Pestic. Sci. 1997, 51, 171È175 Resistance of Barny ardgrass ( Echinochloa crus - galli ) to Atrazine and Quinclorac Nuria Lopez-Martinez,1 George Marshall2 & Rafael De Prado1 1 Departamento de Qu•mica Agr•cola y Edafolog•a, E.T.S.I.A.M., Universidad de Cordoba, Apdo. 3048, Cordoba, Spain 2 Plant Science Department, Scottish Agricultural College, Auchincruive, Ayr KA6 5HW, UK (Received 28 May 1996 ; revised version received 14 February 1997 ; accepted 28 May 1997) Abstract : Two populations of Echinochloa crus-galli (R and I) exhibited resist- ance to quinclorac. Another population (X) exhibited resistance to quinclorac and atrazine. The R and I populations were collected from monocultures of rice in southern Spain. The X population was collected from maize Ðelds subjected to the application of atrazine over several years. The susceptible (S) population of the same genus was collected from locations which had never been treated with herbicides. The quinclorac value (dose causing 50% reduction in shoot ED 50 fresh weight) for the R and I biotypes were 26- and 6-fold greater than for the S biotype. The X biotype was 10 times more tolerant to quinclorac than the S biotype and also showed cross-resistance to atrazine, being 82-fold more resist- ant to atrazine than the R, I and S biotypes. Chlorophyll Ñuorescence and Hill reaction analysis supported the view that the mechanism of resistance to atrazine in the X biotype was modiÐcation of the target site, the DI protein. Quinclorac at 20 mg litre~1 did not inhibit photosynthetic electron transport in any of the test biotypes. The quinclorac values (herbicide dose needed for 50% Hill I 50 reaction reduction) of the S population was over 50 000-fold higher than the atrazine value for the same S population, indicating that quinclorac is not a I 50 PS II inhibiting herbicide. Propanil at doses greater than 0É5 kg ha~1 controlled all the biotypes. Pestic Sci., 51, 171È175, 1997 No. of Figures : 2. No. of Tables : 3. No. of Refs : 27 Key words : quinclorac, atrazine, Echinochloa crus-galli, Echinochloa oryzoides, herbicide resistance management 1 INTRODUCTION The genus Echinochloa includes the most important grass weeds in maize and rice crops. Infestation by weeds of this genus can cause crop yield losses of nearly 50%.1h3 For the control of Echinochloa spp. one of the main herbicides used in maize is atrazine, while in rice quinclorac is widely used. Atrazine is a PS II inhibiting herbicide which displaces plastoquinone at the Q b binding site on the D1 protein, thereby blocking elec- tron Ñow from to The quinolinecarboxylic Q a Q b .4,5 Contract grant sponsor : CICYT ; Contract grant number : AGF 97-1230. acid, quinclorac, causes inhibition of shoot growth accompanied by chlorosis and necrosis ; these symptoms observed in Echinochloa crus-galli (L) Beauv. shoots are closely correlated with an accumulation of endogenous toxic hydrogen cyanide formed as a co-product during quinclorac-stimulated ethylene biosynthesis.6,7 It has been widely demonstrated that the use of the same her- bicide family and/or mode of action, among other factors, could select weeds resistant to herbicides.8,9 Since 1978, only a few cases of triazine resistance in E. crus-galli have been detected.10 The Ðrst herbicide- resistant E. crus-galli was an atrazine-resistant biotype found in the USA10 and later another biotype in France,11 both in maize Ðelds continuously treated with 171 1997 SCI. Pestic. Sci. 0031-613X/97/$17.50. Printed in Great Britain (