food and Drug Regulations, the maximum residue limit of foods for which MRLs have not specifically been established is 0.1 ppm.
For supplemental MRL information regarding the International situation and trade implications, refer to Appendix XII.
The PMRA has determined that to reduce the effects of rimsulfuron in the environment, mitigation in the form of precautionary label statements and terrestrial buffer zones listed in Appendix XI are required.
1 "Consultation statement" as required by subsection 28(2) of the Pest Control Products Act.
2 "Acceptable risks" as defined by subsection 2(2) of the Pest Control Products Act
3 "Value" as defined by subsection 2(1) of the Pest Control Products Act: "the product's actual or potential contribution to pest management, taking into account its conditions or proposed conditions of registration, and includes the product's (a) efficacy; (b) effect on host organisms in connection with which it is intended to be used; and (c) health, safety and environmental benefits and social and economic impact".
4 "Consultation statement" as required by subsection 28(2) of the Pest Control Products Act
5 "Decision statement" as required by subsection 28(5) of the Pest Control Products Act