First population of Black Grass ( myosuroides) resistant to in northern Spain (Navarre). Irache Garnica, Juan A. Lezáun, Arturo Segura and Ricardo Biurrun, INTIA, Instituto Navarro de Tecnologías e Infraestructuras Agroalimentarias de Navarra, Avda. Serapio Huici 20-22, 31610-Villava (Spain). [email protected]

Introduction In Navarre, the most cultivated crops (regarding surface) are cereals ( and ). The application of herbicides to control grass weeds is one of the most important action to get a proper yields. The resistant populations of wild oat, raygrass and other broadleaf species have been already selected. From the last few years, black grass (Alopecurus Myosuroides) became a difficult weed to control due to its density increase. The mode of action most commonly used are HRAC groups A and B, which are being less and less effective. Therefore, a suspect of resistance is patent.

Objectives The main objective of the work reported here was to prove the existence of a resistant population of black grass to A and B herbicide mode of action. Another important objective is to compare the efficacy of A and B groups with other mode of actions.

Materials & methods The trial was carried out in wheat in Ripodas. A randomized blocs was designed with 4 replications. All treatments were applied in a water volume equivalent to 300 l/ha at a pressure of 3 bar. Weed assessments were recorded at specific times after herbicide application and were based on four dime-meter quadrant counts per plot (2 x 10 m2). Weed control results are listed as weed score in this report where 0 indicates no control and 10 indicates total weed control. The cereal herbicides were applied in two times: autumn (1), when the crop had 1 to 3 leaves (BCHH 10-13), and the weed had 1-2,5 leaves; and winter (2), when the crop had 4 tillers detectable (BCHH 24), and the weed had 2-4 tillers detectable.

Results The results of efficacy on black grass of herbicide applications were:

Table 1. Efficacy results. HRAC Weed Herbicide g a.i. x h-1 Spray timing Group* score** 1 Untreated 0 2 Polarpec+Mohican 2400+125 1 N+F 7 3 Polarpec+Mohican+Protur 2400+125+1500 1 N+F+C 9 4 Herold (120+240) 1 K+F 8 5 Herold+Protur (120+240)+1500 1 (K+F)+C 9 6 Herold+Polarpec (120+240)+2400 1 (K+F)+N 8 7 Atlantis (15+3) 2 B+B 2 8 Broadway (188+63) 2 B+B 2 9 Traxos Pro (300+300) 2 A+A 3 * In bold, mode of action with activity on grass weed. * * 0 = no weed control; 10 = 100% weed control The herbicides belonging to HRAC groups A and B failed and their weed control were nearly non-existent. The rest of treatments gave almost complete control of black grass. The best results were when Polarpec and Herold were combined with Protur, or rather, two modes of action were combined.

Discussion The result of this trial indicates that the herbicides commonly used to control black grass were commercially non acceptable. It seems that the replication during the last years of these modes of action (A and B) have finally selected a resistant population. These control problems occur in lands nearby. When mode of action is changed, the result of efficacy is successful. Therefore, mode of action on black grass must be changed or alternated in the coming years, even if population are still sensible to HRAC groups A and B.