Evaluation of Castor bean (Ricinus communis L.) competition with redroot pigweed (Amaranthus retroflxus L.) at different levels of plant density

In order to evaluate the castor bean’s competitive effect with redroot pigweed at different levels of plant density, a factorial experiment based on randomized complete block design was conducted in three replications at Agricultural Research of West Azarbayjan in Urmia in 2011. The first factor was different densities of castor bean in five levels 0 (Pure stand of pigweed), 3, 4, 5 and 6 plant.m-², and second factor was three levels of redroot pigweed 0 (Pure stand of castor), 5, 10 and 15 plant.m-². Results showed that with increasing pigweed density, castor bean grain yield 20-38%, oil percent 39%, and plant height 32% comparing to pure culture decreased. The maximum amount of leaf dry weight (58.7g.m-2), stem dry weight (49.3g.m-2), number of branches (5.3), and height (44.9cm) belonged to pure stand of pigweed, that decreased by increasing castor bean density. With increasing of pigweed density, its leaf area index in pure culture reached the highest level (1.44) which was 50% higher than with 15 plant/m2 of pigweed, and 6 plant.m-² of castor bean. The highest (157cm) and the lowest (107 cm) height of castor bean attained at 6 and 3 plants per m2, respectively. Therefore that increasing pigweed density decrease grain yield and oil yield. The amount of this reduction was related to pigweed density.