Evolution of drought stress and nitrogen rate on redroot pigweed (Amaranthus retroflexus) and corn (Zea mays) competition

In order to evaluate the effect of irrigation intervals and nitrogen rate on corn and redroot pigweed competition، a split- plot experiment was conducted based on strip block at Agricultural Research Station of Ferdowsi University of Mashhad. Treatments were combination of 3 irrigation intervals (7، 14 and 21 days) as main-plots and different rates of urea fertilizer (0، 100، 200 and 300 kg/ha) as sub-plots. Results showed increasing nitrogen rates had a significantly effect (P<0. 01) on corn yield in weed free conditions. But in the presence of redroot pigweed، nitrogen had no significant effect on corn yield and redroot pigweed competition was the main cause of corn yield reduction specially in water stress conditions. Reduction of corn yield by redroot pigweed competition in optimum irrigation (7 day) was 55% that equals to the yield reduction in 14 and 21 days irrigation intervals. It seems that in corn-redroot pigweed competition، water is more important resource than nitrogen and effects of water stress، nitrogen and pig-weed were additive.