Occurrence of Multiple–herbicide Resistance in Penoxsulam–resistant Grasslike Fimbristylis (Fimbristylis miliacea (L.) Vahl.) in Paddy Fields

Abstract

Grasslike fimbristylis is a common weed of paddy fields, which is most often controlled by penoxsulam, acetolactate synthase (ALS) inhibitors. After several years of successful control there were recent failure reports by farmers in Krachan Sub-district, ​​U-Thong District, Suphan Buri Province, This study was conducted to determine whether grasslike fimbristylis populations have developed resistance to penoxsulam in paddy fields. Weed seeds were collected from farmer’s fields to determine the physiological response to penoxsulam. The experiment was designed as a split-plot design in CRD with four replications. The main plots consisted of six penoxsulam dose rates (0, 7.03, 14.06, 28.12, 56.24 and 112.48 g a.i./ha) while sub-plots were comprised of grasslike fimbristylis of susceptible and resistant biotypes. The experimental plots were treated with herbicides 15 days after seed sowing. Reaction of both resistant and susceptible biotypes to penoxulam were assessed for resistance index, I ₅₀ based on visual injury and GR₅₀ based on plant height and fresh weight at 5, 10, 14 and 21 days after application. The resistance index to penoxulam of resistant biotype was approximately 27-66 times higher than that of the susceptible biotype. The cross-herbicide resistance of penoxsulam-resistant biotype to the other ALS-inhibiting herbicides with the same site of action was determined. Results indicated that penoxsulam-resistant grasslike fimbristylis was not controlled at the labelled rate by bispyribac-sodium, pyribenzoxim and pyrazosulfuron-ethyl. In addition, the multiple herbicide resistance of penoxsulam-resistant biotype was subjected to treatments by other herbicides with different modes of action. Results indicated that alternative herbicides which could be used to control penoxsulam resistant grasslike fimbristylis were carfentrazone-ethyl (Contacts-membrane disrupters) and propanil (Inhibitors of photosynthesis at PS II) at the recommended rate of each herbicide. The results also suggested that farmers should rotate the application of carfentrazone-ethyl and propanil, which have different herbicide sites of action, to control and reduce the probability of penoxsulam-resistant grasslike fimbristylis occurrence in paddy fields.