Response of Landrace and Improved Genotypes of Rice to Weed Competition in Direct- Seeded System

In order to evaluate the competitive ability of rice genotypes against weeds in direct seeding system and determination the traits that may confer such attributes, a field experiment was conducted at the Rice Research Institute of Iran in Rasht during 2016-17 cropping season. The experiment was laid out in a split plot based on randomized complete blocks design with three replications. The treatments included two levels of weed management (weedy (under natural weed flora) and weed-free) as the main plots, and ten rice genotypes (Hashemi, Abjiboji, Tazeh Hasani, Gilaneh, Shiroodi, Saleh, RI18430-1, RI18430-60, RI18430-72, RI18430-77) as subplots. The results showed genotypic differences in competitive ability against weeds. Under weed- free conditions, grain yield varied from 3293.5 in Saleh to 6796.1 kg.ha-1 in Shiroodi. Under weedy conditions, grain yield was also registered from 260.6 in breeding line RI18430-72 to 1291.2 kg.ha-1 in Hashemi. The highest and lowest amounts of weed biomass at flowering were also observed in Abjiboji and shiroodi, respectively. Average yield loss due to weed competition ranged from 73.55 to 94.1% in different rice genotypes. The results also indicated a positive significant correlation between competitive ability and grain yield, thus Hashemi, Shiroodi and the breeding line RI18430-60 had the highest competitive ability against weeds and also grain yield (the basis for direct selection under weedy condition). This method can be applied in integrated weed management programs to reduce herbicide use. The study revealed that early growth (vigor), leaf area index, crop biomass and height had significant positive correlation with competitive ability and hence, can be used as useful indirect selection criteria of genotypes with high competitive ability.