Re-evaluation Decision RVD2012-08, MCPB

Pest Management Regulatory Agency
17 October 2012
ISSN: 1925-1025 (PDF version)
Catalogue number: H113-28/2012-8E-PDF (PDF version)

Table of Contents

• Re-evaluation Decision
• What Does Health Canada Consider When Making a Re-evaluation Decision?
• What is MCPB?
• Health Considerations
• Environmental Considerations
• Value Considerations
• Measures to Minimize Risk
• What Additional Information is Being Requested?
• Other Information

Re-evaluation Decision

After a thorough re-evaluation of the herbicide 4-(4-Chloro-2-methylphenoxy) butanoic acid (MCPB), Health Canada's Pest Management Regulatory Agency (PMRA), under the authority of the  Pest Control Products Act and  Regulations, is granting continued registration of products containing MCPB for sale and use in Canada.

An evaluation of available scientific information found that products containing MCPB have value in the food and crop industry and do not pose unacceptable risks to human health or the environment when used according to label directions. As a condition of the continued registration of MCPB uses, new risk-reduction measures must be included on the labels of all products. In addition, registrants are required to submit confirmatory data identified in this document.

The regulatory approach for the re-evaluation of MCPB was first presented in Proposed Re-evaluation Decision PRVD2011-06, MCPB, a consultation document. Re-evaluation Decision RVD2012-08, MCPB describes this stage of the Agency's decision and the reasons for it. Comments received during the consultation process did result in changes to the proposed regulatory decision as described in PRVD2011-06. A summary of these comments and the PMRA's responses to them appear in Appendix I of Re-evaluation Decision RVD2012-08, MCPB. To comply with this decision, registrants of products containing MCPB will be informed of the specific requirements affecting their product registration(s).

What Does Health Canada Consider When Making a Re-evaluation Decision?

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 registration. The Act also requires that products have value when used according to the 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 hazard and risk assessment methods as well as policies that are rigorous and modern. These methods consider the unique characteristics of sensitive subpopulations in both humans (for example, children) and organisms in the environment (for example, 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 Pesticides and Pest Management portion of Health Canada's portion of Health Canada's website.

What is MCPB?

MCPB is a selective systemic broadleaf herbicide. It belongs to the phenoxy acid family and is classified as a Group 4 herbicide. This herbicide mimics the natural plant hormone indole-3-acetic acid (also known as auxin), thereby causing susceptible broadleaf weeds to be controlled. MCPB is registered for the post-emergence control of annual and perennial broadleaf weeds in terrestrial food crops, terrestrial feed crops and industrial oil seed crops and fibre crops. The rate of application of MCPB ranges from 1.031 to 1.594 kg a.i./ha. It is applied once per year. MCPB is available only in Commercial Class products and can be applied by ground and/or aerial equipment.

Health Considerations

Can Approved Uses of MCPB Affect Human Health?

MCPB is unlikely to affect human health when used according to revised label directions.

Potential exposure to MCPB may occur through diet, when handling and applying the product, or when entering or contacting treated sites. When assessing health risks, two key factors are considered: the levels at which no health effects occur in animal testing and the levels to which people may be exposed. The dose levels used to assess risks are established to protect the most sensitive human population (for example, pregnant women, nursing mothers and children). Only the uses for which 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 at which 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 products containing pesticides are used according to label directions.

MCPB is of moderate acute oral toxicity, low acute dermal toxicity, and slight acute inhalation toxicity in laboratory animals. MCPB is non-irritating to the skin, moderately irritating to the eyes and is not a dermal sensitizer.

The most sensitive endpoint for non-pregnant animals from the oral route of exposure is kidney toxicity. Test data indicated that MCPB is not likely to be carcinogenic or mutagenic in humans.

When MCPB is administered to pregnant rats and rabbits, reduced skeletal ossification and increased incidences of cranio-facial malformations are observed. Due to the nature of the effects and their potential implications on the health and survival of the fetus, extra protective factors are applied during the risk assessment to further reduce the allowable level of human exposure to MCPB.

With the proposed mitigation measures, the risk assessment protects against these effects by ensuring that the level of human exposure is well below the lowest dose at which these effects occur in animal tests.

Residues in Water and Food 

Dietary risks from food and water are not of concern.

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 (ARfD) or chronic reference dose (acceptable daily intake). An acceptable daily intake (ADI) is an estimate of the level of daily exposure to a pesticide residue that, over a lifetime, is expected to have no significant harmful effects.

Dietary exposure to MCPB was estimated from residues in treated crops and drinking water for different subpopulations representing different ages, genders and reproductive status. Acute and chronic exposure estimates were determined for the general population and all subpopulations including females of child-bearing age (13 to 49 years old), infants and children.

The aggregate acute exposure (i.e. to MCPB from food and drinking water) represents 39% of the acute reference dose for females 13 to 49 years old and is in the range of 1 to 5% of the acute reference dose for all the other population subgroups when using drinking water concentrations generated from water modelling. The aggregate chronic exposure represents 5% of the chronic reference dose for the general population and is in the range of 4 to 10% of the chronic reference dose for all subpopulations. Thus, acute and chronic aggregate risks are not of concern.

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 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 or on certain foods. Food containing a pesticide residue that is at or below the established MRL does not pose an unacceptable health risk.

MRLs in/on all commodities treated with MCPB are currently regulated under B.15.002(1) of the Food and Drug Regulations which requires that residues do not exceed 0.1 ppm. Details regarding MRLs for MCPB can be found in the Science Evaluation of PRVD2011-06.

Risks in Residential and Other Non-Occupational Environments 

Residential and other non-occupational risks are not of concern.

MCPB is not registered for use in residential areas. Therefore, a non-occupational risk assessment was not required. 

Occupational Risks from Handling MCPB

Occupational risks for handlers are not of concern when used according to the revised label directions.

Based on the precautions and directions for use on the original product labels reviewed for this re-evaluation, the risk assessment for mixing/loading and application activities indicate that target margins of exposure (MOEs) are achieved for all crops, provided that risk mitigation measures are applied. The MOEs for mixing/loading and application reach target MOEs for pastures, cereals (wheat, oats, barley and rye), seedling alfalfa, seedling clover, field corn, seedling grasses, and peas (succulent/processing and dry/field) with the addition of mitigation measures including the use of additional protective equipment and engineering controls. In some scenarios, limiting the amount of kilograms handled per day by each worker will be necessary. Please refer to Appendix II of Re-evaluation Decision RVD2012-08, MCPB for a summary of the calculations.

Occupational post-application risks are not of concern when used according to the revised label directions.

Occupational post-application risk assessments consider exposure to workers entering treated agricultural sites. Based on the precautions and directions for use on the original product labels reviewed for this re-evaluation, post-application risks to workers meet current standards and are not of concern for all crops. A restricted entry interval (REI) set at 12 hours for most crops will mitigate any risk from exposure for post-application workers entering an area that has been treated with MCPB. For field corn, the REI is 9 days for scouting. This REI is considered to be agronomically feasible due to the timing of application. Please refer to Appendix III of Re-evaluation Decision RVD2012-08, MCPB for a summary of the calculations.

Environmental Considerations

What Happens When MCPB is Introduced Into the Environment?

MCPB poses a risk to terrestrial broadleaf plants, birds, small wild mammals and aquatic organisms including macrophyte plants and amphibians; therefore, additional risk-reduction measures need to be observed.

MCPB can enter non-target terrestrial habitats by drift from aerial or ground application such as pasture use, and it can enter aquatic habitats by run-off and leaching. It is water soluble and can move through the soil profile horizontally and vertically, thereby contaminating ground water and surface water, including drinking water sources. MCPB does not accumulate or bioconcentrate in the environment and it is not persistent in soil, having a degradation half-life of 8.3 days depending on the type of soil. In aquatic environments, biotransformation eliminates fifty percent of the chemical in less than 18 days, and degradation by sunlight in surface water can be even more rapid.

Because of the specific mode of action affecting broadleaf plants (MCPB is a synthetic auxin plant hormone similar to other phenoxy herbicides such as 2,4-D), it is highly toxic to terrestrial plants such as trees, shrubs, crops and others. Non-target invertebrates including bees and beneficial insects are not likely to be affected by this chemical. Although vertebrate animals including birds and small wild mammals are not usually affected by MCPB's specific mode of action, some species show slight to moderate toxicity for oral/dietary exposure. In aquatic habitats, fish and invertebrates are not likely to be affected by MCPB based on available data; however, aquatic plants such as duck weed are sensitive.

The use of MCPB poses a risk to terrestrial and aquatic organisms, including plants, birds, mammals, aquatic plants and amphibians. To reduce exposure of terrestrial organisms, environmental hazard label statements are recommended. Terrestrial plants including crops and non-target plant habitats such as shelter belts and riparian zones along streams and ponds can be protected from adverse effects by the observance of specified spray restrictions which provide a spray buffer zone between sites of the application and non-target areas. Furthermore, precautionary label statements will be used to help reduce the potential for surface runoff and for ground water contamination.

Value Considerations

What is the Value of MCPB?

MCPB continues to contribute to weed management in a variety of crops when used in accordance with the label directions.

MCPB is one of the few post-emergent herbicides that control a broad spectrum of annual and perennial broadleaf weeds in peas (succulent/processing and dry/field).MCPB is co-formulated with MCPA to broaden the spectrum of weed control. When formulated with MCPA, it is the only alternative to 2,4-DB registered for use in seedling clovers (wild white, Dutch white, ladino, alsike, and red clovers) alone or with a companion crop (wheat, barley, oats and rye). It is one of the few post-emergent herbicides registered for use in seedling grasses and in seedling alfalfa grown for seed production. Although many herbicides are registered in field corn, MCPB is one of the few that can be applied at a later stage to this crop (up to 60 cm in plant height). MCPB also plays a role in mitigating resistance development in weeds to other herbicide groups when used in rotation with them.

Measures to Minimize Risk

Labels of registered pesticide products 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 MCPB, the PMRA is requiring further risk-reduction measures for product labels.

Additional Key Risk-Reduction Measures

Refer to Appendix IV of Re-evaluation Decision RVD2012-08, MCPB for detailed label mitigation measures. 

Human Health

• Statements reducing dietary exposure relating to:
  • Pre-harvest and grazing restrictions;
  • Rotational plant back interval.
• To protect workers entering treated fields, REIs are required:
  • Most crops: 12 hours
  • Field corn: 9 days (scouting only)
• Precautionary statements to avoid drift to areas of human habitation or areas of human activity.
• Additional personal protective equipment and engineering controls.
• Limitations on the amount of kg of a.i. handled per day.

Environment

• To reduce release of MCPB into the environment: changes to label statements include measures to reduce spray drift to non-target habitats, and to prevent unintentional contamination of such areas. Also to provide measures to reduce contamination of non-target sites resulting from surface runoff and leaching.
• To protect aquatic habitats: the inclusion of spray buffer zones on the label; i.e. the end-use products may not be sprayed within 1 to 175 metres of aquatic or terrestrial habitats. The specific distance depends on the type of spray equipment and the application rate.

Value

• A maximum of one application is allowed per season when applying products containing MCPB.
• The application rate of 1.751 kg a.i./ha, which is used exclusively on peas (succulent/processing and dry/field), is no longer supported by the registrant and must be reduced to 1.594 kg a.i./ha.

What Additional Information is Being Requested?

Although the risks and value have been found to be acceptable when all risk-reduction measures are followed, additional information is required from registrants as a result of this re-evaluation:

Recent analytical data from at least five batches of the technical grade active ingredient (TGAI) must be provided for all identifiable dioxins and furans from a GLP-compliant or government-accredited laboratory.

The report should include data for the 17 substances listed in Table 4 of the Priority Substances List 1 document "Polychlorinated dibenzodioxins and polychlorinated dibenzofurans".

The analytical method(s) used must utilize the lowest practical limits of quantitation and be fully specified, either by reference to a standard method or by inclusion of a detailed description together with validation data.

Other Information

Any person may file a notice of objection regarding this decision on MCPB within 60 days from the date of publication of Re-evaluation Decision RVD2012-08, MCPB. For more information regarding the basis for objecting (which must be based on scientific grounds), please refer to the Pesticides and Pest Management portion of Health Canada's website (Request a Reconsideration of Decision) or contact the PMRA's Pest Management Information Service.