Introducing an easy-to-measure and reliable index that enables researchers and farmers to estimate the crop performance under weed infestation would be of great value. Eight field experiments were conducted at two locations, Karaj and Varamin, in Iran in 2003-2004. At each location, eight Iranian wheat cultivars were studied in competition against four weed species. Weed species were wild oat (Avena fatua), rye (Secale cereale), flixweed (Descurainia sophia) and rocket (Eruca sativa), each of which were studied in a single experiment. Leaf area index (LAI) of the wheat crop was measured destructively throughout the growing season and peak LAI was recorded. Wheat grain yield and biomass were also measured at final harvest. The ratio of weed-infested to weed-free grain yield (R) was used as a criterion for selection of successful wheat cultivars under weed competition. According to this criterion, wheat cultivars were judged from two standpoints; i.e. cultivars with the ability to tolerate weeds (0.90 < R < 1) and cultivars with the ability to suppress weeds (R > 1). The results indicated that cultivars Mahdavi and Pishtaz were able to tolerate weeds while cultivars Roshan, Pishtaz and Karaj2 were able to suppress weeds. In the latter case, the grain yield under weed-infested condition was higher than weed-free condition. It was found that cultivars Roshan, Pishtaz and Karaj2 had higher peak LAI under weed-infested condition compared to other cultivars considering all weed species and locations. Regressing wheat grain yield and biomass against peak LAI resulted in two different responses. Wheat biomass was positively associated with peak LAI while wheat grain yield was negatively associated with this index. However, the association between grain yield and peak LAI differed between weed-free and weed-infested conditions; in the latter, a linear relationship with almost constant slope was observed. Overall, it was concluded that peak LAI could be reliably used as an appropriate index for identification of successful wheat cultivars under weed infestation.

Keywords