PMEP Home Page --> Pesticide Active Ingredient Information --> Herbicides, Growth Regulators, and Desiccants --> Herbicides, F to M --> Imazapyr --> Imazapyr - Lightning® Registration 11/03

Imazapyr - Registration of Lightning® Herbicide 11/03

New York State Department of Environmental Conservation
Division of Solid & Hazardous Materials

Bureau of Pesticides Management
625 Broadway, Albany, New York 12233-7254
Phone 518-402-8788 FAX 518-402-9024

November 17, 2003


Ms. Judy Fersch
BASF Corporation
P.O. Box 13528
Research Triangle Park, North Carolina 27709

Dear Ms. Fersch:

Re: Registration of Lightning® Herbicide (EPA Reg. No. 241-377) Containing the Active Ingredients Imazapyr and Imazethapyr

The New York State Department of Environmental Conservation (Department) has completed its technical review of your application and data packages submitted in support of the registration of Lightning® Herbicide (EPA Reg. No. 241-377) containing the active ingredients imazapyr and imazethapyr. Lightning® Herbicide is a new product which combines two active ingredients from two previously registered products, Pursuit® and Arsenal® Herbicides for use on Clearfield™ corn.

The Department has registered this product for labeled use on Clearfield™ corn hybrids in New York State. Lightning® Herbicide contains two active ingredients which may cause potential contamination of groundwater resources in New York State. In order to better track the use of this product in New York State, this Department has "restricted" it's sale and use. The product labeling also prohibits its use in Nassau and Suffolk Counties on Long Island.

Lightning® Herbicide contains the active ingredients imazapyr and imazethapyr, both ingredients are in the imidazolinone family of compounds which inhibit the actions of plant enzymes, thus stopping plant growth. These compounds generally have a low mammalian toxicity and are applied at very low rates, generally 1/50 of other common herbicides to achieve the same control. The formulated product is used to control postemergent annual weeds in imidazolinone tolerant Clearfield™ corn hybrids only. The maximum application rate is 1.28 ounces per acre per year on Clearfield™ corn (0.042 lb. imazethapyr and 0.014 lb. imazapyr per acre per year).

Lightning® Herbicide is a new product which combines two active ingredients from two previously-registered products, Pursuit® and Arsenal® Herbicides for use on Clearfield™™ corn. The active ingredient imazethapyr recently underwent a full technical review by this Department, which resulted in the approval to use Pursuit® DG Herbicide on Clearfield™ corn hybrids, on August 14, 2000. However, the active ingredient imazapyr was registered by this Department on April 24, 1986, prior to the current review process.

The Lightning® Herbicide major change in labeling (MCL) application was received on January 27, 2003. The Department notified BASF Corporation of an incomplete application via letter dated March 24, 2003. BASF Corporation submitted a data package on April 10, 2003, to complete their application for a MCL review of Lightning® Herbicide. The Lightning® Herbicide MCL application package was declared complete as per Department letter dated June 23, 2003. Pursuant to the review time frame specified in ECL §33-0704.2, a registration decision date of November 20, 2003 was established. The Department conducted the following technical reviews with regard to the registration of Lightning® Herbicide for impacts to human health, nontarget organisms, and the environment. Review summaries are provided below:

Product - Labeled Use:

The formulated product Lightning® Herbicide (EPA Reg. No. 241-377) contains the active ingredients imazethapyr and imazapyr at 52.5% and 17.5%, respectively. This product controls broadleaf, annual grass, and sedge weeds in Clearfield™ corn hybrids. The product is applied as a foliar spray for postemergenf weed control. Ground and aerial applications may use up to 1.28 ounces of product per acre of corn per application with a maximum number of one application per year. Lightning® Herbicide is a water dispersible granule packaged in water soluble bags, which are mixed with no less than ten gallons of water per acre for ground applications and no less than five gallons of water per acre for aerial applications. The Lightning® Herbicide label warns the user that, "this chemical has properties and characteristics associated with chemicals detected in groundwater." The "Environmental Hazards" section of the Lightning® label directs the user to not apply the product through irrigation equipment, or directly to water, and not to contaminate water when disposing of equipment washwaters. The product is most effective when used at temperatures above 50° F and adequate soil moisture to increase plant uptake and photosynthesis. The active ingredients are then better able to inhibit the actions of plant enzymes, thus stopping plant growth and further resulting in plant death. The label prohibits the sale or use of the product on Long Island in New York State.

Human Health Summary:

The New York State Department of Health (DOH) reviewed the application and supporting data submitted by the BASF Corporation to determine potential impacts to human health related to the registration of the pesticide product Lightning Herbicide (EPA Reg. No. 241-377) in New York State.

On an acute basis, Lightning Herbicide is a severe eye irritant (tested on rabbits). The United States Environmental Protection Agency (USEPA) did not require any other acute toxicity, skin irritation or skin sensitization studies on this pesticide product for federal registration.

In the past, DOH has reviewed imazethapyr in three Pursuit Herbicide products. In these reviews, DOH did not identify any significant toxicological concerns. A current search of the toxicological literature did not find any significant new information on the toxicity of imazethapyr.

The active ingredient, imazapyr, had not previously been evaluated, therefore, information on the toxicology and environmental fate of this compound was required to be submitted by BASF for this review. On an acute basis, imazapyr was not very toxic to laboratory animals by the oral, dermal or inhalation routes of exposure. It was not very irritating to rabbit skin nor did it cause dermal sensitization (guinea pigs). It, however, was corrosive to the eyes (rabbits).

Imazapyr caused very few toxicological effects in subchronic, chronic and developmental/reproductive studies in laboratory animals. All of these toxicity studies were conducted at doses of 400 milligrams per kilogram body weight per day (mg/kg/day) or higher, except the chronic dog feeding study where the highest dose tested was 250 mg/kg/day. The USEPA Office of Pesticide Programs established a reference dose (RfD) of 2.5 mg/kg/day for imazapyr based on a no-observed-effect level (NOEL) of 250 mg/kg/day from the chronic feeding study in dogs and an uncertainty factor of 100. This RfD has not yet been adopted by the USEPA's Integrated Risk Information System (IRIS).

Imazapyr did not cause oncogenic effects in either rat or mouse chronic feeding studies. It also was negative in a number of genotoxicity studies. The USEPA classified imazapyr as a Group E (evidence of noncarcinogenicity in humans) carcinogen.

The USEPA established tolerances for imazapyr and imazethapyr residues in or on corn of 0.05 parts per million (ppm) and 0.1 ppm, respectively. The USEPA estimated that the chronic dietary exposure to the residues of imazapyr would be less than one percent of the RfD (2.5 mg/kg/day) for all age groups. For imazethapyr, dietary exposure would be less than 0.2% of the RfD (0.25 mg/kg/day) for the general U.S. population, less than 0.5% percent for non-nursing infants and less than 0.3% for children one to six years old. These chronic exposure analyses are based on the conservative assumptions that 100% of crops are treated and that these crops contain tolerance level residues.

The USEPA did not require a worker risk assessment for either imazapyr or imazethapyr since no toxicological endpoints have been identified for either short- or intermediate-term exposures to these two active ingredients. Furthermore, based on their intermittent use pattern on corn, no chronic exposures are expected to result from their use.

Both imazapyr and imazethapyr have the properties of chemicals that can leach through soil and contaminate groundwater/drinking water. The label for Lightning Herbicide states under Environmental Hazards that, "This chemical [sic] has properties and characteristics associated with chemicals detected in groundwater. The use of this chemical in areas where soils are permeable, particularly where the water table is shallow, may result in groundwater contamination."

There are no chemical-specific federal or State drinking water/groundwater standards for either imazapyr or imazethapyr. Based on their chemical structures, each of these two compounds fall under the 50 microgram per liter New York State drinking water standard for "unspecified organic contaminants" (10 NYCRR Part 5, Public Water Systems).

The available information on Lightning Herbicide and its two active ingredients, imazapyr and imazethapyr, indicates that overall neither the formulated product (for which very limited data were submitted) nor its active ingredients are very toxic. While both the formulated product and imazapyr can cause severe eye irritation, the product's label requires applicators and other handlers to wear protective eyewear. Exposure of the general population to imazapyr and imazethapyr should also be minimal given the use pattern of the Lightning product and the limited consumption from dietary sources. Given the above, the use of Lightning Herbicide should not pose a significant risk to workers or the general public.

Nontarget Organism Summary:

The Department's Bureau of Habitat (BoH) reviewed the data submissions for both the formulated product and each of the active ingredients. The BoH summarized their findings as follows: "Transient effects to non-target aquatic and terrestrial vascular plants are possible but will be minimal given the limited use pattern and low application rate. No other acute or chronic non-target organism effects are likely." The BoH has no objections to the registration of Lightning Herbicide and believes that the use of this product should not pose a significant risk to nontarget organisms in New York State.

Environmental Fate Summary:

Technical Review of Imazethapyr -The following information is taken from the January 10, 2000 technical review:

Hydrolysis: Imazethapyr is stable and persistent.

Solubility: Imazethapyr has a low water solubility of 1 g/l.

Aqueous Photolysis: Imazethapyr has a half-life of 46 hours under artificial light.

Soil Photolysis: Imazethapyr has a half-life of 33 months.

Aerobic Soil Metabolism: The half-life ranges from 33 to 37 months.

Anaerobic Soil Metabolism: The half-life is > two months.

Adsorption/Desorption: Kocs range from 97 to 283 in various soil types.

Field Dissipation: Field dissipation studies consistently show persistence regardless of soil type, agriculture practice and climatic effects. Studies done with an application rate of 2 oz/ai/acre (twice the labeled rate) in a permeable sandy loam indicated that the half-life is in the two- to four-month range at sites that approximate conditions in New York State. The compound was detected in the six- to 12-inch sampling interval, and 14 ppb was detected in the 12- to I 8-inch sampling interval. The USEPA reviewer indicated in the March 9, 1994 memorandum that residues may have leached deeper than 18 inches, but were not detected. It was also stated that, given the low application rate and relatively high limit of analytical sensitivity, analyzing the soil in six-inch increments may have diluted residues.

Degradate: Only one major degradate was found, 5-ethyl-3-pyridine carboxylic acid at 19.8%, but it was found during aqueous photolysis. Aqueous photolysis, however, is not considered to be a significant factor in the breakdown of the parent material.

Groundwater Monitoring: Two small-scale prospective groundwater studies were started in 1997, one in the midwest, one in the southeast. Monitoring wells and lysimeters were installed, and soil samples were collected. Application rate was 1.0 oz ai/a/yr, or 0.063 lb ai/a/yr, the labeled application rate for field corn. The second progress report was submitted in November 1999. Analytical results for DAT 0 (days after treatment) through DAT 180 of imazethapyr in soil and analytical results for DAT 7 though DAT 300 for imazethapyr in soil-water samples were presented. No groundwater sample results from sampling after application were available in the southeast site due to low water levels.

In the southeast site (loamy sand), the soil-water sample concentrations were all less than 0.05 ppb at three-foot, six-foot, nine-foot and 15-foot lysimeters. Soil samples from 0 to six inches reached 5.8 ppb at DAT 8, and declined to less than 1 ppb by DAT 90. Concentrations from the six- to 24-inch depth were less than 1 ppb.

In the midwest site (loamy sand), the soil-water sample concentrations ranged from 0.11 to 0.69 in the three-foot lysimeter. In lysimeter L3, the 15-foot lysimeter had from 0.77 to 0.85 ppb up to DAT 175. For some reason, they did not analyze for imazethapyr after DAT 175 in any lysimeters. Soil samples from 0 to six inches reached 39 ppb at DAT 3, and declined to less than 7.5 ppb by DAT 232. Concentrations from the six- to nine-inch depth were 3.7 to less than 1 ppb, from nine to 12 inches were 1.9 and 1.2 ppb, and less than 1 ppb from 12 to 23 inches. Groundwater sample results were not available.

Computer Modeling: Running LEACHM on Riverhead soil using a Koc of 97, a half-life of three years and an application rate of 0.042 lb ai/acre/year, the model projected breakthrough in year one, and consistent peaks of five to six ppb.

USEPA Comments (1/14/94 USEPA One Line Summary): Imazethapyr is persistent and mobile according to laboratory environmental fate studies. Calculations using data from two aerobic metabolism studies gave the half-life as 33-37 months; adsorption/desorption studies with aged parent showed the compound to be very mobile with Kds of 0.46 to 0.82 for four soils--two sandy loams and two silt loams. Percentages for sand, silt, clay and organic matter did not appear to have an effect on sorption. Field dissipation studies with imazethapyr consistently show persistence regardless of soil type, agriculture practice and climatic effects.

Technical Review of Imazapyr: Please note, no DER was submitted, the information was taken from a 3/15/84 EFB memorandum.

Hydrolysis: No DER was submitted. Imazapyr is stable to hydrolysis at pHs 5, 7, and 9. Aqueous Photolysis: No DER was submitted. Imazapyr photodegraded in aqueous solutions with a half-life ranging from 1.3-2.7 days (based on a continuous 24-hour irradiation) and from 2.5-5.3 days (based on a 12-hour irradiation period).

Aerobic Soil Metabolism: USEPA found these studies acceptable. Imazapyr did not degrade when exposed to aerobic soil conditions. The calculated half-life was 5.9 years.

Anaerobic Soil Metabolism: No DER was submitted, but the USEPA reviewer indicated that this study was acceptable. Imazapyr did not degrade when exposed to a sandy loam soil under anaerobic soil conditions.

Aerobic Aquatic Metabolism: USEPA found this study acceptable. Imazapyr did not degrade when exposed to aquatic conditions maintained in water or sediment. The calculated half-life was 5.9 years.

Adsorption/Desorption: No DER was submitted. Information was taken from the 3/15/84 EFB memorandum indicating a moderate leaching potential. Imazapyr has a low soil adsorption potential and a high potential for desorption. However, acidic soils may have enhanced adsorption. Freundlich adsorption coefficients were 3.8 in a sandy loam, 2.1 in a loamy sand, 1.7 in a clay loam and 4.9 in a silt loam.

Terrestrial Field Dissipation: No DERs were submitted. Five studies were done, but none were acceptable; USEPA was concerned about the characterization of the soils and the depth of sampling. The calculated half-life in a sandy loam soil was seven months. In a silt loam soil, the calculated half-life was 21 days. No half-lives were provided for the remaining three studies.

Computer Modeling: Running LEACHM on Riverhead soil, using a Kf of 2.1, a half-life of 5.9 years and an application rate of 0.014 lb ai/acre/year, the model projected breakthrough in year one, and consistent peaks of two to three ppb.

Groundwater Review Summary: Imazethapyr is currently registered for use on corn at 0.067 lb ai/a/yr, higher than the proposed application rate for Lightening (0.042 lb ai/a/yr). Imazapyr is currently registered for non-crop use at a maximum application rate of 1.5 lb ai/a/yr, which is much higher than the proposed rate for Lightening (0.014 lb ai/a/yr). The Lightening label bears text prohibiting its use in Nassau and Suffolk Counties. While both of these active ingredients have parameters that would indicate that they have the potential to leach into groundwater, the maximum application rate of the active ingredients in Lightening is less than that of the products already registered for use in NYS. The amount of acreage that Lightening would be used on is probably not significantly greater than the acreage that currently receives applications of Arsenal and Pursuit. Therefore, registration of this product for use as labeled, in New York State, will have no significant impact to groundwater resources.

Registration Summary:

The Department will register Lightning Herbicide for use as labeled in New York State but will restrict the use of this product to certified applicators to be protective of groundwater resources. The current labeling also prohibits the sale or use of the product on Long Island.

Enclosed for your record is a copy of the stamped accepted label and the Certificate of Registration for Lightning Herbicide (EPA Reg. No. 241-377). Please note that a proposal by BASF or any other registrant, to register a product that contains imazapyr and/or imazethapyr, and whose labeled uses are likely to increase the potential for significant impact to humans, nontarget organisms, or the environment, would constitute a MCL use pattern. Such an application must be accompanied by a new application fee and meet the requirements listed in Appendix 1.B. of "New York State Pesticide Product Registration Procedures" (August 1996). Such information as well as forms can be accessed at our website as listed in our letterhead.

Please note that the Lightning® Herbicide product, as noted on the "restriction" column on the Certificate, is to be classified as "restricted use" under rules and regulations 6NYCRR 326.23(e). As such, this product is restricted in its purchase, distribution, sale, use and possession in New York State.

According to New York State Department of Environmental Conservation Regulations 6NYCRR 326.3(a): "It shall be unlawful for any person to distribute, sell, offer for sale, purchase for the purpose of resale, or possess for the purpose of resale, any restricted pesticide unless said person shall have applied for, and been issued a commercial permit."

The Pesticide Reporting Law (PRL) requires all certified commercial pesticide applicators to report information annually to the Department regarding each pesticide application they make. Commercial pesticide retailers are required to report all sales of restricted pesticide products and sales of general use pesticide products to private applicators for use in agricultural crop production. If no sales are made within New York State, a report still must be filed with the Department indicating this is the case.

If you need information relating to the Pesticide Reporting Law, or annual report forms, please visit the Department's website at or contact the Pesticide Reporting Section at (518) 402-8765.

Should you require information to obtain a commercial permit, please contact Thomas Lynch, Chief, Pesticide Certification Section, at (518) 402-8748.

Please be aware that any unregistered product may not be sold, offered for sale, distributed, or used in New York State.

Please contact our Pesticide Product Registration Section, at (518) 402-8768, if you have any questions.


Maureen P. Serafini Director
Bureau of Pesticides Management

cc: w/enc. - N. Kim/D. Luttinger - NYS Dept. of Health
R. Zimmerman/R. Mungari - NYS Dept. of Ag. & Markets
W. Smith - Cornell University, PMEP