In order to evaluate the competitive ability (CA) of canola cultivars against wild mustard, two experiments were conducted at the Gorgan Institute in Iran during the 2005–2007 cropping seasons. The experimental factors were canola cultivars (1st year: Zarfam, Option500, Hayola330, Hayola401, Talayh, RGS003 and Sarigol; 2nd year: Zarfam, Hayola330, RGS003 and Option500) and weed density (1st year: control and 30 plants m2 ; 2nd year: control, 4, 8 and 16 plants m2 ). The result of the first year of experiment indicated that the grain yield and competitive indices differed significantly between the cultivars. Cultivar Zarfam showed a high ability to withstand competition (AWC ¼ 47%), high competitive indices (CI ¼ 1.79 and CI2 ¼10 1.83) and low grain yield in the weed-free plots (1729 kg ha1 ). The cultivar Option500, a less competitive cultivar had the lowest ability to withstand competition (AWC ¼ 4%) and the lowest competitive indices (CI ¼ 0.09 and CI2 ¼ 0.11) amongst the cultivars. However, the cultivar Option500 showed more grain yield in the weed-free plots (2333 kg ha1 ) than cultivar Zarfam. In the second year of the experiment, the result of the yield loss models showed that the lowest and highest yield loss belonged to cultivars Zarfam and Option500 (50 and 95% respectively). A comparison of different empirical models revealed that the empirical yield loss model based on weed relative leaf area was more reliable for predicting canola yield loss according to a high coefficient of determination (R2 ¼ 0.99). The relative damage coefficient (q) of the weed relative leaf area model showed that wild mustard was more competitive than canola (q > 1).

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