MP5 Agrochemicals and Pesticides
Monday, 14 November 2005: 8:00 AM - 5:30 PM in Exhibit Hall

(JAB-1117-575453) Dissipation of penoxsulam, a new sulfonamide herbicide, in simulated rice field conditions.

Jabusch, T¹, Tjeerdema, R¹, ¹ University of California, Davis, CA, USA

ABSTRACT- Penoxsulam (trade name Granite) is a new acetolactate synthase (ALS) inhibitor herbicide for post-emergence control of annual grasses, sedges, and broadleaf weeds in rice culture. This laboratory study ascertains microbial and photochemical degradation rates and transformation pathways as well as partition constants under conditions simulating flooded rice field conditions. Soil sorption values were inversely related with the pH and ranged from 0.14 to 5.3, indicating that penoxsulam is qualitatively mobile and there is no significant retention in the soil. Penoxsulam did not partition into air at any measurable rate at 20°C or 40°C and its K_H (pH 7) was estimated at 4.6 x 10^-15 Pa × L × mol^-1 based on available water solubility and vapor pressure data. Preliminary results from biodegradation studies in microcosms simulating flooded conditions in a silty clay soil revealed a DT₅₀ value of 45 h and three detectable but transient transformation products. Additional experiments are in progress for a thorough determination of transformation rates and products in the tested soil and three additional soils. In addition, results from a photodegradation study in a photoreactor and in 'outdoor' experiments will be presented. Based on the molar absorptivity of penoxsulam and the maximum photoreaction rate for spring and summer conditions in California, a minimum half-live of 1.2 h (spring) and 43 min (summer) was calculated. Results imply that dissipation of penoxsulam in flooded rice fields is rapid and controlled by microbial and photochemical degradation rates.

Key words: penoxsulam, aquatic fate, biotransformation, photolysis