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Primisulfuron-methyl

Publication Date: 5/94

TRADE OR OTHER NAMES

Trade names for the compound include Beacon, CGA-136872 and Rifle. Primisulfuron-methyl may be found in other herbicide formulations such as 2,4-D, dicamba, cyanazine, bromoxynil and atrazine.

REGULATORY STATUS

Primisulfuron-methyl is a General Use Pesticide (GUP).

INTRODUCTION

Primisulfuron-methyl is a urea herbicide that is used on crop and non- crop areas for the control of grass weeds and many kinds of broad-leaved weeds. The compound is a selective postemergence systemic herbicide that is rapidly absorbed by plants and transported throughout the plant roots and foliage. The herbicide works by blocking cell growth in the active growing regions of the plant. The herbicide also acts to inhibit photosynthesis (1, 4).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Primisulfuron-methyl is a slightly toxic compound and requires that the signal word CAUTION be used on the label. The acute oral LD50 for primisulfuron-methyl is greater than 5,050 mg/kg in the rat and the LD50 for skin toxicity is greater than 2,010 mg/kg (1, 2). Both of these values indicate that there is little acute toxic threat from exposure or contact with the herbicide. In addition there is little skin irritation from primisulfuron-methyl and it is not a skin sensitizer (4). In rabbits, primisulfuron-methyl caused slight eye irritation that cleared within three days after contact (2). The four hour inhalation LC50 of primisulfuron- methyl was greater than 4.8 mg/l for rats (1).

The trade product Beacon, which contains 75% of this active ingredient, is classified as slightly toxic for skin, inhalation and eye exposure. It is not a skin sensitizer (1).

CHRONIC TOXICITY

Rats and mice fed very large amounts of primisulfuron-methyl for up to ninety days showed no ill effects (2). This indicates that even at relatively high levels of exposure (500 mg/kg) there is little toxic effect from the compound over relatively short exposure periods. Male rats fed higher doses of primisulfuron-methyl (up to 1,000 mg/kg) for three months showed a decrease in body weight and a decrease in spleen weight. The lowest dose at which adverse effects were noted (LEL) was about 150 mg/kg (2). When dogs were fed moderate doses of primisulfuron-methyl for a year, a number of effects were noted at the highest doses tested (125 mg/kg) which include changes in the blood such as increased platelets and anemia. Other changes include decreased cholesterol, pale livers, and thyroid gland changes.

This evidence suggests that the chronic risks of human exposure to moderate levels of the herbicide for extended periods of time are slight.

Reproductive Effects

In a two generation study, rats were fed moderate doses (250 mg/kg) of primisulfuron-methyl. Rats had decreased testicular function and the offspring had decreased body weights. No compound related reproductive effects were noted at doses below 50 mg/kg/day (1).

In a two year dietary study, rats were fed high doses (500 mg/kg) of primisulfuron-methyl. The rats, at the end of the study, had soft and atrophied testes (1). Further study is necessary to fully assess the potential reproductive toxicity of the compound, although it does not appear that such toxicity would occur during normal use.

Teratogenic Effects

Pregnant rabbits fed high doses of primisulfuron-methyl produced normal offspring. Only at doses of 300 mg/kg and above were there changes in maternal body weight, spontaneous abortion and changes in the maternal stool. In another study, pregnant rats were fed moderate doses of primisulfuron-methyl. They showed increases in the number of litters having incomplete bone formations, though this study was inconclusive.

The EPA has indicated that it is unlikely, given the results of the three studies noted above, that primisulfuron-methyl is teratogenic to humans (1).

Mutagenic Effects

Primisulfuron-methyl did not cause mutations in various tests on bacteria, hamster cells, rat liver cells and human cells. This indicates that, at the doses tested, the compound is not mutagenic (1).

Carcinogenic Effects

Two different tests have been conducted to assess the carcinogenicity of primisulfuron-methyl. The first study lasted for two years with a high dose of 500 mg/kg. While there were some adverse effects, atrophied testes and decreased food consumption, no evidence of cancer was found.

In another study, mice were fed up to 1,200 mg/kg/day for a year and a half. Liver tumors in the mice were observed. However, these tumors and other adverse effects were present at doses exceeding the maximum tolerated dose and when mortality occurred (1).

The EPA's Agency Peer Review Committee has determined that primisulfuron-methyl is not classifiable as a human carcinogen.

Organ Toxicity

Several adverse effects have been noted in the organs of rats and mice which were fed large doses of primisulfuron-methyl. The most notable were changes in liver and spleen weights and in the testes of rats (1).

Fate in Humans and Animals

In a rat study, nearly 97% of the primisulfuron-methyl was excreted in the feces and the urine within 9 days. Most of the parent compound was unchanged, indicating little metabolic activity. However, there were small concentrations of five different breakdown products in the urine and feces. Similar patterns of breakdown were noted in the goat and the chicken (1).

ECOLOGICAL EFFECTS

Effects on Birds

Primisulfuron-methyl is practically non-toxic to wildfowl. Both bobwhite quail and mallard ducks show a high tolerance for the compound. The LD50 for primisulfuron-methyl in both species was in excess of 2,150 mg/kg. In addition, mallards fed moderate amounts of the compound showed no adverse reproductive effects at the highest dose tested, 500 ppm (1).

Effects on Aquatic Organisms

Primisulfuron-methyl is slightly toxic to freshwater fish, aquatic organisms and to marine (estuarine) shrimp. The LC50 is greater than 48 mg/l in the bluegill and greater than 13 mg/l in the rainbow trout (1). The compound is practically non-toxic to the freshwater invertebrate Daphnia magna (2).

Primisulfuron-methyl is unlikely to pose a threat to endangered aquatic organisms.

Effects on Other Organisms (Nontarget species)

Primisulfuron-methyl is non-toxic to honey bees (4).

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

In laboratory tests on well aerated soils, half of the initial amount of the compound was broken down within two months. In soils without oxygen, breakdown of the compounds took nearly three months (2). Sunlight has little effect on the breakdown of the compound in soil or in water. Dissipation of the compound under field conditions was much quicker, with a half-life of three to twelve days (2).

Primisulfuron-methyl is very mobile in soil in field and laboratory studies. However, in the field studies, no primisulfuron-methyl was detected below a depth of 9 inches. The mobility of the compound in soils indicates that it has the potential to leach to surface water and to groundwater that lies close to the surface and is in highly vulnerable soils (1).

Breakdown of Chemical in Surface Water

The breakdown of the compound is much quicker in acidic soils and water rather than in neutral or alkaline soils and water (1).

Breakdown in Vegetation

Primisulfuron-methyl has been detected at concentrations up to 0.011 ppm in the non-edible portion of untreated food crops such as wheat, soybeans, sugar beets, corn and lettuce. These crops accumulated the compound from the site in which corn plants were treated with primisulfuron-methyl three to twelve months prior to the untreated crops. Thus, while degradation may be rapid, there is the potential for some compound residues to remain in the soil (1).

PHYSICAL PROPERTIES AND GUIDELINES

Exposure Guidelines:

RfD: NA
HA: NA
ADI: NA
PEL: NA

Physical Properties:

CAS #: 86209-51-0
Chemical name: 2-[4,6-bis(difluoromethoxy)pyrimidin-2-ylcarbamoylsulfamoyl] benzoic acid
Molecular weight: 468.3 g/mol
Solubility in water: 70 mg/l at 20 degrees C and pH 7.
Solubility in solvents: Acetone, 36 g/l; cyclohexanone, 27 g/l; isopropanol 0.4 g/l; methanol, 3.5 g/l; xylene, 0.3 g/l; toluene, 0.5 g/l.
Melting point: 203.1 degrees C
Vapor pressure: < 1 nPa at 20 degrees C
Partition coefficient (octanol/water) (log): 0.2
Adsorption coefficient: NA
Chemical Class/use: urea herbicide

BASIC MANUFACTURER

Ciba-Geigy
Ciba Plant Protection
P.O. Box 18300
Greensboro, NC 27419-8300
FAX: (910) 299-8318
Telephone: (910) 632-6000

Review by Basic Manufacturer:

Comments solicited: April, 1993
Comments received: January, 1994

REFERENCES

  1. Walker, M.M. and L.H. Keith. (1991). EPA's Pesticide Fact Sheet Database. Lewis Publishers. Chelsea, MI.
  2. The Agrochemical Handbook: Third Edition. (1991). Royal Society of Chemistry, Unwin Brothers Ltd., Surrey, England.
  3. The Farm Chemicals Handbook. (1992). Meister Publishing. Willoughby, OH.
  4. Worthing, C.R. and R.J. Hance. (1988). The Pesticide Manual, A World Compendium. Ninth Edition. The British Crop Protection Council