Distribution and resistance of barnyardgrass to quinclorac in rice fields in Thailand

Background: In Thailand, Echinochloa crus-galli (L.) P. Beauv. (barnyardgrass) is the most problematic weed in rice. Quinclorac was first commercialized in 1997 in Thailand and became the primary herbicide option for barnyardgrass control. The intensive use of quinclorac in rice fields increases the risk of the evolution of resistant barnyardgrass. Objective: This study was conducted to survey the occurrence of quinclorac-resistant barnyardgrass in Thailand, investigate the levels of quinclorac resistance, and evaluate alternative control measures for quinclorac-resistant barnyardgrass. Methods: Seeds of barnyardgrass were collected from 165 rice fields, located in 27 provinces of Thailand, and screened for quinclorac resistance. A whole-plant dose-response study was conducted on a susceptible population and three resistant populations to evaluate the resistance level. In addition, effectiveness of alternative herbicides including bispyribac, fenoxaprop, penoxsulam, profoxydim, propanil, and pyribenzoxim were evaluated using the selected populations. Results: Quinclorac-resistant barnyardgrass was identified in 121 rice fields (73%). Five sites (3%) exhibited developing barnyardgrass resistance to quinclorac whereas barnyardgrass populations in thirty-nine sites (24%) were susceptible to quindorac. The evaluated resistant populations were at least 93fold more resistant to quinclorac than the susceptible population. Profoxydim and propanil provided effective control of the two quindorac-resistant barnyardgrass populations. One population (B56) exhibited no shoot biomass reduction after treatment either with quinclorac or bispyribac, suggesting multiple resistance to auxin mimics and ALS-inhibiting herbicides. Conclusions: Quinclorac-resistant barnyardgrass cases were confirmed in Thailand. The resistant barnyardgrass populations were broadly distributed on major rice-production areas of Thailand. Multiple-resistance in a quinclorac resistant population requires further investigation.

Automated summary

This study investigated the occurrence and resistance level of quinclorac-resistant barnyardgrass in Thailand, as well as alternative control measures. The findings showed that quinclorac-resistant barnyardgrass was widely distributed in major rice-production areas of Thailand, with some populations exhibiting multiple resistance to other herbicides.