Current status of resistance Ischaemum rugosum Salisb., to herbicide profoxydim in Venezuela.
Ischaemum rugosum Salisb., is currently the most difficult weed problem in Venezuelan rice fields. The objective of this research was to determine the resistance of I. rugosum to profoxydim. Two experiments were established: (1) bioassay detection and (2) whole-plant dose-response. In the first experiment, 87 accessions and a susceptible control (IR143G) were treated with 160 g a.i. ha-1 of profoxydim (recommended rate); percentage of fresh weight was estimated 21 days after application. To quantify the resistance level, four resistant accessions: IR37P, IR44P, IR75G and IR90G, were treated with 0, 20, 40, 80, 160, 320, 640, 1280 g a.i. ha-1 of profoxidim, and the susceptible accession IR143G with 0, 1.25, 2.5, 5, 10, 20, 40 and 80 g a.i. ha-1 of profoxidim. Thirty-seven accessions from Portuguesa State, six from Guarico, six from Cojedes, two from Barinas and one from Carabobo, were found resistant to profoxidim (59% R). The resistance index (RI=ED50R/ED50S) of IR37P and IR44P were 92.62 and 100.49, respectively, corroborating the resistance of I. rugosum to profoxidim. RI values of IR75G and IR90G could not be estimated because (ED50 R>1280 g i.a. ha-1).