Persistence of pyrazosulfuron in rice-field and laboratory soil under Indian tropical conditions

Pyrazosulfuron ethyl, a new rice herbicide belonging to the sulfonylurea group, has recently been registered in India for weed control in rice crops. Many field experiments revealed the bioefficacy of this herbicide; however, no information is available on the persistence of this herbicide in paddy soil under Indian tropical conditions. Therefore, a field experiment was undertaken to investigate the fate of pyrazosulfuron ethyl in soil and water of rice fields. Persistence studies were also carried out under laboratory conditions in sterile and non-sterile soil to evaluate the microbial contribution to degradation.

High-performance liquid chromatography (HPLC) of pyrazosulfuron ethyl gave a single sharp peak at 3.41 min. The instrument detection limit (IDL) for pyrazosulfuron ethyl by HPLC was 0.1 µg mL(-1) , with a sensitivity of 2 ng. The estimated method detection limit (EMDL) was 0.001 µg mL(-1) and 0.002 µg g(-1) for water and soil respectively. Two applications at an interval of 10 days gave good weed control. The herbicide residues dissipated faster in water than in soil. In the present study, with a field-soil pH of 8.2 and an organic matter content of 0.5%, the pyrazosulfuron ethyl residues dissipated with a half-life of 5.4 and 0.9 days in soil and water respectively. Dissipation followed first-order kinetics. Under laboratory conditions, degradation of pyrazosulfuron ethyl was faster in non-sterile soil (t(1/2) = 9.7 days) than in sterile soil (t(1/2) = 16.9 days).

Pyrazosulfuron ethyl is a short-lived molecule, and it dissipated rapidly in field soil and water. The faster degradation of pyrazosulfuron in non-sterile soil than in sterile soil indicated microbial degradation of this herbicide.