CROPS AND SOILS RESEARCH PAPER Distribution and analysis of the mechanisms of resistance of barnyardgrass (Echinochloa crus-galli) to imidazolinone and quinclorac herbicides F. O. MATZENBACHER 1 , E. D. BORTOLY 1 , A. KALSING 2 AND A. MEROTTO Jr 1 * 1 Graduate Group in Plant Sciences, Federal University of Rio Grande do Sul, UFRGS, Porto Alegre, RS, Brazil 2 Institute Riograndense of Rice, IRGA, Agriculture Sector, Cachoeirinha, RS, Brazil (Received 1 April 2014; revised 24 May 2014; accepted 9 July 2014) SUMMARY The lack of control of barnyardgrass in flooded rice cultivated with imidazolinone-resistant rice cultivars is challenging the utilization of this system, which is continuously expanding for new rice areas worldwide. The objectives of the present study were to evaluate the frequency, distribution and mechanisms of imidazolinone resistance in barnyardgrass to establish the best practices to control and prevent this problem. The distribution of resistance was evaluated in 624 populations collected in Southern Brazil. The frequency of imidazolinone- herbicide resistance was 0·81, broadly distributed in all sampled regions. Resistance to quinclorac was also found in 0·19 of the populations, but all of the evaluated populations were susceptible to cyhalofop-butyl. Further studies were conducted in six populations. The enhanced metabolism was assessed with the metabolic inhibitors that reversed the resistance to quinclorac from 0·54 to 1·00 in two populations and the resistance to imazethapyr from 0·15 to 0·41 in three populations. The acetolactase synthase (ALS) enzyme activity also indicated the occurrence of altered target site resistance in two populations caused by the ALS gene mutations Trp574Leu and Ser653Asn, which is a novel finding in this species. The herbicide resistance in barnyardgrass in Southern Brazil presented a complex basis of resistance because it is associated with resistance to multiple herbicides due to multiple mechanisms and with multiple mutations of the ALS gene. This indicates that it is necessary to adopt specific measures to prevent and control the evolution of multiple herbicide resistance in this species. INTRODUCTION The negative interference caused by weeds is an important limitation to achieving high levels of productivity in rice (Oerke 2006). Several species of the genus Echinochloa are among the major rice weeds and occur worldwide in areas cultivated with different rice establishment systems. The main weed species of this genus are E. crus-galli (barnyardgrass), Echinochloa colona (junglerice), Echinochloa phyllo- pogon (late watergrass), Echinochloa oryzoides (early watergrass) and Echinochloa crus-pavonis (gulf cock- spur). These species have large morphological varia- bility and intermediate traits that make species identification difficult. This problem has resulted in the denomination of ‘Echinochloa complex’ in several studies (Damalas et al. 2008). The high frequency of these species in paddy fields has resulted in the intensive use of herbicides and consequent evolution of resistance to compounds of different mechanisms of action. Barnyardgrass has evolved resistance to propanil (Valverde et al. 2001), atrazine (Lopez- Martinez et al. 1997) and quinclorac (Lopez-Martinez et al. 1997; Concenço et al. 2008). Recently, resistance to glyphosate has been found in junglerice (Alarcón- Reverte et al. 2013). In addition, partial tolerance of junglerice to oxyfluorfen and pendimethalin was reported as a beginning of the evolution of resistance in this species (Jafun et al. 2003). Late watergrass resistant to molinate, thiobencarb, fenoxaprop-P-ethyl, bispyribac-sodium, clomazone (Yasuor et al. 2008) and penoxsulam (Yasuor et al. 2009) was found * To whom all correspondence should be addressed. Email: merotto@ufrgs.br Journal of Agricultural Science, Page 1 of 15. © Cambridge University Press 2014 doi:10.1017/S0021859614000768