Application of ALS inhibitors at pre-emergence is effective in controlling resistant barnyardgrass biotypes depending on the mechanism of resistance

Echinochloa species are one of the most troublesome weeds in rice crops, and their control is hampered due to herbicide resistance. The aim of this study was to identify the cross-resistance pattern and the differential resistance level for pre- and post-emergence applications of imazethapyr and penoxsulam in populations of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] with different ALS gene mutations. Of 26 biotypes, 23 were imazethapyr-resistant, and 10 were cross-resistant to penoxsulam. The resistance index (RI) to imazethapyr was 5.7-19.5 for Ser653Asn, 26.7-68.3 for Ala122Thr and Ala205Asn, and 70.9-252.9 for Trp574Leu. Only Trp574Leu also resulted in resistance to penoxsulam. The double ALS mutation Ala122Asn + Trp574Leu resulted in a RI for imazethapyr and penoxsulam higher than 2800. The ED50 for penoxsulam applied at pre-emergence was three and six times lower than at post-emergence for the susceptible and resistant biotypes, respectively. The application of imazethapyr at pre-emergence was more effective than at post-emergence only for the biotypes with low resistance level mutation Ser653Asn. The efficacy of the herbicide quinclorac was similar for the application at pre- and post-emergence for susceptible and resistant biotypes. The ALS mutations Ala205Asn and Ala122Asn + Trp574Leu are reported for the first time in barnyardgrass. The use of ALS inhibitors at preemergence should be prioritized over post-emergence in fields with resistant barnyardgrass and in need of using these products to control other non-resistant weeds.