Herbicide
HRAC  C1 WSSA  5; 1,2,4-triazinone



NOMENCLATURE
Common name metribuzin (BSI, E-ISO, WSSA); métribuzine ((f) F-ISO)
IUPAC name 4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one; 4-amino-6-tert-butyl-3-methylthio-1,2,4-triazin-5(4H)-one 
Chemical Abstracts name 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one 
CAS RN [21087-64-9]  EEC no. 244-209-7  Development codes Bayer 94 337; DIC 1468 (both Bayer); DPX-G2504 (Du Pont) 

PHYSICAL CHEMISTRY
Mol. wt. 214.3  M.f. C₈H₁₄N₄OS  Form White crystals, with a weak characteristic odour.  M.p. 126.2 ºC  B.p. 132 ºC/2 Pa.  V.p. 0.058 mPa (20 ºC)  K_OW logP = 1.6 (pH 5.6, 20 ºC)  Henry 1 × 10^-5 Pa m³ mol^-1 (20 °C, calc.)  S.g./density 1.28 (20 ºC)  Solubility In water 1.05 g/l (20 ºC). In dimethylformamide 1780, cyclohexanone 1000, chloroform 850, acetone 820, methanol 450, dichloromethane 340, benzene 220, n-butanol 150, ethanol 190, toluene 87, xylene 90, isopropanol 77, hexane 1.0 (all in g/l, 20 ºC).  Stability Relatively stable to u.v. irradiation. At 20 ºC, stable to dilute acids and alkalis; DT₅₀ (37 ºC) 6.7 h (pH 1.2); DT₅₀ (70 ºC) 569 h (pH 4), 47 d (pH 7), 191 h (pH 9). Photodecomposition in water is very rapid (DT₅₀ <1 d). On soil surfaces under natural light conditions, DT₅₀ 14-25 d.

COMMERCIALISATION
History Herbicide reported by W. Draber et al. (Naturwissenschaften, 1968, 55, 446) and reviewed by L. Eue (Pflanzenschutz-Nachr. (Engl. Ed.), 1972, 25, 175). Introduced by Bayer AG and E. I. du Pont de Nemours and Co., and first marketed in 1971.  Patents BE 697083, DE 1795784 both to Bayer; US 3905801 to Du Pont

APPLICATIONS
Biochemistry Photosynthetic electron transport inhibitor at the photosystem II receptor site. Selectivity is due to metabolism (mostly conjugation) within the plant (C. Fedtke, Proc. Br. Crop Prot. Conf. - Weeds, 1993, 1,221).  Mode of action Selective systemic herbicide, absorbed predominantly by the roots, but also by the leaves, with translocation acropetally in the xylem.  Uses Pre- and post-emergence control of many grasses and broad-leaved weeds in soya beans, potatoes, tomatoes, sugar cane, alfalfa, asparagus, maize and cereals, at 0.07-1.05 kg a.i./ha.  Phytotoxicity Phytotoxic to many crops, including crucifers, cucurbits, lettuce, onions, sugar beet, sunflowers, flax, strawberries, sweet potatoes, and tobacco.  Formulation types SC; WG; WP.  Compatibility Compatible with most other herbicides, except in highly concentrated mixtures

ANALYSIS
Product analysis by glc with FID (AOAC Methods, 1995, 984.11; CIPAC Handbook, 1988, 1D, 124) or by i.r. spectrometry (J. W. Betker et al., J. Assoc. Off. Anal. Chem., 1976, 59, 278). Residues determined by glc (C. A. Anderson, Anal. Methods Pestic. Plant Growth Regul., 1976, 8, 453). In drinking water, by gc with FID (AOAC Methods, 1995, 991.07). Details available from Bayer AG.

MAMMALIAN TOXICOLOGY
Reviews Toxikologie der Herbizide, Deutsche Forschungsgemeinschaft [Data collection on toxicology of herbicides], 7. Lieferung, Weinheim, 1981.  Oral Acute oral LD₅₀ for rats c. 2000, mice c. 700, guinea pigs c. 250, cats >500 mg/kg.  Skin and eye Acute percutaneous LD₅₀ for rats >20 000 mg/kg. Not irritating to skin and eyes (rabbits).  Inhalation LC₅₀ (4 h) for rats >0.65 mg/l air (dust).  NOEL (2 y) for rats and dogs 100, mice c. 800 mg/kg diet.  ADI 0.013 mg/kg b.w.  Toxicity class WHO (a.i.) III (Table 5); EPA (formulation) III  EC hazard Xn; R22| N; R50, R53  

ECOTOXICOLOGY
Birds Acute oral LD₅₀ for bobwhite quail 168, mallard ducks 460-680 mg/kg. Dietary LC₅₀ (5 d) for bobwhite quail and mallard ducks >4000 mg/kg diet.  Fish LC₅₀ (96 h) for bluegill sunfish 80, rainbow trout 76, goldfish >10, catfish >10 ppm.  Daphnia LC₅₀ (48 h) 4.5-35 mg/l.  Algae ErC₅₀ for Scenedesmus subspicatus 0.021 mg/l.  Bees Not toxic to bees; LD₅₀ 35 mg/bee.  Worms LC₅₀ for Eisenia foetida 331.8 mg/kg dry soil.

ENVIRONMENTAL FATE
Animals In mammals, following oral administration, 98% elimination occurs within 96 hours, about equally in the urine and the faeces.  Plants In plants, metribuzin undergoes oxidative deamination and further degradation to water-soluble conjugates.  Soil/Environment Metribuzin is rapidly degraded in soil; microbial breakdown is the major mechanism of loss. Degradation involves deamination, followed by further degradation to water-soluble conjugates. Photodecomposition on soil surfaces and in aqueous solution is an important process for the degradation of metribuzin in the environment.