Proposed Registration Decision PRD2023-07, Diflufenican, SC500, SC600, and SC617

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Pest Management Regulatory Agency
3 August 2023
ISSN: 1925-0886 (PDF version)
Catalogue number: H113-9/2023-7E-PDF (PDF version)

Summary

Table of contents

• Proposed registration decision for Diflufenican, SC500, SC600, and SC617
• What does Health Canada consider when making a registration decision?
• What is diflufenican?
• Health considerations
• Environmental considerations
• Value considerations
• Measures to minimize risk
• Next steps
• Other information

Proposed registration decision for Diflufenican, SC500, SC600, and SC617

Health Canada’s Pest Management Regulatory Agency (PMRA), under the authority of the Pest Control Products Act, is proposing registration for the sale and use of Diflufenican Technical and SC500 containing the technical grade active ingredient diflufenican, for pre-plant and pre-emergent weed control in corn and soybean; SC600, containing the technical grade active ingredients diflufenican and metribuzinfor pre-plant and pre-emergent weed control in soybean; and SC617 containing the technical grade active ingredients diflufenican and isoxaflutole for pre-plant and pre-emergent weed control in field corn.

Metribuzin is currently registered as a herbicide on crops, including soybean, and shelterbelt plants. For details, see Proposed Acceptability for Continuing Registration PACR2005-07, Re-evaluation of Metribuzin, and Re-evaluation Registration Decision RRD2006-15, Metribuzin.

Isoxaflutole is currently registered as a broad-spectrum herbicide for use in field corn and isoxaflutole-tolerant soybeans. For details, see Proposed Re-evaluation Decision PRVD2021-02, Isoxaflutole and Its Associated End-use Products, and Re-evaluation Decision Document RVD2022-04, Isoxaflutole and Its Associated End-use Products.

An evaluation of available scientific information found that, under the approved conditions of use, the health and environmental risks and the value of the pest control products are acceptable.

This summary describes the key points of the evaluation, while the Science Evaluation of Proposed Registration Decision PRD2023-07, Diflufenican, SC500, SC600, and SC617 provides detailed technical information on the human health, environmental and value assessments of diflufenican, SC500, SC600, and SC617.

What does Health Canada consider when making a registration 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^{Footnote 1} 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 proposed conditions of registration.

The Act also requires that products have value^{Footnote 2} 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 modern, rigorous risk-assessment methods and policies. These methods consider the unique characteristics of sensitive subpopulations in humans (for example, children) as well as organisms in the environment. These methods and policies also consider the nature of the effects observed and the uncertainties when predicting the impact of pesticides. For more information on how the Health Canada regulates pesticides, the assessment process and risk-reduction programs, please visit the Pesticides portion of the Canada.ca website

Before making a final registration decision on diflufenican, SC500, SC600, and SC617, Health Canada’s PMRA will consider any comments received from the public in response to this consultation document.^{Footnote 3} Health Canada will then publish a Registration Decision^{Footnote 4} on diflufenican, SC500, SC600, and SC617, which will include the decision, the reasons for it, a summary of comments received on the proposed registration decision and Health Canada’s response to these comments.

For more details on the information presented in this summary, please refer to the Science Evaluation of PRD2023-07.

What is diflufenican?

Diflufenican is a herbicide that inhibits synthesis of phytoene desaturase, which causes degradation of chlorophyll and destruction of chloroplast membranes responsible in carotenoid production. Sensitive plants develop symptoms of stunting, discolouration, and necrosis, leading to plant death.

Health considerations

Can approved uses of diflufenican affect human health?

SC500, SC600, and SC617, containing diflufenican, are unlikely to affect your health when used according to label directions.

Potential exposure to diflufenican may occur through the diet (food and drinking water), when handling and applying the end-use product(s), or when coming into contact with treated surfaces. When assessing health risks, two key factors are considered:

• the levels where no health effects occur and
• the levels to which people may be exposed.

The dose levels used to assess risks are selected to protect the most sensitive human population (for example, children and nursing mothers). As such, sex and gender are taken into account in the risk assessment. Only uses for which the exposure is well below levels that cause no effects in animal testing are considered acceptable for registration.

Toxicology studies in laboratory animals describe potential health effects from varying levels of exposure to a chemical and identify the dose level at which no effects are observed. The health effects noted in animals occur at dose levels more than 100-times higher (and often much higher) than levels to which humans are normally exposed when pesticide products are used according to label directions.

In laboratory animals, the technical grade active ingredient diflufenican was of low acute toxicity by the oral, dermal and inhalation routes. Diflufenican was minimally irritating to the eyes and non-irritating to the skin and did not cause an allergic skin reaction.

The end-use product, SC500, containing diflufenican, was of low acute toxicity via the oral, dermal and inhalation routes of exposure. It was minimally irritating to the eyes and skin and did not cause an allergic skin reaction.

The end-use product, SC617, containing diflufenican and isoxaflutole, was of low acute toxicity via the oral and inhalation routes of exposure and considered of low acute dermal toxicity. It was mildly irritating to the eyes and minimally irritating to the skin. It did not cause an allergic skin reaction.
The end-use product, SC600, containing diflufenican and metribuzin, was of slight acute toxicity via the oral route and low acute toxicity via the inhalation route of exposure and considered of slight acute dermal toxicity. It was minimally irritating to the eyes and skin and did not cause an allergic skin reaction.

Registrant-supplied short- and long-term (lifetime) animal toxicity tests, as well as information from the published scientific literature, were assessed for the potential of diflufenican to cause neurotoxicity, immunotoxicity, chronic toxicity, cancer, reproductive and developmental toxicity, and various other effects. The most sensitive endpoints for risk assessment were effects on body weight. There was no evidence of tumourigenicity, nor was there evidence of increased sensitivity of the young compared to adult animals. The risk assessment protects against the effects noted above and other potential effects by ensuring that the level of exposure to humans is well below the lowest dose level at which these effects occurred in animal tests.

Residues in food and drinking water

Dietary risks from food and drinking water are not of health concern.

Studies in laboratory animals showed no acute health effects of diflufenican. Consequently, a single dose of diflufenican is not likely to cause acute health effects in the general population (including infants and children).

Aggregate chronic dietary (food plus drinking water) intake estimates for diflufenican indicated that the general population and all population subgroups are exposed to less than 6%of the acceptable daily intake, and therefore are not of health concern.

Metabolite BCS-BT38895 is a unique soybean seed metabolite for which separate dietary exposure assessments were conducted. Acute dietary (soybean seed commodities alone) intake estimates indicated that the general population and all population subgroups are exposed to less than 1% of the acute reference dose, and therefore are not of health concern. Chronic non-cancer dietary (food alone) intake estimates indicated that the general population and all population subgroups are exposed to less than 1%of the acceptable daily intake, and therefore are not of health concern. The lifetime cancer risk for exposure to the metabolite BCS-BT38895 from the use of diflufenican on soybeans is not of health concern to the general population (including infants and children).

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. Given that dietary risks from the consumption of foods are shown to be acceptable when diflufenican is used according to the proposed label directions, MRLs are being proposed as a result of this assessment (refer to PMRL2023-39, Diflufenican).

MRLs for diflufenican determined from the acceptable residue trials conducted throughout the United States, including growing regions representative of Canada, on soybeans, field corn and seed corn can be found in the Science Evaluation of PRD2023-07.

Some diflufenican products are also formulated with the active ingredients metribuzin or isoxaflutole. These co-active ingredients are already registered for these uses in Canada, and residues in treated commodities will be covered under the existing MRLs for each active ingredient.

Occupational risks from handling SC500, SC617 and SC600

Occupational risks are not of health concern when SC500, SC600, and SC617 are used according to the proposed label directions, which include protective measures.

Workers mixing, loading or applying SC500, SC600, and SC617, and workers entering recently treated fields can be exposed to diflufenican residues through direct skin contact or through inhalation. Therefore, the labels of SC500, SC600 and SC617 specify that anyone mixing, loading and applying these products must wear a long-sleeved shirt, long pants, chemical-resistant gloves, socks and shoes. In addition, the label of SC617 specifies that protective eyewear (goggles or face shield) must be worn during mixing, loading, clean-up and repair activities. The labels of SC500, SC600 and SC617 also require that workers do not enter or be allowed into treated fields during the restricted-entry interval (REI) of 12 hours. Taking into consideration the label statements, the single application permitted per season and the duration of exposure for handlers and postapplication workers, the risks to these individuals from exposure to SC500, SC600, and SC617 are not of health concern when the end-use products are used according to the proposed label directions.

Health risks to bystanders

Bystander risks are not of health concern when SC500, SC600, and SC617 are used according to the proposed label directions and spray drift restrictions are observed.

A standard label statement to protect against drift during application is on the labels. Therefore, health risks to bystanders are not of concern when the end-use products are used according to the proposed label directions.

Environmental considerations

What happens when diflufenican is introduced into the environment?

Diflufenican can enter the environment when it is applied as a pre-plant surface or pre-emergence spray to corn or soybean for the control of redroot pigweed, green pigweed, tall waterhemp and palmer amaranth. Diflufenican is slightly persistent to persistent under most terrestrial and aquatic conditions. Diflufenican can be broken down by microbes in soil but is not broken down by water or sunlight. Diflufenican binds to soil, thus, it is expected to have limited mobility to groundwater. In surface water, diflufenican will move into sediments where it can persist. Diflufenican is not likely to accumulate in tissues of organisms. It is not expected to travel long distances from where it was applied.

Diflufenican presents a negligible risk to earthworms, beneficial arthropods, bees, birds and mammals. Diflufenican may, however, present a risk to non-target terrestrial plants adjacent to treated fields, which could also affect wildlife habitat. In waterbodies, diflufenican may pose a risk to aquatic organisms, such as invertebrates, fish, plants, and amphibians. Spray buffer zones and precautionary label statements, are thus required to minimize the exposure to non-target terrestrial plants and aquatic habitats. When diflufenican is used in accordance with label directions, and when the required risk reduction measures are applied, the risks to the environment are considered to be acceptable.

Value considerations

What is the value of SC500 Herbicide, SC600 Herbicide, and SC617 Herbicide?

SC500 Herbicide provides early-season and season-long residual control of Amaranthus species, which are problematic and highly resistant to many herbicide modes of action in corn (field and seed) and soybean. SC600 Herbicide and SC617 Herbicide, co-formulations of diflufenican with other registered herbicides, control a broader spectrum of weeds with soil residual activity and also aim to manage existing and the future evolution of herbicide resistant weeds in corn (field and seed) and soybean.

SC500 Herbicide is formulated with diflufenican for pre-plant surface and pre-emergent application to corn (field and seed) and soybean. It provides early-season control of redroot pigweed, green pigweed, tall waterhemp, and palmer amaranth, including biotypes resistant to many herbicide modes of action, at 120–180 mL/ha and season-long control of these weeds at 180–360 mL/ha.

SC600 Herbicide is a co-formulation of diflufenican with metribuzin for pre-plant surface and pre-emergent application to soybean. It provides early-season or season-long control of weeds controlled by SC500 Herbicide and weeds controlled by registered metribuzin-based herbicides, applied at similar active ingredient rates.

SC617 Herbicide is a co-formulation of diflufenican with isoxaflutole for pre-plant surface and pre-emergent application to corn (field and seed) in Eastern Canada and British Columbia. It provides early-season or season-long control of weeds controlled by SC500 Herbicide and weeds controlled by registered isoxaflutole-based herbicides, applied at similar active ingredient rates.

Registrations of these herbicides provide users with options for pre-plant or pre-emergent residual control of broadleaf weeds, including Amaranthus species that are problematic and highly resistant to many herbicide modes of action, in corn (field and seed) and/or soybean. The application of these herbicides reduces early-season weed competition to the emerging crop allowing the crop to benefit from additional moisture, nutrients, and light that would otherwise be captured by the weeds. Weed management at this time is critical as the crop does not compete well with weeds until crop canopy closure. As all three end-use products have soil residual activity, the reduction in weed competition with the crop is extended.

Measures to minimize risk

Labels of registered pesticide products include specific instructions for use. Directions include risk-reduction measures to protect human health and the environment. These directions must be followed by law.

The key risk-reduction measures being proposed on the labels of Diflufenican Technical, SC500, SC600 and SC617 to address the potential risks identified in this assessment are as follows.

Key risk-reduction measures - Human health

To reduce the potential exposure of workers to diflufenican through direct skin contact or inhalation of sprays, workers mixing, loading, and applying SC500, SC617 or SC600 and performing cleaning and repair activities must wear a long-sleeved shirt, long pants, chemical-resistant gloves, socks and shoes. The label of SC617 also requires workers to wear protective eyewear (goggles or face shield) during mixing, loading, clean-up and repair activities. In addition, the labels of SC500, SC617 or SC600 require that workers do not enter or be allowed entry into treated fields during the REI of 12 hours. Furthermore, a standard label statement to protect against drift during application is on the labels.

Key risk-reduction measures - Environment

The following risk reduction measures are required to be added to the label:

• Environmental precautionary statements for non-target terrestrial plants and aquatic organisms;
• Spray buffer zones to protect aquatic and non-target terrestrial habitats;
• Standard runoff statements

Next steps

Before making a final registration decision on diflufenican, SC500, SC600, and SC617, Health Canada’s PMRA will consider any comments received from the public in response to PRD2023-07. Health Canada will accept written comments on this proposal up to 45 days from the date of publication of PRD2023-07. Please note that, to comply with Canada's international trade obligations, consultation on the proposed MRLs will also be conducted internationally via a notification to the World Trade Organization. Please forward all comments to Publications. Health Canada will then publish a Registration Decision, which will include its decision, the reasons for it, a summary of comments received on the proposed decision and Health Canada’s response to these comments.

Other information

When the Health Canada makes its registration decision, it will publish a Registration Decision on diflufenican, SC500, SC600, and SC617 (based on the Science evaluation of PRD2023-07). In addition, the test data referenced in this consultation document will be available for public inspection, upon application, in the PMRA’s Reading Room. For more information, please contact the PMRA’s Pest Management Information Service.