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Imazamox - Active Ingredient Registration 3/03

New York State Department of Environmental Conservation
Division of Solid and Hazardous Materials
Bureau of Pesticides Management, 9th Floor
625 Broadway, Albany, New York 12233-7254
Phone: (518) 402-8788     FAX: (518) 402-9024
Website: www.dec.state.ny.us

March 13, 2003

CERTIFIED MAIL
RETURN RECEIPT REQUESTED


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

Dear Ms. Fersch:

Re: Registration of the New Active Ingredient Imazamox, Contained in the Pesticide Product Raptor® Herbicide (EPA Reg. No. 241-379)

    The New York State Department of Environmental Conservation (Department) has reviewed the application, received October 23, 2002, from BASF Corporation Agricultural Products to register the above mentioned product in New York State. The application was deemed complete for purposes of review on December 11, 2002 and a registration decision is due by May 9, 2003.

    Raptor® herbicide is a water-based formulation containing the ammonium salt of imazamox (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid). It is used for post-emergence grass and broadleaf weed control in alfalfa, edible legumes, and soybeans. Raptor herbicide is applied as a postemergence treatment when weeds are actively growing and before they exceed the maximum recommended size. Postemergence applications of Raptor require the addition of an adjuvant and a nitrogen fertilizer solution. When Raptor is applied postemergence, absorption will occur through both the roots and foliage. Susceptible weeds stop growing and either die or are not competitive with the crop. It also provides activity on susceptible weeds that may emerge shortly after application. Raptor may be applied by either ground application or aerial application.

HEALTH EFFECTS:

    Neither imazamox nor the formulated product Raptor Herbicide was very acutely toxic by the oral, dermal and inhalation routes of exposure in laboratory animal studies. Imazamox was moderately irritating to the eyes, but not to skin, whereas Raptor Herbicide was not irritating to either skin or eyes (tested on rabbits). In addition, neither imazamox nor Raptor Herbicide was a skin sensitizer (tested on guinea pigs).

    Imazamox did not cause any toxicity in subchronic or chronic feeding studies. A 28-day dermal toxicity study in rats, a 90-day feeding study in rats, and a 90-day feeding study in dogs reported no systemic toxicity from exposure to this chemical at the highest doses tested (HDT) of 1,000 milligrams per kilogram body weight per day (mg/kg/day), 1,661 mg/kg/day, and 1,400 mg/kg/day, respectively. A chronic feeding study in dogs and chronic feeding/oncogenicity studies in rats and mice also reported no toxicity from imazamox at HDTs of 1,165 mg/kg/day, 1,284 mg/kg/day, and 1,348 mg/kg/day, respectively. There was no evidence of carcinogenicity in either the rat or mouse study. In addition, imazamox was negative in a number of genotoxicity studies. Based on these findings, the United States Environmental Protection Agency (USEPA) designated imazamox as "not likely to be carcinogenic to humans."

    Imazamox caused some maternal effects in developmental toxicity studies in laboratory animals. Pregnant rats given imazamox had reduced body weight at 1,000 mg/kg/day; the no-observed-effect-level (NOEL) was 500 mg/kg/day. Reduced food consumption at 600 mg/kg/day was the only reported maternal effect in rabbits fed imazamox; the NOEL was 300 mg/kg/day. No developmental toxicity was reported in either the rat or rabbit studies at doses up to 1,000 and 900 mg/kg/day, respectively. Imazamox caused no reproductive or parental toxicity in a multi-generation reproduction study in rats at the HDT of 1,705 mg/kg/day. The USEPA, Office of Pesticide Programs established a reference dose (RfD) of 3.0 mg/kg/day for imazamox based on a NOEL of 300 mg/kg/day from the developmental toxicity study in rabbits and an uncertainty factor of 100 (10 for interspecies extrapolation and 10 for intraspecies variation). This RfD has not yet been adopted by the USEPA's Integrated Risk Information System (IRIS). A search of the current literature did not identify any toxicological concerns.

    The USEPA established tolerances for imazamox residues in or on soybeans at 0.1 parts per million (ppm); edible legumes (0.05 ppm); alfalfa seed, forage, and hay (0.4, 2.0, and 4.0 ppm, respectively). Because of this chemical's low toxicity, the USEPA did not conduct a dietary risk assessment for imazamox residues in or on the other above noted crops.

    There are no chemical specific federal or State drinking water/groundwater standards for imazamox. Based on its chemical structure, imazamox falls 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 imazamox and Raptor Herbicide indicates that they are not very acutely toxic or irritating to the skin and eyes. Imazamox also did not cause any significant toxicity in subchronic, chronic, developmental or reproductive toxicity studies. The USEPA did not conduct either a worker or dietary risk assessment for imazamox because an appropriate toxicological endpoint was not identified for the dermal or inhalation routes of exposure. Given the low potential for risks to workers and the general public, we do not object to the registration of Raptor Herbicide in New York State based on direct health concerns.

ECOLOGICAL EFFECTS:

    CHEMICAL DESCRIPTION: Raptor Herbicide is a water-based formulation containing a new active ingredient (AI), the ammonium salt of imazamox. Raptor is used for post-emergence control of broadleaf weeds and grass in alfalfa, edible legumes, and soybeans. One gallon of Raptor contains a pound of imazamox acid equivalent. The highest application rate of formulated product is 6 fluid ounces per acre, which is approximately 0.05 lbs AI/acre. Only one Raptor application is allowed per year.

    Imazamox belongs to the imidazolinone class of pesticides. Their mode of action is to inhibit acetohydroxyacid synthesis, an enzyme involved with the biosynthesis of the amino acids leucine, isoleucine, and valine. Animals do not synthesize these amino acids via this pathway, so imidazolinone herbicides generally exhibit very little toxicity to animals, birds, fish, or insects. They are potent herbicides however, and if not used carefully, they can be harmful to non-target plants adjacent to treated fields.

    Imazamox can be mobile and is fairly persistent. Microbial degradation is the primary fate process in soil, where the aerobic metabolism half-life is about 28 days. In five field dissipation studies on various soil types, imazamox dissipated with half-lives ranging from 35 to 118 days; the geometric mean being 59 days. In water, the photolytic half-life was found to be 6.8 hours (0.23 days). If not degraded photolytically, imazamox is very stable and persistent in aquatic sediments.

    MODEL PARAMETERS: The standard Pesticide Screening System models were run. For PONDTOX, the assumption was made that 25% of the applied product would be lost to foliar intercept. The estimated runoff percentage was 5%.

    MODEL RESULTS: All models showed that labeled application rates of imazamox did not cause an exceedance of any toxicity threshold.

    MODEL ANALYSIS/RISK ASSESSMENT: No toxicity threshold was exceeded, indicating that imazamox was unlikely to harm birds, mammals, fish, aquatic insects or plants. The greatest concern about imazamox was its resistance to degradation in aquatic sediments, where the half-life exceeds two years (761 days). On the other hand, imazamox degrades very rapidly in water, with a half-life of less than seven hours. Unfortunately, the degradation process is photolysis. Degradation will be much slower in turbid waters or deep waters. Imazamox is an ionic compound, so it is likely to bind with charged clay particles in sediments. Bound imazamox might be biologically inactivated. It also might be subject to photolysis even when bound to sediment particles. The greatest risk is to non-target aquatic macrophytes, which could be impacted if imazamox concentrations were to build up in aquatic sediments. However, the low application rate and label restriction of one application per season suggests that the accumulation of biologically active imazamox in aquatic sediments is likely to be very limited. When used as labeled, Raptor Herbicide is unlikely to harm non-target fish, birds, mammals or plants.

ENVIRONMENTAL FATE AND GROUNDWATER IMPACTS:

    BASF Corporation is applying to register Raptor Herbicide (also referred to as AC 299,263) for post-emergence grass and broadleaf weed control in alfalfa, edible legumes and soybeans. It is foliarly applied either by ground or aerial application. Raptor is a liquid and contains 12.1% by weight active ingredient, or 1 lb ai /gal as the free acid. The maximum application rate is 4-6 fluid ounces of product per acre per year or 0.031 to 0.047 lb ai/a/yr, depending on crop type. None of the inerts appear to be solvent carriers.

Technical Review

    Hydrolysis: Imazamox is stable to hydrolysis in pHs 5, 7, and 9.

    Solubility: No DER is available. Imazamox has a solubility of 4.5 X 10+5 mg/L (.45 g/ml) according to the MSDS.

    Aqueous Photolysis: Imazamox degraded with an average half-life of 6.8 hours. There were six major degradates: 2,3,5-pyridine tricarboxylic acid (CL 351,543) at 34% of applied at 30 days; 2-carbamoyl-5-(ethoxy-methyl) nicotinic acid (CL 359-770) at 22.1% of applied at 30 days; 2-[(1-carbamoyl-1,2-dimethylpropyl)carbamoyl]-5-(methoxymethyl)nicotinic acid (CL 336,554) at 36.4% of applied at one day; 2-carbamoyl-3,5-pyradine dicarboxylic acid (CL 359,712) at 11.1% at three days; 5-methoxymethyl-2-3-pyridine dicarboxylic acid (CL 334,151) at 28.7% of applied at one day; and 2-formyl-5-(methoxymethyl) nicotinic acid (AC9960-139A) at 14.6% of applied at seven days. One unidentified degradate reached 12.3% of applied at 21 days.

    Soil Photolysis: Imazamox degraded slowly with a calculated half-life of 65 days. One major degradate formed, the diacid AC 312,622, at a maximum of 15.4% at 30 days.

    Aerobic Soil Metabolism: Two studies were performed. The first was only partially acceptable, but had a half-life of 30 days, and degradate CL312,622 at 45% at 6 weeks. In the second study, which USEPA found acceptable, imazamox degraded with a registrant-calculated half-life of 28 days in a sandy loam. Major degradates included the diacid AC 312,622 at 43.5% at day 44 (decreasing to 3.4 at 365 days), which then degraded to AC 354,825 which reached 55.2% at 365 days after treatment. The CL312,622 residues never reach 50% of the applied dose because as parent residues are converted to CL 312,622, residues of CL 312,622 are being converted to CL 354,825.

    Anaerobic Aquatic Metabolism: USEPA found this study scientifically valid, but it did not meet Subdivision N Guidelines. The parent is essentially stable with a registrant-calculated half-life of 654 days. The majority of the "degradation" was due to parent compound leaving the aqueous phase and associating with the soil. The USEPA reported half-life was 654 days.

    Adsorption/Desorption: This study partially satisfied the Subdivision N Guidelines. Imazamox appears to be very mobile to mobile in 6 soils, with Freundlich Kd's of 0.05 (sandy loam), 0.13 (silty clay loam), 0.24 (loam), 0,3 (sandy loam), 1.43 (silt loam), and 2.71 (clay loam). Mobility does not appear to be correlated with any soil characteristics, such as percent organic carbon, pH or CEC.

    Degradate Adsorption/Desorption: AC 312,622 is very mobile with linear Kocads of 278 in a loamy sand, 93 in a sandy loam, 59 in a loam, 208 in a silt loam, 27 in a silty clay loam and 69 in a clay loam soil. Linear Kocdes values were 28 in a loamy sand, 6 in a sandy loam, 8 in a loam, 157 in a silt loam, 1 in a silty clay loam and 45 in a clay loam soil.

    Degradate Adsorption/Desorption: AC 354,825 is moderately mobile to immobile with linear Kocads of 1364 in a loamy sand,1259 in a sandy loam, 1051 in a loam, 1924 in a silt loam, 331 in a silty clay loam and 1415 in a clay loam soil. Linear Kocdes values were 3228 in a loamy sand, 2658 in a sandy loam, 1905 in a loam, 3400 in a silt loam, 640 in a silty clay loam and 1415 in a clay loam soil.

    Field Dissipation: USEPA found studies conducted at two of the five sites to be acceptable. Imazamox dissipated with registrant-calculated half-lives of 130, 50, 35, 15 and 50 days for five different studies. In the acceptable studies, in Arkansas the half-life was 35 days and in California the half-live was 50 days. AR and CA had parent detections at three- and nine-inch depths, respectively. The CA site also had CL 354,825 at six inches which remained until day 557 at 5.5% of applied.

    Computer Modeling: Modeling by the Department on Riverhead sand, using 0.047 lb ai/a/yr, a Kf of 0.3 for the parent, and a Koc of 93 for degradate CL 312,622 (43.5%) and a Koc of 1364 for degradate CL 354,825 (55.2%), and an aerobic half-life of 28 days projected breakthrough of the parent in the first year, with peaks ranging up to 0.035 ppb throughout the ten year cycle. The model projected breakthrough of degradate CL312,622 in the first year with peaks ranging up to 0.2 ppb. The model projected breakthrough of degradate CL354,825 in the first year with peaks ranging up to 0.015 ppb.

    While the parent and degradate CL 312,622 have low Kocs and are fairly mobile, the half-life of the parent is 28 days and the application rate is very low. The short half-life shows up in the modeling with cyclical peaks for the parent and both degradates, but the most mobile degradate, CL 312,622 is only projected to reach an occasional peak of 0.2 ppb during the ten-year modeling cycle. Mobility does not appear to be correlated with any soil characteristics, such as percent organic carbon, pH or CEC. However, even though the parent and one degradate are fairly mobile, they are not persistent, so impact to ground water should be minimal.

    The Department concludes that Raptor® herbicide should not have an adverse effect on the health of workers or the general public, the fish and wildlife resources, or the ground and surface water of New York State when used as labeled.

    Therefore, the Department hereby accepts for general use registration in New York State Raptor® herbicide (EPA Reg. No. 241-379) which contains the new active ingredient imazamox.

    Enclosed are your Certificate of Registration and New York State stamped "ACCEPTED" label.

    If you have any questions, please contact Mr. Samuel Jackling, Chief of our Pesticide Product Registration Section, at (518) 402-8768.

Sincerely,

Maureen P. Serafini
Director
Bureau of Pesticides Management

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