BACKGROUND:Bifora radians, an annual weed in winter wheat, is distributed mainly in the Mediterranean area, Asia Minor and the Caucasus. It infests winter-sown crops of the Central Anatolia and Middle Black Sea regions of Turkey. Field experiments in heavily B. radians-infested fields were conducted over 3 years in Samsun, Turkey, to determine the response of B. radians to ALS-inhibiting herbicides, because growers had complained of a decrease in herbicide effect.

RESULTS: The efficacy of ALS inhibitors on a putatively tolerant population sprayed annually with ALS inhibitors and an adjacent allegedly sensitive population was estimated at the ED50 and ED90 response levels. The recommended rates of herbicides controlled 90% of the weed (ED90) in the sensitive population at the early stage of B. radians development, but not in the tolerant population. The relative potencies (EDx(tolerant)/EDx(sensitive)) of herbicides on the two populations were estimated by assuming years as being random effects. The relative potency was on average about 1.7, irrespective of the EDx levels.

CONCLUSION: Although the relative potencies were not large, they were large enough to be noted by growers. In field experiments it would be important to establish tools demonstrating when farmers recognise loss of herbicide efficacy. There has been no indication of evolution of resistant biotypes so far, but continuous spraying favours biotypes with increased levels of tolerance. Copyright © 2011 Society of Chemical Industry.