Abstract
To evaluate the effect of different levels of nitrogen fertilizer on competitive ability of canola against wild mustard interference, a split plot trial based on a randomized complete block design with 3 replications was carried out. Experimental treatment were amounts of nitrogen fertilizer of urea (46% nitrogen) in 4 levels (100, 150, 200 and 250 kgN/ha) and 5 wild mustard densities (0, 4, 8, 16 and 32 plant/m2). Linear reciprocal, yield loss and one and two parameter relative leaf area index models were fitted on data obtained from experiment, coefficients of these equations were used to evaluate changes in competitiveness of canola in nitrogen application levels. The results showed that all models well described the trend of canola yield loss in wild mustard interference. More nitrogen application led to increase biological and grain yield of canola on interference conditions. Biological and grain yield of canola decreased significantly with increasing plant density of wild mustard in four levels of nitrogen. Coefficients obtained from reciprocal model showed that increasing of nitrogen to 200 kg/ha led to increase relative competitive ability of canola against wild mustard, also grain yield was more affected by competition at all levels of nitrogen than biological yield. The lowest initial slope of yield loss (I) and the highest economic threshold of wild mustard and minimum relative damage coefficient (q) of two-parameter relative leaf area model were obtained in 200 kgN/ha. Overall results of this study showed that the optimum level of nitrogen fertilizer was 200 kgN/ha.

Keywords: Canola, Competition, Wild mustard, Yield, Empirical model