Competitive Interaction of Canola (Brassica napus) against Wild Mustard (Sinapis arvensis) using Replacement Series Method

Introduction: Increasing costs of herbicide inputs in intensive crop production systems and the incidence of herbicide resistance in weeds have renewed interest in exploiting crop competitiveness to reduced herbicide use. Two factors contribute to crop competitiveness against weeds: ability to withstand competition (AWC), or the ability to maintain high yields in the presence of weeds, and weed suppressive ability (WSA), the ability of the crop to reduce weed biomass and seed production. Wild mustard is a dominant weed in rapeseed fields of Iran bringing about major yield losses. A strongly persistent seedbank, competitive growth habit, and high fecundity all contribute to its weedy nature ensuring that it will be a continuing problem. In addition to yield losses in rapeseed, wild mustard can reduce crop quality even at its low densities. The main objective of the current paper is to investigate the competitive ability of the canola against wild mustard, and evaluating of empirical yield loss model in predicting the effect of different densities of wild mustard on canola yield. Materials and Methods: The experiment was performed in a randomized complete blocks design with four replications using replacement series in which wild mustard and rapeseed were planted in different ratios of 8:0, 6:2, 4:4, 2:6 and 0:8 plants per pot in 2014. Wild mustard and rapeseed seeds were planted in 35 cm diameter plastic pots filled with a sandy clay loam soil and 1 and 2 cm deep, respectively. Plants were harvested from the soil surface at maturity and were oven dried at 75 ͦ C for 48h, while total shoot biomass for each species being determined. Measurements included shoot and root dry weight, plant height, number of branches per plant, number of pod per plant, number seed per pod and plant seed yield in rapeseed. Relative Yield (RY), Relative Yield Total (RYT) and Relative Crowding Coefficient (RCC) were calculated. Relative yield (RY) is a measure of the relative competitive ability of the two species. RY was calculated using the equation: where Ymix and Ymon are yields in mixture and monoculture. Relative Yield Total (RYT) describes how the species pair utilizes resources. RYT was calculated using the equation: RYT= Relative Crowding Coefficient (RCC) is a measure of competitiveness between the two species. The RCC was calculated using the equation: RCC= Where YAmix and YBmix are average yield per plant of A and of B grown in mixture, respectively, YAmon and YBmon are average yield per plant of A and B grown in monoculture, respectively. Means were compared using Duncans, Multiple Range Test (P 0.05) (SAS, 2002). Results and Discussion: Results showed that the relative yield of rapeseed decreased in the density ratio of 25 and 50 percent compared to same densities of wild mustard. In comparison, rapeseed in a lower or even equal density was more sensitive to competition than wild mustard and hence it faced to sharp yield decrease. However, in the higher planting densities of 75 percent the relative yield of rapeseed increased and the value reached to 0.497. Regarding the higher values of wild mustard compared to rapeseed’s relative yield in higher density ratios of 50 and 75 percent it can be concluded that wild mustard possesses a higher competitive strength, as a consequence, was able to better use nutrition resources. Grain yield influenced markedly by density ratios (