Rice flatsedge (Cyperus iria L.) is Resistant to the Auxinic Herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA)

Kirija Nishanthan, Samantha Dissanayaka, Tahananthan Sivananthawerl, Loha Pradheeban and Buddhi Marambe

Keywords:

Cyperus iria, herbicide resistance, MCPA, paddy fields, Sri Lanka

Abstract:

This study assessed the potential resistance build-up in rice flatsedge (Cyperus iria L.), a dominant sedge weed in paddy fields of Sri Lanka, to MCPA (2-methyl-4-chlorophenoxyacetic acid; WSSA/HRAC Group 4). Mature seeds of C. iria were collected from susceptible (S) and suspectedresistant (R) biotypes in five districts of the Northern Province of Sri Lanka. Experiments were conducted in a completely randomized design with five replicates. A knapsack sprayer fitted with a flat-fan nozzle was used to spray MCPA on C. iria seedlings at the three-leaf stage at ten dosages ranging from 25% (0.54 kg a.i. ha-1) to 300% (3.24 kg a.i. ha-1) of the recommended dose (1.08 kg a.i. ha-1), with a water-only control. Seedling survival % was estimated at ten days after treatment. The seed germination of MCPA-S and MCPA-R biotypes of C. iria did not significantly differ across districts (P>0.05). Probit analysis and ED50 values calculated using log-logistic model-fitting showed that C. iria has developed resistance against MCPA (Resistance Index ranging from 1.59 to 1.62), with no significant difference among districts (P>0.05). Auxin-mimicking Florpyrauxifen-benzyl effectively controlled the MCPA-R biotype with no indication of cross-resistance. The other four herbicides, namely, Bispyribac sodium, Carfentrazone ethyl, Pretilachlor + Pyribenzoxim and Fenoxaprop-p-ethyl + Ethoxysulfuron, did not effectively control MCPA-R and MCPA-S biotypes. However, multiple resistance in C. iria cannot be ruled out. Interestingly, MCPA at the recommended dosage also showed relatively poor control of MCPA-S biotypes, indicating the need to revisit the herbicide recommendation against the sedge weed in paddy fields.